llvm dev - Dec 2018 - [RFC][llvm-mca] Adding binary support to llvm-mca.

+1 to what Clement said.
I believe the intrinsics are a better design to support many architectures.

IACA users are probably decorating their code with IACA_START / IACA_END
macros. One possibility is to provide a header that define these macros in
terms of the new intrinsics.

On Mon, Dec 10, 2018 at 3:59 PM Clement Courbet <courbet at google.com>
wrote:
> Hi Matt/Andrea,
>
> I see pros and cons for IACA-style markers vs intrinsics.
> On the one hand, IACA-style markers are very magical, and not very visible
> in both the source and object code. Using IACA-style markers has the
> advantage that you can use llvm-mca as a drop-in replacement for IACA, or
> even to compare their outputs on the exact same binary. They also do not
> require tooling on the compiler side and allow comparing the output of
> several compilers.
>
On the other hand, IACA-style markers do not have a equivalent on
other
> architectures, and I'm not sure inventing new ones is a good idea :) I
> think the latter makes them pretty much a no-go for llvm-mca as I don't
> think we'll want to teach each target how to parse code regions.
That's
> much better handled in a target-agnostic way by the object. Intel got away
> with them because they only had to support one architecture.
>
> tl;dr: In the case of llvm-mca, I like your design better than the markers.
>
> In terms of future-proofness of only allowing regions within a basic
> block, are we confident we can actually ever simulate branches apart from
> "always taken, perfectly predicated" loop ? Even this simple need
requires
> knowing quite a few details on the frontend. The current design could
> handle this use case with the addition of an external "loop mode"
option to
> MCA. If there are no other strong use cases, I would advocate for
> experimental intrinsics unless people can contribute other example use
> cases.
>
> On Mon, Dec 3, 2018 at 11:38 PM Matt Davis <matthew.davis at
sony.com> wrote:
>
>> Hi Andrea,
>>
>> On Mon, Dec 03, 2018 at 01:21:33PM +0000, Andrea Di Biagio wrote:
>> > So, I have been thinking a bit more about this whole design.
>> >
>> > The more I think about your suggested design, the more I am
convinced
>> that
>> > we should do something more to support ranges in binary object
files
>> too.
>> > My understanding is that the reason why we don't support
object files in
>> > general, is because of the presence of relocations. That is
because a
>> > region start marker is effectively symbol relative, and the symbol
(a
>> > function) would be relocated in the final executable.
>> > You mentioned to me that resolving even a 'simple'
symbol-relative
>> > relocation is not trivial, beause it requires specific knowledge
about
>> the
>> > binary format, and the target (i.e. how relocations are encoded is
>> target
>> > specific). I am surprised that there is not a utility library for
>> resolving
>> > relocations.. but I am not familiar with that part of the
compiler. I
>> was
>> > hoping that there was a target specific interface to use in this
case...
>>
>> There might be a better way of resolving the relocs, but from what I
saw
>> looking at llvm-objdump and other related tools, it seems that
resolving
>> the relocated symbol is a target specific effort.  I also spent
sometime
>> sniffing around ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp which also
>> performs the reloc resolution.  I should clarify that I too am not an
>> expert in llvm's utilities for performing symbol/reloc resolution,
and
>> perhaps someone in the community can point me in the right direction. 
I
>> can clearly see the reloc data in the object file via tools like
>> objdump; however, accessing the relocs via
>> llvm::object::ObjectFile::relocations() did not produce address values
>> that we could use (values of zero).
>>
>> I was hoping that, for a first pass at this patch, supporting just
>> executables would be okay.  That keeps this initial patch set simple,
>> and hopefully will encourage others to take a peek at it, since
it's
>> less daunting than what it might otherwise be.  Of course, there is the
>> concern that this initial patch will lock us into a design that will be
>> more complicated to unravel later.
>>
>> > An alternative approach would require that you define your own
>> > "symbol-relative" reference. After all, ranges are just
a sequences of
>> > instructions in a function. If a function symbol is described by
the
>> symbol
>> > table, then you should be able to obtain its offset in the .text
>> section.
>> > So, you could potentially encode your own symbol+offset. However,
the
>> > linker would not be able to understand your "custom
relocation", and
>> > information about regions in the final elf would be basically
broken.
>> > So,that would not be a solution...
>> >
>> > I don't know honestly what is the best approach to use in this
case.
>> > As a compromise, it would not be a bad idea to add the ability to
>> specify
>> > ranges from command line. What do you think?
>> > Still, from a user point of view, the idea that we don't
support object
>> > files in general sounds like a big limitation.
>>
>> I agree, only supporting executables is a limitation.  However, I'd
>> like to land the base support now and add in the additional
>> features/support after this large patch set lands.  But I can see
>> where landing the whole thing entirely also makes sense.
>>
>> > About the new experimental intrinsics: those would definitely work
well
>> for
>> > the simple case where instructions are from the same basic block.
>> > However, some/most of the constraints that you plan to add will
have to
>> > change if in future we decide to allow ranges that potentially
cross
>> > multiple basic blocks. How will the rules/constraints on those new
>> > intrinsics change? I just want to make sure that the suggested
design is
>> > future-proof.
>>
>> Since the llvm/clang parts of the code are just responsible for
>> collecting where a range starts/ends, I hope that we can remove some
>> of the baked-in constraints that are specified in IR/Verifier.cpp.
>> As you pointed out earlier in this thread, we might want to
>> introduce a dominance check if/when we lift the one-basic-block
>> restriction.
>>
>> -Matt
>>
>> >
>> > -Andrea
>> >
>> > On Tue, Nov 27, 2018 at 5:08 PM Andrea Di Biagio <
>> andrea.dibiagio at gmail.com>
>> > wrote:
>> >
>> > > Thanks for clarifying it Matt.
>> > >
>> > > In general, I quite like your suggested design.
>> > >
>> > > My only concern is about the semantic of the two new
intrinsics. You
>> > > design doesn't allow mca ranges to span through multiple
basic
>> blocks. That
>> > > constraint is acceptable for now, since llvm-mca doesn't
know how to
>> deal
>> > > with control flow.
>> > > However, I am a bit concerned about what might happen in
future if we
>> > > decide to let users specify code regions that span through
multiple
>> basic
>> > > blocks. Basically, I don't particularly like the idea of
changing the
>> > > semantic of already existing intrinsic. A design that already
>> accounts for
>> > > that particular scenario/future work would be ideal. That
being said,
>> > > marking those new intrinsics as 'experimental' may be
a good
>> compromise (at
>> > > least for now).
>> > >
>> > > So, I am quite happy overall with the direction of this RFC.
>> > > However, I am interesting to hear from other developers about
your
>> > > suggested design.
>> > >
>> > > > This initial patch only targets ELF object files, and
does not
>> handle
>> > > relocatable addresses. Since the start of a code region is
>> represented as
>> > > an
>> > > assembly label, and referenced in the .mca_code_regions
section, that
>> > > address
>> > > is relocatable.
>> > >
>> > > This may be okay for now. However, it would be nice to remove
that
>> > > constraint in future and add support to generic object files.
>> > >
>> > > -Andrea
>> > >
>> > > On Thu, Nov 22, 2018 at 7:21 PM <Matthew.Davis at
sony.com> wrote:
>> > >
>> > >> I want to clarify a few restrictions of llvm-mca code
regions that
>> this
>> > >> RFC proposes:
>> > >>
>> > >> 1) All llvm-mca code regions must start with an
>> > >> llvm.mca.code.region.start intrinsic and end with
>> > >> an llvm.mca.code.region.end intrinsic.  This rule is
enforced at the
>> IR
>> > >> level in the IR verifier.
>> > >>
>> > >> 2) llvm-mca code regions cannot nest.  This restriction
implies that
>> an
>> > >> llvm.mca.code.region.start
>> > >> must have a llvm.mca.code.region.end intrinsic without
any other
>> llvm.mca
>> > >> start intrinsics
>> > >> between the two. The current implementation in the patch
enforces
>> this
>> > >> restriction at the
>> > >> IR level via the IR Verifier.
>> > >>
>> > >> 3) An llvm-mca code region cannot span multiple basic
blocks.
>> llvm-mca
>> > >> does not follow
>> > >> branches (yet).  Instead, a branch instruction is treated
by llvm-mca
>> > >> like any other instruction.
>> > >> The current patch associated with this RFC does not
enforce this
>> > >> restriction.  I plan on updating
>> > >> the patch to enforce that a code region can only belong
to a single
>> basic
>> > >> block.  This is a simple
>> > >> check, ensuring that both the llvm.mca.code.region.start
and
>> accompanying
>> > >> end intrinsics live
>> > >> in the same basic block. I imagine adding this check at
the IR level
>> when
>> > >> we also verify points 1 and 2
>> > >> above.  That will keep the code-region verification logic
isolated
>> to the
>> > >> IR verifier.  The start/end
>> > >> intrinsics should not have any uses, so I'm not sure
that they would
>> be
>> > >> moved/sunk on behalf
>> > >> of any other instruction.  In other words, I do not
imagine that a
>> start
>> > >> and end would be split
>> > >> apart due to later MI optimizations.  If I discover that
such a case
>> > >> occurs, then I might add the
>> > >> basic-block check prior to emitting the code region data
to the
>> object
>> > >> file.    Once  llvm-mca  is
>> > >> updated to handle branches, then we can remove this
constraint.
>> > >>
>> > >> -Matt
>> > >>
>> > >> > -----Original Message-----
>> > >> > From: llvm-dev <llvm-dev-bounces at
lists.llvm.org> On Behalf Of Matt
>> > >> Davis via llvm-
>> > >> > dev
>> > >> > Sent: Wednesday, November 21, 2018 8:47 AM
>> > >> > To: Andrea Di Biagio <andrea.dibiagio at
gmail.com>
>> > >> > Cc: llvm-dev <llvm-dev at lists.llvm.org>; Di
Biagio, Andrea
>> > >> > <Andrea.Dibiagio at sony.com>; cfe-dev at
lists.llvm.org
>> > >> > Subject: Re: [llvm-dev] [RFC][llvm-mca] Adding
binary support to
>> > >> llvm-mca.
>> > >> >
>> > >> > Hi Andrea,
>> > >> >
>> > >> > Thanks for your input.
>> > >> >
>> > >> > On Wed, Nov 21, 2018 at 12:43:52PM +0000, Andrea Di
Biagio wrote:
>> > >> > [... snip ...]
>> > >> > > About the suggested design:
>> > >> > > I like the idea of being able to identify code
regions using a
>> numeric
>> > >> > > identifier.
>> > >> > > However, what happens if a code region spans
through multiple
>> basic
>> > >> blocks?
