llvm dev - Dec 2015 - RFC: Reducing Instr PGO size overhead

On Fri, Oct 9, 2015 at 3:58 PM, Sean Silva <chisophugis at gmail.com>
wrote:
>
>
> On Wed, Oct 7, 2015 at 11:12 PM, Xinliang David Li <davidxl at
google.com>
> wrote:
>>
>> There is no further response to this, so I will assume general
>> direction of solution-3 is acceptable ;)
> No response does not mean "LGTM".
>
What I meant is that the discussion can be moved on to the formal code
review. I have not yet submitted the final patch for review yet.
Before that is ready, continue using this thread to voice your
concerns.

David



> -- Sean Silva
>
>>
>>
>> Solution-3 can be further improved. Instead of using static symbol
>> table (with zero size __llvm_prf_nm symbols) to store function names
>> for profile display and coverage mapping, the function names can be
>> stored compressed in a non-allocatable section. The compression ratio
>> for function name strings can be very high (~5x).  The covmapping data
>> can also be made non-allocatable.
>>
>> thanks,
>>
>> David
>>
>> On Tue, Sep 29, 2015 at 9:52 AM, Xinliang David Li <davidxl at
google.com>
>> wrote:
>> > Sorry for the late update. I finally found time to implement a
solution
>> > (Solution-3) that has the best size savings (for both PGO and
coverage
>> > testing) with symbolic information available.  Here is a re-cap of
what
>> > we
>> > have discussed so far:
>> >
>> > Solution-1:
>> >
>> > This is the original solution proposed. In this solution, A
function
>> > name's
>> > MD5 hash is used as the primary key (combined with IR hash) for
function
>> > lookup. __llvm_prf_names section won't be emitted into the
binary nor
>> > dumped
>> > into the profile data unless -fcoverage-mapping is also specified.
>> >
>> >     Pros:
>> >         1. maximal reduction of instrumentation binary process
image
>> > size
>> >         2. maximal reduction of object and unstripped binary size
>> >         3. maximal reduction of raw profile data size
>> >         4. maximal reduction of indexed profile data size
>> >
>> >    Cons:
>> >         1. -fcoverage-mapping use case is not handled -- the size
>> > problem
>> > still exist
>> >         2. profile dump with llvm-profdata no longer have function
names
>> > associated -- user needs to use postprocessing tool to get the
>> > functionality
>> >         3. function filtering with partial function name is not
>> > supported
>> >         4. Requires incompatible format change
>> >
>> >
>> > Solution-2: (http://reviews.llvm.org/D12715)
>> >
>> > In this solution, the MD5 hash string is used to replace the raw
name
>> > string
>> >       Pros:
>> >           1. Very simple to implement
>> >           2. have good reduction of all sizes for typical large
C++
>> > applications
>> >           3. No profile data format change is required.
>> >
>> >       Cons:
>> >           1. Still requires 16byte overhead per-function -- can
actually
>> > hurt C programs
>> >           2. -fcoverage-mapping use case is still not handled
>> >           3. The problem with llvm-profdata still exists (no
symbolic
>> > info,
>> > partial filtering support)
>> >
>> >
>> > Solution-3:
>> >
>> > This is the new solution I am proposing. It is basically an
enhancement
>> > of
>> > Solution-1 with most of the weakness resolved.  The difference
with
>> > Solution-1 is
>> >       1. Function name symbols are emitted into the symbol table
as weak
>> > externs. They don't occupy any space at runtime and can be
easily
>> > stripped.
>> >       2. -fcoverage-mapping does not need special handling -- it
>> > automatically benefit from the same size saving.
>> >       3. llvm-cov is changed to read symbol info from the symtab
instead
>> > of
>> > reading them from the name section data
>> >       4. llvm-profdata is enhanced to take a binary as input and
dump
>> > profile with names attached. Function filtering is fully supported
>> > (option
>> > can also be introduced to force dumping names into binary and
profile
>> > data,
>> > so that llvm-profdata use case is not changed at all).
>> >
>> > Pros:
>> >        1. All the pros from Solution-1
>> >        2. Size savings for coverage-mapping case
>> > Cons:
>> >        Format change is required for profile data and coverage
mapping.
>> >
>> > The initial patch is here: http://reviews.llvm.org/D13251
>> >
>> > With this patch, the size of a release clang binary with coverage
>> > mapping is
>> > reduced from 986M to 569M.
>> >
>> > If there are no major concerns,  I will carve out the patch into
smaller
>> > ones for review.
>> >
>> > thanks,
>> >
>> > David
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> > On Tue, Sep 8, 2015 at 3:47 PM, Xinliang David Li <davidxl at
google.com>
>> > wrote:
>> >>
>> >> >> >>
>> >> >> >> yes -- it is fixed length (8byte) blob which
may include null
>> >> >> >> byte
>> >> >> >> in
>> >> >> >> the middle.
>> >> >> >
>> >> >> >
>> >> >> > For reference, MD5 sum is 16 bytes (128-bit):
>> >> >> > https://en.wikipedia.org/wiki/MD5
>> >> >>
>> >> >> yes, LLVM's MD5 hash only takes the lower 64bit.
>> >> >>
>> >> >>
>> >> >> >
>> >> >> >>
>> >> >> >>
>> >> >> >> >
>> >> >> >> > Or to say it another way, suppose that
Itanium mangling
>> >> >> >> > required
>> >> >> >> > as a
>> >> >> >> > final
>> >> >> >> > step to replace the string with its md5
sum in hex. Therefore
>> >> >> >> > all
>> >> >> >> > symbol
>> >> >> >> > names are "small". My
understanding is that this is effectively
>> >> >> >> > all
>> >> >> >> > your
>> >> >> >> > patch is doing.
>> >> >> >>
>> >> >> >> The key type before the change is StringRef,
while the after the
>> >> >> >> key
>> >> >> >> type is uint64_t. Are you suggesting
treating uint64_t md5 sum
>> >> >> >> key
>> >> >> >> as
>> >> >> >> a string of 8 bytes or storing md5 has in
text form which will
>> >> >> >> double
>> >> >> >> the size?
>> >> >> >
>> >> >> >
>> >> >> > How much does this change the benefit? If most
of the benefit is
>> >> >> > avoiding
>> >> >> > extraordinarily long mangled names then it may
be sufficient.
>> >> >> >
>> >> >> > With IR-level instrumentation like Rong is
pursuing the size may
>> >> >> > be
>> >> >> > reduced
>> >> >> > sufficiently that we do not need the
optimization proposed in this
>> >> >> > thread.
>> >> >> > For example, Rong found >2x size reduction on
Google's C++
>> >> >> > benchmarks,
>> >> >> > which
>> >> >> > I assume are representative of the extremely
large Google binaries
>> >> >> > that
>> >> >> > are
>> >> >> > causing the problems addressed by your proposal
in this thread.
>> >> >> > The
>> >> >> > measurements you mention for Clang in this
thread provide similar
>> >> >> > size
>> >> >> > reductions, so Rong's approach may be
sufficient (especially
>> >> >> > because
>> >> >> > functions with extremely large mangled names
tend to be small
>> >> >> > inline
>> >> >> > functions in header-only template libraries).
>> >> >>
>> >> >> Late instrumentation helps many cases. In some cases
(as shown in
>> >> >> SPEC), the reduction in size is not as large.
Reducing PGO overhead
>> >> >> will lower the bar for its adoption.
>> >> >>
>> >> >> >
>> >> >> > Of the points you mention in "Large size of
overhead can limit the
>> >> >> > usability
>> >> >> > of PGO greatly", many of the issues are
hard limits that prevent
>> >> >> > the
>> >> >> > use
>> >> >> > of
>> >> >> > PGO. Do you have a lower bound on how much the
size of the PGO
>> >> >> > data
>> >> >> > must
>> >> >> > be
>> >> >> > reduced in order to overcome the hard limits?
>> >> >>
>> >> >> This is a static view:  Think about the situation
where application
>> >> >> size is ever increasing; also think about situation
where we want to
>> >> >>
>> >> >> collect more types of profile data. Think about
situation where user
>> >> >> want to run pgo binaries on small devices with tiny
memory/storage
>> >> >> ..
>> >> >
>> >> >
>> >> > If we want to reduce memory overhead at runtime and
reduce the size
>> >> > of
>> >> > the
>> >> > raw profile data extracted from the target, there are
clear
>> >> > solutions.
>> >> > Consider that debug info does not need to be loaded into
the memory
>> >> > image of
>> >> > the target; why should information identifying each
counter need to
>> >> > be?
>> >> > A file containing raw profile counters is a subset of a
core dump; in
>> >> > most
>> >> > environments, a core dump does not need to have debug
info or symbol
>> >> > names
>> >> > in it, but can be still be read in full detail in
conjunction with
>> >> > the
>> >> > original binary.