>> > >> >
>> > >> > The current patch does not take into consideration
cases where the
>> > >> > region start and end intrinsics are placed in
different basic
>> blocks.
>> > >> > Such would be the case if a region is defined to
span multiple
>> blocks.
>> > >> > This would be similar to the current case where a
user places a
>> > >> > #LLVM-MCA-BEGIN assembly comment in one block and an
#LLVM-MCA-END
>> in
>> > >> > another.  However, as you point out below, if the
user does this
>> in the
>> > >> > source code via intrinsics (just what this patch is
proposing),
>> then
>> > >> > there is a chance that optimizations might change
the layout of the
>> > >> > instructions and confuse the ordering of the MCA
intrinsics.
>> > >> >
>> > >> > Since MCA does not follow branches (MCA just treats
a branch as it
>> would
>> > >> > a non-branching instruction), it seems that a user
should be aware
>> that
>> > >> > defining MCA code regions that span multiple blocks
might result
>> in an
>> > >> > unexpected analysis.  While we do not discourage
this, it seems
>> like
>> > >> > such a case will probably not produce an expected
result for the
>> user.
>> > >> > We could introduce a warning, or automatically
divide the regions
>> so
>> > >> > that a single region can only contain a single
block.
>> > >> >
>> > >> > > My understanding is that code regions are not
allowed to
>> overlap. So,
>> > >> it
>> > >> > > makes sense if ` __mca_code_region_end()`
doesn't take an ID as
>> input.
>> > >> > > However, what if ` __mca_code_region_end()`
ends in a different
>> basic
>> > >> block?
>> > >> > >
>> > >> > > `__mca_code_region_start()` has to always
dominate `
>> > >> > > __mca_code_region_end()`. This is trivial to
verify when both
>> calls
>> > >> are in
>> > >> > > a same basic block; however, we need to make
sure that the
>> > >> relationship is
>> > >> > > still the same when the `end()` call is in a
different basic
>> block.
>> > >> > > That would not be enough. I think we should
also verify  that `
>> > >> > > __mca_code_region_end()` always post-dominates
the call to
>> > >> > > `__mca_code_region_start()`.
>> > >> >
>> > >> > In any case this patch should probably check
dominance of the
>> > >> > intrinsics, even though MCA does not follow branches
and MCA does
>> not
>> > >> > not explicitly forbid a region from containing
multiple blocks.
>> > >> >
>> > >> > >
>> > >> > > My question is: what happens with basic block
reordering? We
>> don't
>> > >> know the
>> > >> > > layout of basic blocks until we reach code
emission. How does it
>> work
>> > >> for
>> > >> > > regions that span through multiple basic
blocks?. I think your
>> RFC
>> > >> should
>> > >> > > clarify this aspect.
>> > >> > >
>> > >> > > As a side note: at the moment, llvm-mca
doesn't know how to deal
>> with
>> > >> > > branches. So, for simplicity we could force
code regions to only
>> > >> contain
>> > >> > > instructions from a single basic block.
>> > >> > >
>> > >> > > However, In future we may want to teach
llvm-mca how to analyze
>> > >> branchy
>> > >> > > code too. For example, we could introduce a
simple control-flow
>> > >> analysis in
>> > >> > > llvm-mca, and use an external "branch
trace" information (for
>> > >> example, a
>> > >> > > perf trace generated by an external tool) to
decorate branches
>> with
>> > >> with
>> > >> > > branch probabilities (similarly to what we
currently do in LLVM
>> with
>> > >> PGO).
>> > >> > > We could then use that knowledge to model
branch prediction and
>> > >> simulate
>> > >> > > what happens in the presence of multiple
branches.
>> > >> > >
>> > >> > > So, the idea of having regions that potentially
span multiple
>> basic
>> > >> blocks
>> > >> > > is not bad in general. However, I think you
should better clarify
>> > >> what are
>> > >> > > the constraints (at least, you should answer to
my questions from
>> > >> before).
>> > >> >
>> > >> > I agree! Thanks for pointing that out.
>> > >> >
>> > >> > > If we decide to use those new intrinsics, then
those should be
>> > >> experimental
>> > >> > > (at least to start).
>> > >> >
>> > >> > Agreed.
>> > >> >
>> > >> > -Matt
>> > >> >
>> > >> > > On Thu, Nov 15, 2018 at 11:07 PM via llvm-dev
<
>> > >> llvm-dev at lists.llvm.org>
>> > >> > > wrote:
>> > >> > >
>> > >> > > > Introduction
>> > >> > > > -----------------
>> > >> > > > Currently llvm-mca only accepts assembly
code as input. We
>> would
>> > >> like to
>> > >> > > > extend llvm-mca to support object files,
allowing users to
>> analyze
>> > >> the
>> > >> > > > performance of binaries. The proposed
changes (which involve
>> both
>> > >> > > > clang and llvm) optionally introduce an
object file section,
>> but
>> > >> this can
>> > >> > > > be
>> > >> > > > stripped-out if desired.
>> > >> > > >
>> > >> > > > For the llvm-mca binary support feature to
be useful, a user
>> needs
>> > >> to tell
>> > >> > > > llvm-mca which portions of their code they
would like analyzed.
>> > >> Currently,
>> > >> > > > this is accomplished via assembly
comments. However, assembly
>> > >> comments are
>> > >> > > > not
>> > >> > > > preserved in object files, and this has
encouraged this RFC.
>> For the
>> > >> > > > proposed
>> > >> > > > binary support, we need to introduce
changes to clang and llvm
>> to
>> > >> allow the
>> > >> > > > user's object code to be recognized by
llvm-mca:
>> > >> > > >
>> > >> > > > * We need a way for a user to identify a
region/block of code
>> they
>> > >> want
>> > >> > > >    analyzed by llvm-mca.
>> > >> > > > * We need the information defining the
user's region of code
>> to be
>> > >> > > > maintained
>> > >> > > >    in the object file so that llvm-mca can
analyze the desired
>> > >> region(s)
>> > >> > > > from the
>> > >> > > >    object file.
>> > >> > > >
>> > >> > > > We define a "code region" as a
subset of a user's program that
>> is
>> > >> to be
>> > >> > > > analyzed via llvm-mca. The sequence of
instructions to be
>> analyzed
>> > >> is
>> > >> > > > represented as a pair: <start, end>
where the 'start' marks the
>> > >> beginning
>> > >> > > > of
>> > >> > > > the user's source code and
'end' terminates the sequence. The
>> > >> instructions
>> > >> > > > between 'start' and 'end'
form the region that can be analyzed
>> by
>> > >> llvm-mca
>> > >> > > > at a
>> > >> > > > later time.
>> > >> > > >
>> > >> > > > Example
>> > >> > > > -----------
>> > >> > > > Before we go into the details of this
proposed change, let's
>> first
>> > >> look at
>> > >> > > > a
>> > >> > > > simple example:
>> > >> > > >
>> > >> > > > // example.c -- Analyze a dot-product
expression.
>> > >> > > > double test(double x, double y) {
>> > >> > > >    double result = 0.0;
>> > >> > > >    __mca_code_region_start(42);
>> > >> > > >    result += x * y;
>> > >> > > >    __mca_code_region_end();
>> > >> > > >    return result;
>> > >> > > > }
>> > >> > > >
>> > >> > > > In the example above, we have identified a
code region, in this
>> > >> case a
>> > >> > > > single
>> > >> > > > dot-product expression. For the sake of
brevity and simplicity,
>> > >> we've
>> > >> > > > chosen
>> > >> > > > a very simple example, but in reality a
more complicated
>> example
>> > >> could use
>> > >> > > > multiple expressions. We have also denoted
this region as
>> number
>> > >> 42. That
>> > >> > > > identifier is only for the user, and
simplifies reading an
>> llvm-mca
>> > >> > > > analysis
>> > >> > > > report later.
>> > >> > > >
>> > >> > > > When this code is compiled, the region
markers (the
>> mca_code_region
>> > >> > > > markers)
>> > >> > > > are transformed into assembly labels.
While the markers are
>> > >> presented as
>> > >> > > > function calls, in reality they are
no-ops.
>> > >> > > >
>> > >> > > > test:
>> > >> > > > pushq   %rbp
>> > >> > > > movq    %rsp, %rbp
>> > >> > > > movsd   %xmm0, -8(%rbp)
>> > >> > > > movsd   %xmm1, -16(%rbp)
>> > >> > > > .Lmca_code_region_start_0: #
LLVM-MCA-START ID: 42
>> > >> > > > xorps   %xmm0, %xmm0
>> > >> > > > movsd   %xmm0, -24(%rbp)
>> > >> > > > movsd   -8(%rbp), %xmm0
>> > >> > > > mulsd   -16(%rbp), %xmm0
>> > >> > > > addsd   -24(%rbp), %xmm0
>> > >> > > > movsd   %xmm0, -24(%rbp)
>> > >> > > > .Lmca_code_region_end_0: # LLVM-MCA-END
ID: 42
>> > >> > > > movsd   -24(%rbp), %xmm0
>> > >> > > > popq    %rbp
>> > >> > > > retq
>> > >> > > > .section       
.mca_code_regions,"", at progbits
>> > >> > > > .quad   42
>> > >> > > > .quad   .Lmca_code_region_start_0
>> > >> > > > .quad  
.Lmca_code_region_end_0-.Lmca_code_region_start_0
>> > >> > > >
>> > >> > > > The assembly has been trimmed to show the
portions relevant to
>> this
>> > >> RFC.
>> > >> > > > Notice the labels enclose the user's
defined region, and that
>> they
>> > >> > > > preserve the
>> > >> > > > user's arbitrary region identifier,
the ever-so-important
>> region 42.
>> > >> > > >
>> > >> > > > In the object file section
.mca_code_regions, we have noted the
>> > >> user's
>> > >> > > > region
>> > >> > > > identifier (.quad 42), start address, and
region size. A more
>> > >> complicated
>> > >> > > > example can have multiple regions defined
within a single
>> > >> .mca_code_regions
>> > >> > > > section. This section can be read by
llvm-mca, allowing
>> llvm-mca to
>> > >> take
>> > >> > > > object files as input instead of assembly
source.
>> > >> > > >
>> > >> > > > Details
>> > >> > > > ---------
>> > >> > > > We need a way for a user to identify a
region/block of code
>> they
>> > >> want
>> > >> > > > analyzed
>> > >> > > > by llvm-mca. We solve this problem by
introducing two
>> intrinsics
>> > >> that a
>> > >> > > > user can
>> > >> > > > specify, for identifying regions of code
for analysis.
>> > >> > > >
>> > >> > > > The two intrinsics are:
llvm.mca.code.regions.start and
>> > >> > > > llvm.mca.code.regions.end. A user can
identify a code region by
>> > >> inserting
>> > >> > > > the
>> > >> > > > mca_code_region_start and
mca_code_region_end markers. These
>> are
>> > >> simply
>> > >> > > > clang builtins and are transformed into
the aforementioned
>> > >> intrinsics
>> > >> > > > during
>> > >> > > > compilation. The code between the
intrinsics are what we call
>> "code
>> > >> > > > regions"
>> > >> > > > and are to be easily identifiable by
llvm-mca; any code
>> between a
>> > >> start/end
>> > >> > > > pair can be analyzed by llvm-mca at a
later time. A user can
>> define
>> > >> > > > multiple
>> > >> > > > non-overlapping code regions within their
program.
>> > >> > > >
>> > >> > > > The llvm.mca.code.region.start intrinsic
takes an integer
>> constant
>> > >> as its
>> > >> > > > only
>> > >> > > > argument. This argument is implemented as
a metadata i32, and
>> is
>> > >> only used
>> > >> > > > when generating llvm-mca reports. This
value allows a user to
>> more
>> > >> easily
>> > >> > > > identify a specific code region.
llvm.mca.code.region.end
>> takes no
>> > >> > > > arguments.
>> > >> > > > Since we disallow nesting of regions, the
first 'end' intrinsic
>> > >> lexically
>> > >> > > > following a 'start' intrinsic
represents the end of that code
>> > >> region.
>> > >> > > >
>> > >> > > > Now that we have a solution for
identifying regions for
>> analysis,
>> > >> we now
>> > >> > > > need a
>> > >> > > > way for preserving that information to be
read at a later
>> time. To
>> > >> > > > accomplish
>> > >> > > > this we propose adding a new section
(.mca_code_regions) to the
>> > >> object file
>> > >> > > > generated by llvm. During code generation,
the start/end
>> intrinsics
>> > >> > > > described
>> > >> > > > above will be transformed into start/end
labels in assembly.
>> When
>> > >> llvm
>> > >> > > > generates the object file from the
user's code, these start/end
>> > >> labels
>> > >> > > > form a
>> > >> > > > pair of values identifying the start of
the user's code
>> region, and
>> > >> size.
>> > >> > > > The
>> > >> > > > size represents the number of bytes
between the start and end
>> > >> address of
>> > >> > > > the
>> > >> > > > labels. Note that the labels are emitted
during assembly
>> printing.
>> > >> We hope
>> > >> > > > that these labels have no influence on
code generation or
>> > >> basic-block
>> > >> > > > placement. However, the target assembler
strategy for handling
>> > >> labels is
>> > >> > > > outside of our control.
>> > >> > > >
>> > >> > > > This proposed change affects the size of a
binary, but only if
>> the
>> > >> user
>> > >> > > > calls
>> > >> > > > the start/end builtins mentioned above.
The additional size of
>> the
>> > >> > > > .mca_code_regions section, which we
imagine to be very small
>> (to
>> > >> the order
>> > >> > > > of a
>> > >> > > > few bytes), can trivially be stripped by
tools like 'strip' or
>> > >> 'objcopy'.
>> > >> > > >
>> > >> > > > Implementation Status
>> > >> > > > ------------------------------
>> > >> > > > We currently have the proposed changes
implemented at the url
>> > >> posted below.
>> > >> > > > This initial patch only targets ELF object
files, and does not
>> > >> handle
>> > >> > > > relocatable addresses. Since the start of
a code region is
>> > >> represented as
>> > >> > > > an
>> > >> > > > assembly label, and referenced in the
.mca_code_regions
>> section,
>> > >> that
>> > >> > > > address
>> > >> > > > is relocatable. That value can be
represented as
>> section-relative
>> > >> > > > relocatable
>> > >> > > > symbol (.text + addend), but we are not
handling that case yet.
>> > >> Instead,
>> > >> > > > the
>> > >> > > > proposed changes only handle
linked/executable object files.
>> > >> > > >
>> > >> > > > For purposes of review and to communicate
the idea, the change
>> is
>> > >> > > > presented as a monolithic patch here:
>> > >> > > >
>> > >> > > > https://reviews.llvm.org/D54603
>> > >> > > >
>> > >> > > > The change is presented as a monolithic
patch; however, if
>> accepted
>> > >> > > > the patch will be split into three smaller
patches:
>> > >> > > > 1. The introduction of the builtins to
clang.
>> > >> > > > 2. The llvm portion (the added
intrinsics).
>> > >> > > > 3. The llvm-mca portion.
>> > >> > > >
>> > >> > > > Thanks!
>> > >> > > >
>> > >> > > > -Matt
>> > >> > > >
_______________________________________________
>> > >> > > > LLVM Developers mailing list
>> > >> > > > llvm-dev at lists.llvm.org
>> > >> > > >
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> > >> > > >
>> > >> > _______________________________________________
>> > >> > LLVM Developers mailing list
>> > >> > llvm-dev at lists.llvm.org
>> > >> >
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> > >>
>> > >
>>
>
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Thanks for the feedback Guillaume and Clement!

In response to Clement:
> > In terms of future-proofness of only allowing regions within a basic
> > block, are we confident we can actually ever simulate branches apart
from
> > "always taken, perfectly predicated" loop ? Even this simple
need requires
> > knowing quite a few details on the frontend. The current design could
> > handle this use case with the addition of an external "loop
mode" option to
> > MCA. If there are no other strong use cases, I would advocate for
> > experimental intrinsics unless people can contribute other example use
> > cases.
In short, I am in agreement and think that handling of branching or loop
constructs should be isolated to the llvm-mca driver/front-end.  The
only thing the code regions should be concerned with is identifying
blocks of instructions that will later be used by the front end.

We can place limitations to how those blocks are formed. For example the
current implementation forces regions to be isolated to a single basic
block.  However, we anticipate lifting this restriction once branching
is handled.

-Matt


On Mon, Dec 10, 2018 at 04:15:46PM +0100, Guillaume Chatelet
wrote:
> +1 to what Clement said.
> I believe the intrinsics are a better design to support many architectures.
> 
> IACA users are probably decorating their code with IACA_START / IACA_END
> macros. One possibility is to provide a header that define these macros in
> terms of the new intrinsics.
> 
> On Mon, Dec 10, 2018 at 3:59 PM Clement Courbet <courbet at
google.com> wrote:
> 
> > Hi Matt/Andrea,
> >
> > I see pros and cons for IACA-style markers vs intrinsics.
> > On the one hand, IACA-style markers are very magical, and not very
visible
> > in both the source and object code. Using IACA-style markers has the
> > advantage that you can use llvm-mca as a drop-in replacement for IACA,
or
> > even to compare their outputs on the exact same binary. They also do
not
> > require tooling on the compiler side and allow comparing the output of
> > several compilers.
> >
> On the other hand, IACA-style markers do not have a equivalent on other
> > architectures, and I'm not sure inventing new ones is a good idea
:) I
> > think the latter makes them pretty much a no-go for llvm-mca as I
don't
> > think we'll want to teach each target how to parse code regions.
That's
> > much better handled in a target-agnostic way by the object. Intel got
away
> > with them because they only had to support one architecture.
> >
> > tl;dr: In the case of llvm-mca, I like your design better than the
markers.
> >
> > In terms of future-proofness of only allowing regions within a basic
> > block, are we confident we can actually ever simulate branches apart
from
> > "always taken, perfectly predicated" loop ? Even this simple
need requires
> > knowing quite a few details on the frontend. The current design could
> > handle this use case with the addition of an external "loop
mode" option to
> > MCA. If there are no other strong use cases, I would advocate for
> > experimental intrinsics unless people can contribute other example use
> > cases.
> >
> > On Mon, Dec 3, 2018 at 11:38 PM Matt Davis <matthew.davis at
sony.com> wrote:
> >
> >> Hi Andrea,
> >>
> >> On Mon, Dec 03, 2018 at 01:21:33PM +0000, Andrea Di Biagio wrote:
> >> > So, I have been thinking a bit more about this whole design.
> >> >
> >> > The more I think about your suggested design, the more I am
convinced
> >> that
> >> > we should do something more to support ranges in binary
object files
> >> too.
> >> > My understanding is that the reason why we don't support
object files in
> >> > general, is because of the presence of relocations. That is
because a
> >> > region start marker is effectively symbol relative, and the
symbol (a
> >> > function) would be relocated in the final executable.
> >> > You mentioned to me that resolving even a 'simple'
symbol-relative
> >> > relocation is not trivial, beause it requires specific
knowledge about
> >> the
> >> > binary format, and the target (i.e. how relocations are
encoded is
> >> target
> >> > specific). I am surprised that there is not a utility library
for
> >> resolving
> >> > relocations.. but I am not familiar with that part of the
compiler. I
> >> was
> >> > hoping that there was a target specific interface to use in
this case...
> >>
> >> There might be a better way of resolving the relocs, but from what
I saw
> >> looking at llvm-objdump and other related tools, it seems that
resolving
> >> the relocated symbol is a target specific effort.  I also spent
sometime
> >> sniffing around ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp which
also
> >> performs the reloc resolution.  I should clarify that I too am not
an
> >> expert in llvm's utilities for performing symbol/reloc
resolution, and
> >> perhaps someone in the community can point me in the right
direction.  I
> >> can clearly see the reloc data in the object file via tools like
> >> objdump; however, accessing the relocs via
> >> llvm::object::ObjectFile::relocations() did not produce address
values
> >> that we could use (values of zero).
> >>
> >> I was hoping that, for a first pass at this patch, supporting just
> >> executables would be okay.  That keeps this initial patch set
simple,
> >> and hopefully will encourage others to take a peek at it, since
it's
> >> less daunting than what it might otherwise be.  Of course, there
is the
> >> concern that this initial patch will lock us into a design that
will be
> >> more complicated to unravel later.
> >>
> >> > An alternative approach would require that you define your
own
> >> > "symbol-relative" reference. After all, ranges are
just a sequences of
> >> > instructions in a function. If a function symbol is described
by the
> >> symbol
> >> > table, then you should be able to obtain its offset in the
.text
> >> section.
> >> > So, you could potentially encode your own symbol+offset.
However, the
> >> > linker would not be able to understand your "custom
relocation", and
> >> > information about regions in the final elf would be basically
broken.
> >> > So,that would not be a solution...
> >> >
> >> > I don't know honestly what is the best approach to use in
this case.
> >> > As a compromise, it would not be a bad idea to add the
ability to
> >> specify
> >> > ranges from command line. What do you think?
> >> > Still, from a user point of view, the idea that we don't
support object
> >> > files in general sounds like a big limitation.
> >>
> >> I agree, only supporting executables is a limitation.  However,
I'd
> >> like to land the base support now and add in the additional
> >> features/support after this large patch set lands.  But I can see
> >> where landing the whole thing entirely also makes sense.