>> >>
>> >> Yes -- there are many alternatives:
>> >> 1) emit the name key mapping as a side data at compile time,
or
>> >> 2) emit them into nonloadable sections of the object file.
>> >>
>> >> Compared with the above, LLVM's existing design does have
its own
>> >> advantage -- making it easier for tool to access
'debug' info for
>> >> counters.
>> >>
>> >> LLVM's coverage testing, on the other hand, take a hybrid
approach: It
>> >> emits the coverage map as rodata, but does not pass it to the
profile
>> >> dumper. I think it is better to emit covmap as a side data not
>> >> attached to target binary.
>> >>
>> >>
>> >> >
>> >> > Thus, as we require that the binary be passed to
llvm-profdata, there
>> >> > is
>> >> > no
>> >> > fundamental reason that the memory image of the program,
or the raw
>> >> > data
>> >> > extracted from the program, must have any size overhead
besides the
>> >> > raw
>> >> > values of the counters themselves and any text size
increase for
>> >> > incrementing them. If we are willing to impose this
requirement on
>> >> > users,
>> >> > then as far as reducing memory overhead at runtime and
reducing the
>> >> > size
>> >> > of
>> >> > the raw profile data extracted from the target, using
hashed function
>> >> > names
>> >> > is clearly the wrong direction.
>> >> >
>> >> > *Without* imposing the requirement of passing the binary
to
>> >> > llvm-profdata, I
>> >> > do like the ability to use hashed function names like you
are
>> >> > proposing.
>> >> > It
>> >> > is a simple solution for reducing size overhead of
function name
>> >> > strings
>> >> > with little complexity, as it is just swapping one string
for
>> >> > another.
>> >>
>> >> Agree. The good news is that the overhead of hashed function
names is
>> >> small enough that makes this approach attractive.
>> >>
>> >> thanks,
>> >>
>> >> David
>> >> >
>> >> >>
>> >> >>
>> >> >> >
>> >> >> > Obviously LLVM must be able to support the
extremely large
>> >> >> > binaries
>> >> >> > in
>> >> >> > your
>> >> >> > configuration (otherwise what use is LLVM as a
compiler ;) My
>> >> >> > questions
>> >> >> > are
>> >> >> > primarily aimed at establishing which tradeoffs
are acceptable for
>> >> >> > supporting this (both for LLVM and for you
guys).
>> >> >>
>> >> >> As I said, with the modified proposal (after getting
your feedback),
>> >> >> no PGO users in LLVM land is going to lose
anything/functionality.
>> >> >> The
>> >> >> end result will be net win for general users of LLVM
(even though
>> >> >> your
>> >> >> customers don't care about it), not just
'us' as you have mentioned
>> >> >> many times.
>> >> >>
>> >> >> >
>> >> >> > Btw, for us, the issue of PGO data size is not
completely
>> >> >> > immaterial
>> >> >> > but
>> >> >> > is
>> >> >> > very different from your use case. For us, the
primary issue is
>> >> >> > the
>> >> >> > additional memory use at run time, since PS4
games usually use
>> >> >> > "all"
>> >> >> > available memory. We had a problem with UBSan
where the large
>> >> >> > amount
>> >> >> > of
>> >> >> > memory required for storing the UBSan diagnostic
data at runtime
>> >> >> > required
>> >> >> > the game programmers to manually change their
memory map to make
>> >> >> > room.
>> >> >> > +Filipe, do you remember how much memory UBSan
was using that
>> >> >> > caused
>> >> >> > a
>> >> >> > problem?
>> >> >> >
>> >> >>
>> >> >> My proposal does help reducing rodata size
significantly.
>> >> >
>> >> >
>> >> > Yes, that is why I think that this is a useful thing to
do. I just
>> >> > want
>> >> > to
>> >> > be careful about existing use cases and the relevant
workflow issues.
>> >> >
>> >> > -- Sean Silva
>> >> >
>> >> >>
>> >> >>
>> >> >> David
>> >> >>
>> >> >>
>> >> >> > -- Sean Silva
>> >> >> >
>> >> >> >>
>> >> >> >>
>> >> >> >> In the raw format, md5 sum key can be an
embedded field in the
>> >> >> >> prf_data variable instead of as different
var referenced by
>> >> >> >> prf_data.
>> >> >> >>
>> >> >> >> >
>> >> >> >> > If this is not the case, you should
show your current patch so
>> >> >> >> > that
>> >> >> >> > we
>> >> >> >> > can
>> >> >> >> > discuss things concretely.
>> >> >> >>
>> >> >> >> It is not. See above about the difference.