> >>
> >> > About the new experimental intrinsics: those would definitely
work well
> >> for
> >> > the simple case where instructions are from the same basic
block.
> >> > However, some/most of the constraints that you plan to add
will have to
> >> > change if in future we decide to allow ranges that
potentially cross
> >> > multiple basic blocks. How will the rules/constraints on
those new
> >> > intrinsics change? I just want to make sure that the
suggested design is
> >> > future-proof.
> >>
> >> Since the llvm/clang parts of the code are just responsible for
> >> collecting where a range starts/ends, I hope that we can remove
some
> >> of the baked-in constraints that are specified in IR/Verifier.cpp.
> >> As you pointed out earlier in this thread, we might want to
> >> introduce a dominance check if/when we lift the one-basic-block
> >> restriction.
> >>
> >> -Matt
> >>
> >> >
> >> > -Andrea
> >> >
> >> > On Tue, Nov 27, 2018 at 5:08 PM Andrea Di Biagio <
> >> andrea.dibiagio at gmail.com>
> >> > wrote:
> >> >
> >> > > Thanks for clarifying it Matt.
> >> > >
> >> > > In general, I quite like your suggested design.
> >> > >
> >> > > My only concern is about the semantic of the two new
intrinsics. You
> >> > > design doesn't allow mca ranges to span through
multiple basic
> >> blocks. That
> >> > > constraint is acceptable for now, since llvm-mca
doesn't know how to
> >> deal
> >> > > with control flow.
> >> > > However, I am a bit concerned about what might happen in
future if we
> >> > > decide to let users specify code regions that span
through multiple
> >> basic
> >> > > blocks. Basically, I don't particularly like the
idea of changing the
> >> > > semantic of already existing intrinsic. A design that
already
> >> accounts for
> >> > > that particular scenario/future work would be ideal.
That being said,
> >> > > marking those new intrinsics as 'experimental'
may be a good
> >> compromise (at
> >> > > least for now).
> >> > >
> >> > > So, I am quite happy overall with the direction of this
RFC.
> >> > > However, I am interesting to hear from other developers
about your
> >> > > suggested design.
> >> > >
> >> > > > This initial patch only targets ELF object files,
and does not
> >> handle
> >> > > relocatable addresses. Since the start of a code region
is
> >> represented as
> >> > > an
> >> > > assembly label, and referenced in the .mca_code_regions
section, that
> >> > > address
> >> > > is relocatable.
> >> > >
> >> > > This may be okay for now. However, it would be nice to
remove that
> >> > > constraint in future and add support to generic object
files.
> >> > >
> >> > > -Andrea
> >> > >
> >> > > On Thu, Nov 22, 2018 at 7:21 PM <Matthew.Davis at
sony.com> wrote:
> >> > >
> >> > >> I want to clarify a few restrictions of llvm-mca
code regions that
> >> this
> >> > >> RFC proposes:
> >> > >>
> >> > >> 1) All llvm-mca code regions must start with an
> >> > >> llvm.mca.code.region.start intrinsic and end with
> >> > >> an llvm.mca.code.region.end intrinsic.  This rule is
enforced at the
> >> IR
> >> > >> level in the IR verifier.
> >> > >>
> >> > >> 2) llvm-mca code regions cannot nest.  This
restriction implies that
> >> an
> >> > >> llvm.mca.code.region.start
> >> > >> must have a llvm.mca.code.region.end intrinsic
without any other
> >> llvm.mca
> >> > >> start intrinsics
> >> > >> between the two. The current implementation in the
patch enforces
> >> this
> >> > >> restriction at the
> >> > >> IR level via the IR Verifier.
> >> > >>
> >> > >> 3) An llvm-mca code region cannot span multiple
basic blocks.
> >> llvm-mca
> >> > >> does not follow
> >> > >> branches (yet).  Instead, a branch instruction is
treated by llvm-mca
> >> > >> like any other instruction.
> >> > >> The current patch associated with this RFC does not
enforce this
> >> > >> restriction.  I plan on updating
> >> > >> the patch to enforce that a code region can only
belong to a single
> >> basic
> >> > >> block.  This is a simple
> >> > >> check, ensuring that both the
llvm.mca.code.region.start and
> >> accompanying
> >> > >> end intrinsics live
> >> > >> in the same basic block. I imagine adding this check
at the IR level
> >> when
> >> > >> we also verify points 1 and 2
> >> > >> above.  That will keep the code-region verification
logic isolated
> >> to the
> >> > >> IR verifier.  The start/end
> >> > >> intrinsics should not have any uses, so I'm not
sure that they would
> >> be
> >> > >> moved/sunk on behalf
> >> > >> of any other instruction.  In other words, I do not
imagine that a
> >> start
> >> > >> and end would be split
> >> > >> apart due to later MI optimizations.  If I discover
that such a case
> >> > >> occurs, then I might add the
> >> > >> basic-block check prior to emitting the code region
data to the
> >> object
> >> > >> file.    Once  llvm-mca  is
> >> > >> updated to handle branches, then we can remove this
constraint.
> >> > >>
> >> > >> -Matt
> >> > >>
> >> > >> > -----Original Message-----
> >> > >> > From: llvm-dev <llvm-dev-bounces at
lists.llvm.org> On Behalf Of Matt
> >> > >> Davis via llvm-
> >> > >> > dev
> >> > >> > Sent: Wednesday, November 21, 2018 8:47 AM
> >> > >> > To: Andrea Di Biagio <andrea.dibiagio at
gmail.com>
> >> > >> > Cc: llvm-dev <llvm-dev at
lists.llvm.org>; Di Biagio, Andrea
> >> > >> > <Andrea.Dibiagio at sony.com>; cfe-dev at
lists.llvm.org
> >> > >> > Subject: Re: [llvm-dev] [RFC][llvm-mca] Adding
binary support to
> >> > >> llvm-mca.
> >> > >> >
> >> > >> > Hi Andrea,
> >> > >> >
> >> > >> > Thanks for your input.
> >> > >> >
> >> > >> > On Wed, Nov 21, 2018 at 12:43:52PM +0000,
Andrea Di Biagio wrote:
> >> > >> > [... snip ...]
> >> > >> > > About the suggested design:
> >> > >> > > I like the idea of being able to identify
code regions using a
> >> numeric
> >> > >> > > identifier.
> >> > >> > > However, what happens if a code region
spans through multiple
> >> basic
> >> > >> blocks?
> >> > >> >
> >> > >> > The current patch does not take into
consideration cases where the
> >> > >> > region start and end intrinsics are placed in
different basic
> >> blocks.
> >> > >> > Such would be the case if a region is defined
to span multiple
> >> blocks.
> >> > >> > This would be similar to the current case where
a user places a
> >> > >> > #LLVM-MCA-BEGIN assembly comment in one block
and an #LLVM-MCA-END
> >> in
> >> > >> > another.  However, as you point out below, if
the user does this
> >> in the
> >> > >> > source code via intrinsics (just what this
patch is proposing),
> >> then
> >> > >> > there is a chance that optimizations might
change the layout of the
> >> > >> > instructions and confuse the ordering of the
MCA intrinsics.
> >> > >> >
> >> > >> > Since MCA does not follow branches (MCA just
treats a branch as it
> >> would
> >> > >> > a non-branching instruction), it seems that a
user should be aware
> >> that
> >> > >> > defining MCA code regions that span multiple
blocks might result
> >> in an
> >> > >> > unexpected analysis.  While we do not
discourage this, it seems
> >> like
> >> > >> > such a case will probably not produce an
expected result for the
> >> user.
> >> > >> > We could introduce a warning, or automatically
divide the regions
> >> so
> >> > >> > that a single region can only contain a single
block.
> >> > >> >
> >> > >> > > My understanding is that code regions are
not allowed to
> >> overlap. So,
> >> > >> it
> >> > >> > > makes sense if ` __mca_code_region_end()`
doesn't take an ID as
> >> input.
> >> > >> > > However, what if `
__mca_code_region_end()` ends in a different
> >> basic
> >> > >> block?
> >> > >> > >
> >> > >> > > `__mca_code_region_start()` has to always
dominate `
> >> > >> > > __mca_code_region_end()`. This is trivial
to verify when both
> >> calls
> >> > >> are in
> >> > >> > > a same basic block; however, we need to
make sure that the
> >> > >> relationship is
> >> > >> > > still the same when the `end()` call is in
a different basic
> >> block.
> >> > >> > > That would not be enough. I think we
should also verify  that `
> >> > >> > > __mca_code_region_end()` always
post-dominates the call to
> >> > >> > > `__mca_code_region_start()`.
> >> > >> >
> >> > >> > In any case this patch should probably check
dominance of the
> >> > >> > intrinsics, even though MCA does not follow
branches and MCA does
> >> not
> >> > >> > not explicitly forbid a region from containing
multiple blocks.
> >> > >> >
> >> > >> > >
> >> > >> > > My question is: what happens with basic
block reordering? We
> >> don't
> >> > >> know the
> >> > >> > > layout of basic blocks until we reach code
emission. How does it
> >> work
> >> > >> for
> >> > >> > > regions that span through multiple basic
blocks?. I think your
> >> RFC
> >> > >> should
> >> > >> > > clarify this aspect.
> >> > >> > >
> >> > >> > > As a side note: at the moment, llvm-mca
doesn't know how to deal
> >> with
> >> > >> > > branches. So, for simplicity we could
force code regions to only
> >> > >> contain
> >> > >> > > instructions from a single basic block.
> >> > >> > >
> >> > >> > > However, In future we may want to teach
llvm-mca how to analyze
> >> > >> branchy
> >> > >> > > code too. For example, we could introduce
a simple control-flow
> >> > >> analysis in
> >> > >> > > llvm-mca, and use an external "branch
trace" information (for
> >> > >> example, a
> >> > >> > > perf trace generated by an external tool)
to decorate branches
> >> with
> >> > >> with
> >> > >> > > branch probabilities (similarly to what we
currently do in LLVM
> >> with
> >> > >> PGO).
> >> > >> > > We could then use that knowledge to model
branch prediction and
> >> > >> simulate
> >> > >> > > what happens in the presence of multiple
branches.
> >> > >> > >
> >> > >> > > So, the idea of having regions that
potentially span multiple
> >> basic
> >> > >> blocks
> >> > >> > > is not bad in general. However, I think
you should better clarify
> >> > >> what are
> >> > >> > > the constraints (at least, you should
answer to my questions from
> >> > >> before).
> >> > >> >
> >> > >> > I agree! Thanks for pointing that out.
> >> > >> >
> >> > >> > > If we decide to use those new intrinsics,
then those should be
> >> > >> experimental
> >> > >> > > (at least to start).
> >> > >> >
> >> > >> > Agreed.