>> >> >> >>
>> >> >> >> >
>> >> >> >> >>
>> >> >> >> >> >
>> >> >> >> >> >
>> >> >> >> >> >>
>> >> >> >> >> >> It will be
>> >> >> >> >> >> very messy to support
multiple formats in instr-codegen and
>> >> >> >> >> >> instr-runtime.  For
compatibility concerns, the reader is
>> >> >> >> >> >> taught
>> >> >> >> >> >> to
>> >> >> >> >> >> support previous format,
but the changes there are isolated
>> >> >> >> >> >> (also
>> >> >> >> >> >> expected to be removed in
the future).
>> >> >> >> >> >>
>> >> >> >> >> >> >
>> >> >> >> >> >> > My primary concern is
that if the function name are not
>> >> >> >> >> >> > kept
>> >> >> >> >> >> > at
>> >> >> >> >> >> > all
>> >> >> >> >> >> > stages,
>> >> >> >> >> >> > then it becomes
difficult to analyze the profile data in
>> >> >> >> >> >> > a
>> >> >> >> >> >> > standalone
>> >> >> >> >> >> > way.
>> >> >> >> >> >> > Many times, I have
used `llvm-profdata show
>> >> >> >> >> >> > -all-functions
>> >> >> >> >> >> > foo.profdata`
>> >> >> >> >> >> > on
>> >> >> >> >> >> > the resulting profile
data and then imported that data
>> >> >> >> >> >> > into
>> >> >> >> >> >> > Mathematica
>> >> >> >> >> >> > for
>> >> >> >> >> >> > analysis.
>> >> >> >> >> >>
>> >> >> >> >> >> This is certainly a very
valid use case.
>> >> >> >> >> >>
>> >> >> >> >> >> >My understanding of
your proposal is that `llvm-profdata
>> >> >> >> >> >> > show
>> >> >> >> >> >> > -all-functions
foo.profdata` will not show the actual
>> >> >> >> >> >> > function
>> >> >> >> >> >> > names
>> >> >> >> >> >> > but
>> >> >> >> >> >> > instead MD5 hashes,
>> >> >> >> >> >>
>> >> >> >> >> >> Yes.
>> >> >> >> >> >>
>> >> >> >> >> >> To support your use case,
there are two solutions:
>> >> >> >> >> >>
>> >> >> >> >> >> 1) user can add
-fcoverage-mapping option in the build
>> >> >> >> >> >> 2) introduce a new option
-fprofile-instr-names that force
>> >> >> >> >> >> the
>> >> >> >> >> >> emission of the name
sections in the .o file. This is
>> >> >> >> >> >> similar
>> >> >> >> >> >> to
>> >> >> >> >> >> 1),
>> >> >> >> >> >> but no covmap section is
needed.
>> >> >> >> >> >>
>> >> >> >> >> >> llvm-profdata tool  will
be taught to read the name section
>> >> >> >> >> >> and
>> >> >> >> >> >> attach
>> >> >> >> >> >> function names to the
profile records.
>> >> >> >> >> >
>> >> >> >> >> >
>> >> >> >> >> > Needing to pass the executable
to llvm-profdata would cause
>> >> >> >> >> > deployment
>> >> >> >> >> > issues for my customers in
practice.
>> >> >> >> >>
>> >> >> >> >> Why? The deployment needs to pass
the profile data anyway
>> >> >> >> >> right?
>> >> >> >> >
>> >> >> >> >
>> >> >> >> > Yes, but not the executable.
>> >> >> >> >
>> >> >> >> > The PGO training run is likely being
run by a gameplay tester
>> >> >> >> > (non-programmer). In general the binary
will not be lying
>> >> >> >> > around
>> >> >> >> > as a
>> >> >> >> > loose
>> >> >> >> > file anywhere, it will be part of a
full package of the
>> >> >> >> > binary+assets
>> >> >> >> > (think
>> >> >> >> > like what will end up on a bluray
disc). A game's binary
>> >> >> >> > *completely
>> >> >> >> > useless* without the assets, so except
locally on a
>> >> >> >> > programmer's
>> >> >> >> > machine
>> >> >> >> > while they iterate/debug, there is no
reason for a binary to
>> >> >> >> > ever
>> >> >> >> > exist
>> >> >> >> > as a
>> >> >> >> > standalone file.
>> >> >> >> >
>> >> >> >> > I'm not saying that needing the
binary is insurmountable in any
>> >> >> >> > particular
>> >> >> >> > scenario. Just that it will cause a
strict increase in the
>> >> >> >> > number
>> >> >> >> > of
>> >> >> >> > issues
>> >> >> >> > to deploying PGO.