> >> > >> >
> >> > >> > -Matt
> >> > >> >
> >> > >> > > On Thu, Nov 15, 2018 at 11:07 PM via
llvm-dev <
> >> > >> llvm-dev at lists.llvm.org>
> >> > >> > > wrote:
> >> > >> > >
> >> > >> > > > Introduction
> >> > >> > > > -----------------
> >> > >> > > > Currently llvm-mca only accepts
assembly code as input. We
> >> would
> >> > >> like to
> >> > >> > > > extend llvm-mca to support object
files, allowing users to
> >> analyze
> >> > >> the
> >> > >> > > > performance of binaries. The proposed
changes (which involve
> >> both
> >> > >> > > > clang and llvm) optionally introduce
an object file section,
> >> but
> >> > >> this can
> >> > >> > > > be
> >> > >> > > > stripped-out if desired.
> >> > >> > > >
> >> > >> > > > For the llvm-mca binary support
feature to be useful, a user
> >> needs
> >> > >> to tell
> >> > >> > > > llvm-mca which portions of their code
they would like analyzed.
> >> > >> Currently,
> >> > >> > > > this is accomplished via assembly
comments. However, assembly
> >> > >> comments are
> >> > >> > > > not
> >> > >> > > > preserved in object files, and this
has encouraged this RFC.
> >> For the
> >> > >> > > > proposed
> >> > >> > > > binary support, we need to introduce
changes to clang and llvm
> >> to
> >> > >> allow the
> >> > >> > > > user's object code to be
recognized by llvm-mca:
> >> > >> > > >
> >> > >> > > > * We need a way for a user to
identify a region/block of code
> >> they
> >> > >> want
> >> > >> > > >    analyzed by llvm-mca.
> >> > >> > > > * We need the information defining
the user's region of code
> >> to be
> >> > >> > > > maintained
> >> > >> > > >    in the object file so that
llvm-mca can analyze the desired
> >> > >> region(s)
> >> > >> > > > from the
> >> > >> > > >    object file.
> >> > >> > > >
> >> > >> > > > We define a "code region"
as a subset of a user's program that
> >> is
> >> > >> to be
> >> > >> > > > analyzed via llvm-mca. The sequence
of instructions to be
> >> analyzed
> >> > >> is
> >> > >> > > > represented as a pair: <start,
end> where the 'start' marks the
> >> > >> beginning
> >> > >> > > > of
> >> > >> > > > the user's source code and
'end' terminates the sequence. The
> >> > >> instructions
> >> > >> > > > between 'start' and
'end' form the region that can be analyzed
> >> by
> >> > >> llvm-mca
> >> > >> > > > at a
> >> > >> > > > later time.
> >> > >> > > >
> >> > >> > > > Example
> >> > >> > > > -----------
> >> > >> > > > Before we go into the details of this
proposed change, let's
> >> first
> >> > >> look at
> >> > >> > > > a
> >> > >> > > > simple example:
> >> > >> > > >
> >> > >> > > > // example.c -- Analyze a dot-product
expression.
> >> > >> > > > double test(double x, double y) {
> >> > >> > > >    double result = 0.0;
> >> > >> > > >    __mca_code_region_start(42);
> >> > >> > > >    result += x * y;
> >> > >> > > >    __mca_code_region_end();
> >> > >> > > >    return result;
> >> > >> > > > }
> >> > >> > > >
> >> > >> > > > In the example above, we have
identified a code region, in this
> >> > >> case a
> >> > >> > > > single
> >> > >> > > > dot-product expression. For the sake
of brevity and simplicity,
> >> > >> we've
> >> > >> > > > chosen
> >> > >> > > > a very simple example, but in reality
a more complicated
> >> example
> >> > >> could use
> >> > >> > > > multiple expressions. We have also
denoted this region as
> >> number
> >> > >> 42. That
> >> > >> > > > identifier is only for the user, and
simplifies reading an
> >> llvm-mca
> >> > >> > > > analysis
> >> > >> > > > report later.
> >> > >> > > >
> >> > >> > > > When this code is compiled, the
region markers (the
> >> mca_code_region
> >> > >> > > > markers)
> >> > >> > > > are transformed into assembly labels.
While the markers are
> >> > >> presented as
> >> > >> > > > function calls, in reality they are
no-ops.
> >> > >> > > >
> >> > >> > > > test:
> >> > >> > > > pushq   %rbp
> >> > >> > > > movq    %rsp, %rbp
> >> > >> > > > movsd   %xmm0, -8(%rbp)
> >> > >> > > > movsd   %xmm1, -16(%rbp)
> >> > >> > > > .Lmca_code_region_start_0: #
LLVM-MCA-START ID: 42
> >> > >> > > > xorps   %xmm0, %xmm0
> >> > >> > > > movsd   %xmm0, -24(%rbp)
> >> > >> > > > movsd   -8(%rbp), %xmm0
> >> > >> > > > mulsd   -16(%rbp), %xmm0
> >> > >> > > > addsd   -24(%rbp), %xmm0
> >> > >> > > > movsd   %xmm0, -24(%rbp)
> >> > >> > > > .Lmca_code_region_end_0: #
LLVM-MCA-END ID: 42
> >> > >> > > > movsd   -24(%rbp), %xmm0
> >> > >> > > > popq    %rbp
> >> > >> > > > retq
> >> > >> > > > .section       
.mca_code_regions,"", at progbits
> >> > >> > > > .quad   42
> >> > >> > > > .quad   .Lmca_code_region_start_0
> >> > >> > > > .quad  
.Lmca_code_region_end_0-.Lmca_code_region_start_0
> >> > >> > > >
> >> > >> > > > The assembly has been trimmed to show
the portions relevant to
> >> this
> >> > >> RFC.
> >> > >> > > > Notice the labels enclose the
user's defined region, and that
> >> they
> >> > >> > > > preserve the
> >> > >> > > > user's arbitrary region
identifier, the ever-so-important
> >> region 42.
> >> > >> > > >
> >> > >> > > > In the object file section
.mca_code_regions, we have noted the
> >> > >> user's
> >> > >> > > > region
> >> > >> > > > identifier (.quad 42), start address,
and region size. A more
> >> > >> complicated
> >> > >> > > > example can have multiple regions
defined within a single
> >> > >> .mca_code_regions
> >> > >> > > > section. This section can be read by
llvm-mca, allowing
> >> llvm-mca to
> >> > >> take
> >> > >> > > > object files as input instead of
assembly source.
> >> > >> > > >
> >> > >> > > > Details
> >> > >> > > > ---------
> >> > >> > > > We need a way for a user to identify
a region/block of code
> >> they
> >> > >> want
> >> > >> > > > analyzed
> >> > >> > > > by llvm-mca. We solve this problem by
introducing two
> >> intrinsics
> >> > >> that a
> >> > >> > > > user can
> >> > >> > > > specify, for identifying regions of
code for analysis.
> >> > >> > > >
> >> > >> > > > The two intrinsics are:
llvm.mca.code.regions.start and
> >> > >> > > > llvm.mca.code.regions.end. A user can
identify a code region by
> >> > >> inserting
> >> > >> > > > the
> >> > >> > > > mca_code_region_start and
mca_code_region_end markers. These
> >> are
> >> > >> simply
> >> > >> > > > clang builtins and are transformed
into the aforementioned
> >> > >> intrinsics
> >> > >> > > > during
> >> > >> > > > compilation. The code between the
intrinsics are what we call
> >> "code
> >> > >> > > > regions"
> >> > >> > > > and are to be easily identifiable by
llvm-mca; any code
> >> between a
> >> > >> start/end
> >> > >> > > > pair can be analyzed by llvm-mca at a
later time. A user can
> >> define
> >> > >> > > > multiple
> >> > >> > > > non-overlapping code regions within
their program.
> >> > >> > > >
> >> > >> > > > The llvm.mca.code.region.start
intrinsic takes an integer
> >> constant
> >> > >> as its
> >> > >> > > > only
> >> > >> > > > argument. This argument is
implemented as a metadata i32, and
> >> is
> >> > >> only used
> >> > >> > > > when generating llvm-mca reports.
This value allows a user to
> >> more
> >> > >> easily
> >> > >> > > > identify a specific code region.
llvm.mca.code.region.end
> >> takes no
> >> > >> > > > arguments.
> >> > >> > > > Since we disallow nesting of regions,
the first 'end' intrinsic
> >> > >> lexically
> >> > >> > > > following a 'start' intrinsic
represents the end of that code
> >> > >> region.
> >> > >> > > >
> >> > >> > > > Now that we have a solution for
identifying regions for
> >> analysis,
> >> > >> we now
> >> > >> > > > need a
> >> > >> > > > way for preserving that information
to be read at a later
> >> time. To
> >> > >> > > > accomplish
> >> > >> > > > this we propose adding a new section
(.mca_code_regions) to the
> >> > >> object file
> >> > >> > > > generated by llvm. During code
generation, the start/end
> >> intrinsics
> >> > >> > > > described
> >> > >> > > > above will be transformed into
start/end labels in assembly.
> >> When
> >> > >> llvm
> >> > >> > > > generates the object file from the
user's code, these start/end
> >> > >> labels
> >> > >> > > > form a
> >> > >> > > > pair of values identifying the start
of the user's code
> >> region, and
> >> > >> size.
> >> > >> > > > The
> >> > >> > > > size represents the number of bytes
between the start and end
> >> > >> address of
> >> > >> > > > the
> >> > >> > > > labels. Note that the labels are
emitted during assembly
> >> printing.
> >> > >> We hope
> >> > >> > > > that these labels have no influence
on code generation or
> >> > >> basic-block
> >> > >> > > > placement. However, the target
assembler strategy for handling
> >> > >> labels is
> >> > >> > > > outside of our control.
> >> > >> > > >
> >> > >> > > > This proposed change affects the size
of a binary, but only if
> >> the
> >> > >> user
> >> > >> > > > calls
> >> > >> > > > the start/end builtins mentioned
above. The additional size of
> >> the
> >> > >> > > > .mca_code_regions section, which we
imagine to be very small
> >> (to
> >> > >> the order
> >> > >> > > > of a
> >> > >> > > > few bytes), can trivially be stripped
by tools like 'strip' or
> >> > >> 'objcopy'.
> >> > >> > > >
> >> > >> > > > Implementation Status
> >> > >> > > > ------------------------------
> >> > >> > > > We currently have the proposed
changes implemented at the url
> >> > >> posted below.
> >> > >> > > > This initial patch only targets ELF
object files, and does not
> >> > >> handle
> >> > >> > > > relocatable addresses. Since the
start of a code region is
> >> > >> represented as
> >> > >> > > > an
> >> > >> > > > assembly label, and referenced in the
.mca_code_regions
> >> section,
> >> > >> that
> >> > >> > > > address
> >> > >> > > > is relocatable. That value can be
represented as
> >> section-relative
> >> > >> > > > relocatable
> >> > >> > > > symbol (.text + addend), but we are
not handling that case yet.