>> >> >> >>
>> >> >> >>  Your concern is acknowledged.
>> >> >> >>
>> >> >> >> >
>> >> >> >> > These are much bigger
"compatibility concerns" for me than for
>> >> >> >> > newer
>> >> >> >> > toolchains to accept the old format.
For a change in format I
>> >> >> >> > can
>> >> >> >> > easily
>> >> >> >> > tell my users to replace an exe with a
newer one and that is
>> >> >> >> > all
>> >> >> >> > they
>> >> >> >> > need
>> >> >> >> > to do and it takes 10 seconds,
guaranteed. A workflow change is
>> >> >> >> > potentially
>> >> >> >> > a massive disruption and guaranteed to
take more than 10
>> >> >> >> > seconds
>> >> >> >> > to
>> >> >> >> > fix
>> >> >> >> > (perhaps hours or days).
>> >> >> >>
>> >> >> >> ok.
>> >> >> >>
>> >> >> >> >
>> >> >> >> >
>> >> >> >> >>
>> >> >> >> >>   This
>> >> >> >> >> is no different from llvm-cov usage
model.
>> >> >> >> >
>> >> >> >> >
>> >> >> >> > In practice, getting the performance of
PGO is a higher
>> >> >> >> > priority
>> >> >> >> > for
>> >> >> >> > my
>> >> >> >> > users, so we should not assume that
llvm-cov is being used.
>> >> >> >>
>> >> >> >> Glad to hear that :)
>> >> >> >>
>> >> >> >> thanks,
>> >> >> >>
>> >> >> >> David
>> >> >> >>
>> >> >> >> >
>> >> >> >> > -- Sean Silva
>> >> >> >> >
>> >> >> >> >>
>> >> >> >> >>
>> >> >> >> >> David
>> >> >> >> >>
>> >> >> >> >>
>> >> >> >> >>
>> >> >> >> >> >
>> >> >> >> >> >>
>> >> >> >> >> >>
>> >> >> >> >> >> Note that with 1) or 2),
the user can still benefit from
>> >> >> >> >> >> the
>> >> >> >> >> >> reduced
>> >> >> >> >> >> profile size.
>> >> >> >> >> >
>> >> >> >> >> >
>> >> >> >> >> > Let me reiterate that the size
of the profile is not a
>> >> >> >> >> > problem
>> >> >> >> >> > I
>> >> >> >> >> > have
>> >> >> >> >> > observed in practice (nor have
I heard of this being a
>> >> >> >> >> > problem
>> >> >> >> >> > in
>> >> >> >> >> > practice
>> >> >> >> >> > until this thread). Therefore
I'm skeptical of any changes
>> >> >> >> >> > to
>> >> >> >> >> > our
>> >> >> >> >> > default
>> >> >> >> >> > behavior or any new
requirements that are not opt-in.
>> >> >> >> >> >
>> >> >> >> >> > -- Sean Silva
>> >> >> >> >> >
>> >> >> >> >> >>
>> >> >> >> >> >>
>> >> >> >> >> >> thanks,
>> >> >> >> >> >>
>> >> >> >> >> >> David
>> >> >> >> >> >>
>> >> >> >> >> >>
>> >> >> >> >> >>
>> >> >> >> >> >>
>> >> >> >> >> >> >which will make it
more difficult for me to do this kind
>> >> >> >> >> >> > of analysis (would
require using nm on the original
>> >> >> >> >> >> > binary,
>> >> >> >> >> >> > hashing
>> >> >> >> >> >> > everything, etc.).
>> >> >> >> >> >> >
>> >> >> >> >> >> > btw, feel free to
attach the patch even if it in a rough
>> >> >> >> >> >> > state.
>> >> >> >> >> >> > It
>> >> >> >> >> >> > can
>> >> >> >> >> >> > still
>> >> >> >> >> >> > help to clarify the
proposal and be a good talking point.
>> >> >> >> >> >> > Fine-grained
>> >> >> >> >> >> > patch
>> >> >> >> >> >> > review for caring
about the rough parts will happen on
>> >> >> >> >> >> > llvm-commits;
>> >> >> >> >> >> > the
>> >> >> >> >> >> > rough parts will not
distract the discussion here on
>> >> >> >> >> >> > llvm-dev.