> >> > >> Instead,
> >> > >> > > > the
> >> > >> > > > proposed changes only handle
linked/executable object files.
> >> > >> > > >
> >> > >> > > > For purposes of review and to
communicate the idea, the change
> >> is
> >> > >> > > > presented as a monolithic patch here:
> >> > >> > > >
> >> > >> > > > https://reviews.llvm.org/D54603
> >> > >> > > >
> >> > >> > > > The change is presented as a
monolithic patch; however, if
> >> accepted
> >> > >> > > > the patch will be split into three
smaller patches:
> >> > >> > > > 1. The introduction of the builtins
to clang.
> >> > >> > > > 2. The llvm portion (the added
intrinsics).
> >> > >> > > > 3. The llvm-mca portion.
> >> > >> > > >
> >> > >> > > > Thanks!
> >> > >> > > >
> >> > >> > > > -Matt
> >> > >> > > >
_______________________________________________
> >> > >> > > > LLVM Developers mailing list
> >> > >> > > > llvm-dev at lists.llvm.org
> >> > >> > > >
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >> > >> > > >
> >> > >> > _______________________________________________
> >> > >> > LLVM Developers mailing list
> >> > >> > llvm-dev at lists.llvm.org
> >> > >> >
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >> > >>
> >> > >
> >>
> >

Hi Matt,

I can see a near future where perf-analysis tooling uses branch history 
profiler captures to determine how often loops/branches are taken and 
feeds that into llvm-mca, especially for hot/branchy loop analysis 
reports etc. Are you confident that your approach will be easily 
extendable for this?

Similarly, being able to generally embed the profile markers in object 
libraries for reuse is going to be important for some people - I'd like 
to see more of a plan of how this will be achieved. I understand that it 
might not be easy for some exe formats.

Sorry if I'm being too critical, but I'm a bit worried that we end up 
with an initial implementation that will take a lot of reworking to meet 
our final aims.

Thanks, Simon.

On 10/12/2018 19:32, Matt Davis wrote:
> Thanks for the feedback Guillaume and Clement!
>
> In response to Clement:
>
>>> In terms of future-proofness of only allowing regions within a
basic
>>> block, are we confident we can actually ever simulate branches
apart from
>>> "always taken, perfectly predicated" loop ? Even this
simple need requires
>>> knowing quite a few details on the frontend. The current design
could
>>> handle this use case with the addition of an external "loop
mode" option to
>>> MCA. If there are no other strong use cases, I would advocate for
>>> experimental intrinsics unless people can contribute other example
use
>>> cases.
> In short, I am in agreement and think that handling of branching or loop
> constructs should be isolated to the llvm-mca driver/front-end.  The
> only thing the code regions should be concerned with is identifying
> blocks of instructions that will later be used by the front end.
>
> We can place limitations to how those blocks are formed. For example the
> current implementation forces regions to be isolated to a single basic
> block.  However, we anticipate lifting this restriction once branching
> is handled.
>
> -Matt
>
>
> On Mon, Dec 10, 2018 at 04:15:46PM +0100, Guillaume Chatelet wrote:
>> +1 to what Clement said.
>> I believe the intrinsics are a better design to support many
architectures.
>>
>> IACA users are probably decorating their code with IACA_START /
IACA_END
>> macros. One possibility is to provide a header that define these macros
in
>> terms of the new intrinsics.
>>
>> On Mon, Dec 10, 2018 at 3:59 PM Clement Courbet <courbet at
google.com> wrote:
>>
>>> Hi Matt/Andrea,
>>>
>>> I see pros and cons for IACA-style markers vs intrinsics.
>>> On the one hand, IACA-style markers are very magical, and not very
visible
>>> in both the source and object code. Using IACA-style markers has
the
>>> advantage that you can use llvm-mca as a drop-in replacement for
IACA, or
>>> even to compare their outputs on the exact same binary. They also
do not
>>> require tooling on the compiler side and allow comparing the output
of
>>> several compilers.
>>>
>> On the other hand, IACA-style markers do not have a equivalent on other
>>> architectures, and I'm not sure inventing new ones is a good
idea :) I
>>> think the latter makes them pretty much a no-go for llvm-mca as I
don't
>>> think we'll want to teach each target how to parse code
regions. That's
>>> much better handled in a target-agnostic way by the object. Intel
got away
>>> with them because they only had to support one architecture.
>>>
>>> tl;dr: In the case of llvm-mca, I like your design better than the
markers.
>>>
>>> In terms of future-proofness of only allowing regions within a
basic
>>> block, are we confident we can actually ever simulate branches
apart from
>>> "always taken, perfectly predicated" loop ? Even this
simple need requires
>>> knowing quite a few details on the frontend. The current design
could
>>> handle this use case with the addition of an external "loop
mode" option to
>>> MCA. If there are no other strong use cases, I would advocate for
>>> experimental intrinsics unless people can contribute other example
use
>>> cases.
>>>
>>> On Mon, Dec 3, 2018 at 11:38 PM Matt Davis <matthew.davis at
sony.com> wrote:
>>>
>>>> Hi Andrea,
>>>>
>>>> On Mon, Dec 03, 2018 at 01:21:33PM +0000, Andrea Di Biagio
wrote:
>>>>> So, I have been thinking a bit more about this whole
design.
>>>>>
>>>>> The more I think about your suggested design, the more I am
convinced
>>>> that
>>>>> we should do something more to support ranges in binary
object files
>>>> too.
>>>>> My understanding is that the reason why we don't
support object files in
>>>>> general, is because of the presence of relocations. That is
because a
>>>>> region start marker is effectively symbol relative, and the
symbol (a
>>>>> function) would be relocated in the final executable.
>>>>> You mentioned to me that resolving even a 'simple'
symbol-relative
>>>>> relocation is not trivial, beause it requires specific
knowledge about
>>>> the
>>>>> binary format, and the target (i.e. how relocations are
encoded is
>>>> target
>>>>> specific). I am surprised that there is not a utility
library for
>>>> resolving
>>>>> relocations.. but I am not familiar with that part of the
compiler. I
>>>> was
>>>>> hoping that there was a target specific interface to use in
this case...
>>>> There might be a better way of resolving the relocs, but from
what I saw
>>>> looking at llvm-objdump and other related tools, it seems that
resolving
>>>> the relocated symbol is a target specific effort.  I also spent
sometime
>>>> sniffing around ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
which also
>>>> performs the reloc resolution.  I should clarify that I too am
not an
>>>> expert in llvm's utilities for performing symbol/reloc
resolution, and
>>>> perhaps someone in the community can point me in the right
direction.  I
>>>> can clearly see the reloc data in the object file via tools
like
>>>> objdump; however, accessing the relocs via
>>>> llvm::object::ObjectFile::relocations() did not produce address
values
>>>> that we could use (values of zero).
>>>>
>>>> I was hoping that, for a first pass at this patch, supporting
just
>>>> executables would be okay.  That keeps this initial patch set
simple,
>>>> and hopefully will encourage others to take a peek at it, since
it's
>>>> less daunting than what it might otherwise be.  Of course,
there is the
>>>> concern that this initial patch will lock us into a design that
will be
>>>> more complicated to unravel later.
>>>>
>>>>> An alternative approach would require that you define your
own
>>>>> "symbol-relative" reference. After all, ranges
are just a sequences of
>>>>> instructions in a function. If a function symbol is
described by the
>>>> symbol
>>>>> table, then you should be able to obtain its offset in the
.text
>>>> section.
>>>>> So, you could potentially encode your own symbol+offset.
However, the
>>>>> linker would not be able to understand your "custom
relocation", and
>>>>> information about regions in the final elf would be
basically broken.
>>>>> So,that would not be a solution...
>>>>>
>>>>> I don't know honestly what is the best approach to use
in this case.
>>>>> As a compromise, it would not be a bad idea to add the
ability to
>>>> specify
>>>>> ranges from command line. What do you think?
>>>>> Still, from a user point of view, the idea that we
don't support object
>>>>> files in general sounds like a big limitation.
>>>> I agree, only supporting executables is a limitation.  However,
I'd
>>>> like to land the base support now and add in the additional
>>>> features/support after this large patch set lands.  But I can
see
>>>> where landing the whole thing entirely also makes sense.
>>>>
>>>>> About the new experimental intrinsics: those would
definitely work well
>>>> for
>>>>> the simple case where instructions are from the same basic
block.
>>>>> However, some/most of the constraints that you plan to add
will have to
>>>>> change if in future we decide to allow ranges that
potentially cross
>>>>> multiple basic blocks. How will the rules/constraints on
those new
>>>>> intrinsics change? I just want to make sure that the
suggested design is
>>>>> future-proof.
>>>> Since the llvm/clang parts of the code are just responsible for
>>>> collecting where a range starts/ends, I hope that we can remove
some
>>>> of the baked-in constraints that are specified in
IR/Verifier.cpp.
>>>> As you pointed out earlier in this thread, we might want to
>>>> introduce a dominance check if/when we lift the one-basic-block
>>>> restriction.
>>>>
>>>> -Matt
>>>>
>>>>> -Andrea
>>>>>
>>>>> On Tue, Nov 27, 2018 at 5:08 PM Andrea Di Biagio <
>>>> andrea.dibiagio at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Thanks for clarifying it Matt.
>>>>>>
>>>>>> In general, I quite like your suggested design.
>>>>>>
>>>>>> My only concern is about the semantic of the two new
intrinsics. You
>>>>>> design doesn't allow mca ranges to span through
multiple basic
>>>> blocks. That
>>>>>> constraint is acceptable for now, since llvm-mca
doesn't know how to
>>>> deal
>>>>>> with control flow.
>>>>>> However, I am a bit concerned about what might happen
in future if we
>>>>>> decide to let users specify code regions that span
through multiple
>>>> basic
>>>>>> blocks. Basically, I don't particularly like the
idea of changing the
>>>>>> semantic of already existing intrinsic. A design that
already
>>>> accounts for
>>>>>> that particular scenario/future work would be ideal.
That being said,
>>>>>> marking those new intrinsics as 'experimental'
may be a good
>>>> compromise (at
>>>>>> least for now).
>>>>>>
>>>>>> So, I am quite happy overall with the direction of this
RFC.
>>>>>> However, I am interesting to hear from other developers
about your
>>>>>> suggested design.
>>>>>>
>>>>>>> This initial patch only targets ELF object files,
and does not
>>>> handle
>>>>>> relocatable addresses. Since the start of a code region
is
>>>> represented as
>>>>>> an
>>>>>> assembly label, and referenced in the .mca_code_regions
section, that
>>>>>> address
>>>>>> is relocatable.
>>>>>>
>>>>>> This may be okay for now. However, it would be nice to
remove that
>>>>>> constraint in future and add support to generic object
files.
>>>>>>
>>>>>> -Andrea
>>>>>>
>>>>>> On Thu, Nov 22, 2018 at 7:21 PM <Matthew.Davis at
sony.com> wrote:
>>>>>>
>>>>>>> I want to clarify a few restrictions of llvm-mca
code regions that
>>>> this
>>>>>>> RFC proposes:
>>>>>>>
>>>>>>> 1) All llvm-mca code regions must start with an
>>>>>>> llvm.mca.code.region.start intrinsic and end with
>>>>>>> an llvm.mca.code.region.end intrinsic.  This rule
is enforced at the
>>>> IR
>>>>>>> level in the IR verifier.
>>>>>>>
>>>>>>> 2) llvm-mca code regions cannot nest.  This
restriction implies that
>>>> an
>>>>>>> llvm.mca.code.region.start
>>>>>>> must have a llvm.mca.code.region.end intrinsic
without any other
>>>> llvm.mca
>>>>>>> start intrinsics
>>>>>>> between the two. The current implementation in the
patch enforces
>>>> this
>>>>>>> restriction at the
>>>>>>> IR level via the IR Verifier.
>>>>>>>
>>>>>>> 3) An llvm-mca code region cannot span multiple
basic blocks.
>>>> llvm-mca
>>>>>>> does not follow
>>>>>>> branches (yet).  Instead, a branch instruction is
treated by llvm-mca
>>>>>>> like any other instruction.
>>>>>>> The current patch associated with this RFC does not
enforce this
>>>>>>> restriction.  I plan on updating
>>>>>>> the patch to enforce that a code region can only
belong to a single
>>>> basic
>>>>>>> block.  This is a simple
>>>>>>> check, ensuring that both the
llvm.mca.code.region.start and
>>>> accompanying
>>>>>>> end intrinsics live
>>>>>>> in the same basic block. I imagine adding this
check at the IR level
>>>> when
>>>>>>> we also verify points 1 and 2
>>>>>>> above.  That will keep the code-region verification
logic isolated
>>>> to the
>>>>>>> IR verifier.  The start/end
>>>>>>> intrinsics should not have any uses, so I'm not
sure that they would
>>>> be
>>>>>>> moved/sunk on behalf
>>>>>>> of any other instruction.  In other words, I do not
imagine that a
>>>> start
>>>>>>> and end would be split
>>>>>>> apart due to later MI optimizations.  If I discover
that such a case
>>>>>>> occurs, then I might add the
>>>>>>> basic-block check prior to emitting the code region
data to the
>>>> object
>>>>>>> file.    Once  llvm-mca  is
>>>>>>> updated to handle branches, then we can remove this
constraint.
>>>>>>>
>>>>>>> -Matt
>>>>>>>
>>>>>>>> -----Original Message-----
>>>>>>>> From: llvm-dev <llvm-dev-bounces at
lists.llvm.org> On Behalf Of Matt
>>>>>>> Davis via llvm-
>>>>>>>> dev
>>>>>>>> Sent: Wednesday, November 21, 2018 8:47 AM
>>>>>>>> To: Andrea Di Biagio <andrea.dibiagio at
gmail.com>
>>>>>>>> Cc: llvm-dev <llvm-dev at
lists.llvm.org>; Di Biagio, Andrea
>>>>>>>> <Andrea.Dibiagio at sony.com>; cfe-dev at
lists.llvm.org
>>>>>>>> Subject: Re: [llvm-dev] [RFC][llvm-mca] Adding
binary support to
>>>>>>> llvm-mca.
>>>>>>>> Hi Andrea,
>>>>>>>>
>>>>>>>> Thanks for your input.
>>>>>>>>
>>>>>>>> On Wed, Nov 21, 2018 at 12:43:52PM +0000,
Andrea Di Biagio wrote:
>>>>>>>> [... snip ...]
>>>>>>>>> About the suggested design:
>>>>>>>>> I like the idea of being able to identify
code regions using a
>>>> numeric
>>>>>>>>> identifier.
>>>>>>>>> However, what happens if a code region
spans through multiple
>>>> basic
>>>>>>> blocks?
>>>>>>>> The current patch does not take into
consideration cases where the
>>>>>>>> region start and end intrinsics are placed in
different basic
>>>> blocks.
>>>>>>>> Such would be the case if a region is defined
to span multiple
>>>> blocks.
>>>>>>>> This would be similar to the current case where
a user places a
>>>>>>>> #LLVM-MCA-BEGIN assembly comment in one block
and an #LLVM-MCA-END
>>>> in
>>>>>>>> another.  However, as you point out below, if
the user does this
>>>> in the
>>>>>>>> source code via intrinsics (just what this
patch is proposing),
>>>> then
>>>>>>>> there is a chance that optimizations might
change the layout of the
>>>>>>>> instructions and confuse the ordering of the
MCA intrinsics.
>>>>>>>>
>>>>>>>> Since MCA does not follow branches (MCA just
treats a branch as it
>>>> would
>>>>>>>> a non-branching instruction), it seems that a
user should be aware
>>>> that
>>>>>>>> defining MCA code regions that span multiple
blocks might result
>>>> in an
>>>>>>>> unexpected analysis.  While we do not
discourage this, it seems
>>>> like
>>>>>>>> such a case will probably not produce an
expected result for the
>>>> user.
>>>>>>>> We could introduce a warning, or automatically
divide the regions
>>>> so
>>>>>>>> that a single region can only contain a single
block.
>>>>>>>>
>>>>>>>>> My understanding is that code regions are
not allowed to
>>>> overlap. So,
>>>>>>> it
>>>>>>>>> makes sense if ` __mca_code_region_end()`
doesn't take an ID as
>>>> input.
>>>>>>>>> However, what if ` __mca_code_region_end()`
ends in a different
>>>> basic
>>>>>>> block?
>>>>>>>>> `__mca_code_region_start()` has to always
dominate `
>>>>>>>>> __mca_code_region_end()`. This is trivial
to verify when both
>>>> calls
>>>>>>> are in
>>>>>>>>> a same basic block; however, we need to
make sure that the
>>>>>>> relationship is
>>>>>>>>> still the same when the `end()` call is in
a different basic
>>>> block.
>>>>>>>>> That would not be enough. I think we should
also verify  that `
>>>>>>>>> __mca_code_region_end()` always
post-dominates the call to
>>>>>>>>> `__mca_code_region_start()`.
>>>>>>>> In any case this patch should probably check
dominance of the
>>>>>>>> intrinsics, even though MCA does not follow
branches and MCA does
>>>> not
>>>>>>>> not explicitly forbid a region from containing
multiple blocks.
>>>>>>>>
>>>>>>>>> My question is: what happens with basic
block reordering? We
>>>> don't
>>>>>>> know the
>>>>>>>>> layout of basic blocks until we reach code
emission. How does it
>>>> work
>>>>>>> for
>>>>>>>>> regions that span through multiple basic
blocks?. I think your
>>>> RFC
>>>>>>> should
>>>>>>>>> clarify this aspect.
>>>>>>>>>
>>>>>>>>> As a side note: at the moment, llvm-mca
doesn't know how to deal
>>>> with
>>>>>>>>> branches. So, for simplicity we could force
code regions to only
>>>>>>> contain
>>>>>>>>> instructions from a single basic block.
>>>>>>>>>
>>>>>>>>> However, In future we may want to teach
llvm-mca how to analyze
>>>>>>> branchy
>>>>>>>>> code too. For example, we could introduce a
simple control-flow
>>>>>>> analysis in
>>>>>>>>> llvm-mca, and use an external "branch
trace" information (for
>>>>>>> example, a
>>>>>>>>> perf trace generated by an external tool)
to decorate branches
>>>> with
>>>>>>> with
>>>>>>>>> branch probabilities (similarly to what we
currently do in LLVM
>>>> with
>>>>>>> PGO).
>>>>>>>>> We could then use that knowledge to model
branch prediction and
>>>>>>> simulate
>>>>>>>>> what happens in the presence of multiple
branches.
>>>>>>>>>
>>>>>>>>> So, the idea of having regions that
potentially span multiple
>>>> basic
>>>>>>> blocks
>>>>>>>>> is not bad in general. However, I think you
should better clarify
>>>>>>> what are
>>>>>>>>> the constraints (at least, you should
answer to my questions from
>>>>>>> before).
>>>>>>>> I agree! Thanks for pointing that out.
>>>>>>>>
>>>>>>>>> If we decide to use those new intrinsics,
then those should be
>>>>>>> experimental
>>>>>>>>> (at least to start).
>>>>>>>> Agreed.
>>>>>>>>
>>>>>>>> -Matt
>>>>>>>>
>>>>>>>>> On Thu, Nov 15, 2018 at 11:07 PM via
llvm-dev <
>>>>>>> llvm-dev at lists.llvm.org>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>>> Introduction
>>>>>>>>>> -----------------
>>>>>>>>>> Currently llvm-mca only accepts
assembly code as input. We
>>>> would
>>>>>>> like to
>>>>>>>>>> extend llvm-mca to support object
files, allowing users to
>>>> analyze
>>>>>>> the
>>>>>>>>>> performance of binaries. The proposed
changes (which involve
>>>> both
>>>>>>>>>> clang and llvm) optionally introduce an
object file section,
>>>> but
>>>>>>> this can
>>>>>>>>>> be
>>>>>>>>>> stripped-out if desired.
>>>>>>>>>>
>>>>>>>>>> For the llvm-mca binary support feature
to be useful, a user
>>>> needs
>>>>>>> to tell
>>>>>>>>>> llvm-mca which portions of their code
they would like analyzed.
>>>>>>> Currently,
>>>>>>>>>> this is accomplished via assembly
comments. However, assembly
>>>>>>> comments are
>>>>>>>>>> not
>>>>>>>>>> preserved in object files, and this has
encouraged this RFC.
>>>> For the
>>>>>>>>>> proposed
>>>>>>>>>> binary support, we need to introduce
changes to clang and llvm
>>>> to
>>>>>>> allow the
>>>>>>>>>> user's object code to be recognized
by llvm-mca:
>>>>>>>>>>
>>>>>>>>>> * We need a way for a user to identify
a region/block of code
>>>> they
>>>>>>> want
>>>>>>>>>>     analyzed by llvm-mca.
>>>>>>>>>> * We need the information defining the
user's region of code
>>>> to be
>>>>>>>>>> maintained
>>>>>>>>>>     in the object file so that llvm-mca
can analyze the desired
>>>>>>> region(s)
>>>>>>>>>> from the
>>>>>>>>>>     object file.
>>>>>>>>>>
>>>>>>>>>> We define a "code region" as
a subset of a user's program that
>>>> is
>>>>>>> to be
>>>>>>>>>> analyzed via llvm-mca. The sequence of
instructions to be
>>>> analyzed
>>>>>>> is
>>>>>>>>>> represented as a pair: <start,
end> where the 'start' marks the
>>>>>>> beginning
>>>>>>>>>> of
>>>>>>>>>> the user's source code and
'end' terminates the sequence. The
>>>>>>> instructions
>>>>>>>>>> between 'start' and
'end' form the region that can be analyzed
>>>> by
>>>>>>> llvm-mca
>>>>>>>>>> at a
>>>>>>>>>> later time.
>>>>>>>>>>
>>>>>>>>>> Example
>>>>>>>>>> -----------
>>>>>>>>>> Before we go into the details of this
proposed change, let's
>>>> first
>>>>>>> look at
>>>>>>>>>> a
>>>>>>>>>> simple example:
>>>>>>>>>>
>>>>>>>>>> // example.c -- Analyze a dot-product
expression.
>>>>>>>>>> double test(double x, double y) {
>>>>>>>>>>     double result = 0.0;
>>>>>>>>>>     __mca_code_region_start(42);
>>>>>>>>>>     result += x * y;
>>>>>>>>>>     __mca_code_region_end();
>>>>>>>>>>     return result;
>>>>>>>>>> }
>>>>>>>>>>
>>>>>>>>>> In the example above, we have
identified a code region, in this
>>>>>>> case a
>>>>>>>>>> single
>>>>>>>>>> dot-product expression. For the sake of
brevity and simplicity,
>>>>>>> we've
>>>>>>>>>> chosen
>>>>>>>>>> a very simple example, but in reality a
more complicated
>>>> example
>>>>>>> could use
>>>>>>>>>> multiple expressions. We have also
denoted this region as
>>>> number
>>>>>>> 42. That
>>>>>>>>>> identifier is only for the user, and
simplifies reading an
>>>> llvm-mca
>>>>>>>>>> analysis
>>>>>>>>>> report later.
>>>>>>>>>>
>>>>>>>>>> When this code is compiled, the region
markers (the
>>>> mca_code_region
>>>>>>>>>> markers)
>>>>>>>>>> are transformed into assembly labels.
While the markers are
>>>>>>> presented as
>>>>>>>>>> function calls, in reality they are
no-ops.
>>>>>>>>>>
>>>>>>>>>> test:
>>>>>>>>>> pushq   %rbp
>>>>>>>>>> movq    %rsp, %rbp
>>>>>>>>>> movsd   %xmm0, -8(%rbp)
>>>>>>>>>> movsd   %xmm1, -16(%rbp)
>>>>>>>>>> .Lmca_code_region_start_0: #
LLVM-MCA-START ID: 42
>>>>>>>>>> xorps   %xmm0, %xmm0
>>>>>>>>>> movsd   %xmm0, -24(%rbp)
>>>>>>>>>> movsd   -8(%rbp), %xmm0
>>>>>>>>>> mulsd   -16(%rbp), %xmm0
>>>>>>>>>> addsd   -24(%rbp), %xmm0
>>>>>>>>>> movsd   %xmm0, -24(%rbp)
>>>>>>>>>> .Lmca_code_region_end_0: # LLVM-MCA-END
ID: 42
>>>>>>>>>> movsd   -24(%rbp), %xmm0
>>>>>>>>>> popq    %rbp
>>>>>>>>>> retq
>>>>>>>>>> .section       
.mca_code_regions,"", at progbits
>>>>>>>>>> .quad   42
>>>>>>>>>> .quad   .Lmca_code_region_start_0
>>>>>>>>>> .quad  
.Lmca_code_region_end_0-.Lmca_code_region_start_0
>>>>>>>>>>
>>>>>>>>>> The assembly has been trimmed to show
the portions relevant to
>>>> this
>>>>>>> RFC.
>>>>>>>>>> Notice the labels enclose the
user's defined region, and that
>>>> they
>>>>>>>>>> preserve the
>>>>>>>>>> user's arbitrary region identifier,
the ever-so-important
>>>> region 42.
>>>>>>>>>> In the object file section
.mca_code_regions, we have noted the
>>>>>>> user's
>>>>>>>>>> region
>>>>>>>>>> identifier (.quad 42), start address,
and region size. A more
>>>>>>> complicated
>>>>>>>>>> example can have multiple regions
defined within a single
>>>>>>> .mca_code_regions
>>>>>>>>>> section. This section can be read by
llvm-mca, allowing
>>>> llvm-mca to
>>>>>>> take
>>>>>>>>>> object files as input instead of
assembly source.
>>>>>>>>>>
>>>>>>>>>> Details
>>>>>>>>>> ---------
>>>>>>>>>> We need a way for a user to identify a
region/block of code
>>>> they
>>>>>>> want
>>>>>>>>>> analyzed
>>>>>>>>>> by llvm-mca. We solve this problem by
introducing two
>>>> intrinsics
>>>>>>> that a
>>>>>>>>>> user can
>>>>>>>>>> specify, for identifying regions of
code for analysis.
>>>>>>>>>>
>>>>>>>>>> The two intrinsics are:
llvm.mca.code.regions.start and
>>>>>>>>>> llvm.mca.code.regions.end. A user can
identify a code region by
>>>>>>> inserting
>>>>>>>>>> the
>>>>>>>>>> mca_code_region_start and
mca_code_region_end markers. These
>>>> are
>>>>>>> simply
>>>>>>>>>> clang builtins and are transformed into
the aforementioned
>>>>>>> intrinsics
>>>>>>>>>> during
>>>>>>>>>> compilation. The code between the
intrinsics are what we call
>>>> "code
>>>>>>>>>> regions"
>>>>>>>>>> and are to be easily identifiable by
llvm-mca; any code
>>>> between a
>>>>>>> start/end
>>>>>>>>>> pair can be analyzed by llvm-mca at a
later time. A user can
>>>> define
>>>>>>>>>> multiple
>>>>>>>>>> non-overlapping code regions within
their program.
>>>>>>>>>>
>>>>>>>>>> The llvm.mca.code.region.start
intrinsic takes an integer
>>>> constant
>>>>>>> as its
>>>>>>>>>> only
>>>>>>>>>> argument. This argument is implemented
as a metadata i32, and
>>>> is
>>>>>>> only used
>>>>>>>>>> when generating llvm-mca reports. This
value allows a user to
>>>> more
>>>>>>> easily
>>>>>>>>>> identify a specific code region.
llvm.mca.code.region.end
>>>> takes no
>>>>>>>>>> arguments.
>>>>>>>>>> Since we disallow nesting of regions,
the first 'end' intrinsic
>>>>>>> lexically
>>>>>>>>>> following a 'start' intrinsic
represents the end of that code
>>>>>>> region.
>>>>>>>>>> Now that we have a solution for
identifying regions for
>>>> analysis,
>>>>>>> we now
>>>>>>>>>> need a
>>>>>>>>>> way for preserving that information to
be read at a later
>>>> time. To
>>>>>>>>>> accomplish
>>>>>>>>>> this we propose adding a new section
(.mca_code_regions) to the
>>>>>>> object file
>>>>>>>>>> generated by llvm. During code
generation, the start/end
>>>> intrinsics
>>>>>>>>>> described
>>>>>>>>>> above will be transformed into
start/end labels in assembly.
>>>> When
>>>>>>> llvm
>>>>>>>>>> generates the object file from the
user's code, these start/end
>>>>>>> labels
>>>>>>>>>> form a
>>>>>>>>>> pair of values identifying the start of
the user's code
>>>> region, and
>>>>>>> size.
>>>>>>>>>> The
>>>>>>>>>> size represents the number of bytes
between the start and end
>>>>>>> address of
>>>>>>>>>> the
>>>>>>>>>> labels. Note that the labels are
emitted during assembly
>>>> printing.
>>>>>>> We hope
>>>>>>>>>> that these labels have no influence on
code generation or
>>>>>>> basic-block
>>>>>>>>>> placement. However, the target
assembler strategy for handling
>>>>>>> labels is
>>>>>>>>>> outside of our control.
>>>>>>>>>>
>>>>>>>>>> This proposed change affects the size
of a binary, but only if
>>>> the
>>>>>>> user
>>>>>>>>>> calls
>>>>>>>>>> the start/end builtins mentioned above.
The additional size of
>>>> the
>>>>>>>>>> .mca_code_regions section, which we
imagine to be very small
>>>> (to
>>>>>>> the order
>>>>>>>>>> of a
>>>>>>>>>> few bytes), can trivially be stripped
by tools like 'strip' or
>>>>>>> 'objcopy'.
>>>>>>>>>> Implementation Status
>>>>>>>>>> ------------------------------
>>>>>>>>>> We currently have the proposed changes
implemented at the url
>>>>>>> posted below.
>>>>>>>>>> This initial patch only targets ELF
object files, and does not
>>>>>>> handle
>>>>>>>>>> relocatable addresses. Since the start
of a code region is
>>>>>>> represented as
>>>>>>>>>> an
>>>>>>>>>> assembly label, and referenced in the
.mca_code_regions
>>>> section,
>>>>>>> that
>>>>>>>>>> address
>>>>>>>>>> is relocatable. That value can be
represented as
>>>> section-relative
>>>>>>>>>> relocatable
>>>>>>>>>> symbol (.text + addend), but we are not
handling that case yet.
>>>>>>> Instead,
>>>>>>>>>> the
>>>>>>>>>> proposed changes only handle
linked/executable object files.
>>>>>>>>>>
>>>>>>>>>> For purposes of review and to
communicate the idea, the change
>>>> is
>>>>>>>>>> presented as a monolithic patch here:
>>>>>>>>>>
>>>>>>>>>> https://reviews.llvm.org/D54603
>>>>>>>>>>
>>>>>>>>>> The change is presented as a monolithic
patch; however, if
>>>> accepted
>>>>>>>>>> the patch will be split into three
smaller patches:
>>>>>>>>>> 1. The introduction of the builtins to
clang.
>>>>>>>>>> 2. The llvm portion (the added
intrinsics).
>>>>>>>>>> 3. The llvm-mca portion.
>>>>>>>>>>
>>>>>>>>>> Thanks!
>>>>>>>>>>
>>>>>>>>>> -Matt
>>>>>>>>>>
_______________________________________________
>>>>>>>>>> LLVM Developers mailing list
>>>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>>>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>>>>>
>>>>>>>> _______________________________________________
>>>>>>>> LLVM Developers mailing list
>>>>>>>> llvm-dev at lists.llvm.org
>>>>>>>>
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