>> >> >> >> >> >> >
>> >> >> >> >> >> > -- Sean Silva
>> >> >> >> >> >> >
>> >> >> >> >> >> >>
>> >> >> >> >> >> >>
>> >> >> >> >> >> >> thanks,
>> >> >> >> >> >> >>
>> >> >> >> >> >> >> David
>> >> >> >> >> >> >>
_______________________________________________
>> >> >> >> >> >> >> LLVM Developers
mailing list
>> >> >> >> >> >> >> llvm-dev at
lists.llvm.org
>> >> >> >> >> >> >>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >> >> >> >> >> >
>> >> >> >> >> >> >
>> >> >> >> >> >
>> >> >> >> >> >
>> >> >> >> >
>> >> >> >> >
>> >> >> >
>> >> >> >
>> >> >
>> >> >
>> >
>> >
>
>

I plan to divide the patch into series of small patches. The
preparation patches will mostly be refactoring changes with NFC. This
will minimize the size of final patch(es) with functional changes to
help discussions.

thanks,

David

On Fri, Oct 9, 2015 at 4:06 PM, Xinliang David Li <davidxl at google.com>
wrote:
> On Fri, Oct 9, 2015 at 3:58 PM, Sean Silva <chisophugis at gmail.com>
wrote:
>>
>>
>> On Wed, Oct 7, 2015 at 11:12 PM, Xinliang David Li <davidxl at
google.com>
>> wrote:
>>>
>>> There is no further response to this, so I will assume general
>>> direction of solution-3 is acceptable ;)
>
>> No response does not mean "LGTM".
>>
>
> What I meant is that the discussion can be moved on to the formal code
> review. I have not yet submitted the final patch for review yet.
> Before that is ready, continue using this thread to voice your
> concerns.
>
> David
>
>
>
>
>> -- Sean Silva
>>
>>>
>>>
>>> Solution-3 can be further improved. Instead of using static symbol
>>> table (with zero size __llvm_prf_nm symbols) to store function
names
>>> for profile display and coverage mapping, the function names can be
>>> stored compressed in a non-allocatable section. The compression
ratio
>>> for function name strings can be very high (~5x).  The covmapping
data
>>> can also be made non-allocatable.
>>>
>>> thanks,
>>>
>>> David
>>>
>>> On Tue, Sep 29, 2015 at 9:52 AM, Xinliang David Li <davidxl at
google.com>
>>> wrote:
>>> > Sorry for the late update. I finally found time to implement a
solution
>>> > (Solution-3) that has the best size savings (for both PGO and
coverage
>>> > testing) with symbolic information available.  Here is a
re-cap of what
>>> > we
>>> > have discussed so far:
>>> >
>>> > Solution-1:
>>> >
>>> > This is the original solution proposed. In this solution, A
function
>>> > name's
>>> > MD5 hash is used as the primary key (combined with IR hash)
for function
>>> > lookup. __llvm_prf_names section won't be emitted into the
binary nor
>>> > dumped
>>> > into the profile data unless -fcoverage-mapping is also
specified.
>>> >
>>> >     Pros:
>>> >         1. maximal reduction of instrumentation binary process
image
>>> > size
>>> >         2. maximal reduction of object and unstripped binary
size
>>> >         3. maximal reduction of raw profile data size
>>> >         4. maximal reduction of indexed profile data size
>>> >
>>> >    Cons:
>>> >         1. -fcoverage-mapping use case is not handled -- the
size
>>> > problem
>>> > still exist
>>> >         2. profile dump with llvm-profdata no longer have
function names
>>> > associated -- user needs to use postprocessing tool to get the
>>> > functionality
>>> >         3. function filtering with partial function name is
not
>>> > supported
>>> >         4. Requires incompatible format change
>>> >
>>> >
>>> > Solution-2: (http://reviews.llvm.org/D12715)
>>> >
>>> > In this solution, the MD5 hash string is used to replace the
raw name
>>> > string
>>> >       Pros:
>>> >           1. Very simple to implement
>>> >           2. have good reduction of all sizes for typical
large C++
>>> > applications
>>> >           3. No profile data format change is required.
>>> >
>>> >       Cons:
>>> >           1. Still requires 16byte overhead per-function --
can actually
>>> > hurt C programs
>>> >           2. -fcoverage-mapping use case is still not handled
>>> >           3. The problem with llvm-profdata still exists (no
symbolic
>>> > info,
>>> > partial filtering support)
>>> >
>>> >
>>> > Solution-3:
>>> >
>>> > This is the new solution I am proposing. It is basically an
enhancement
>>> > of
>>> > Solution-1 with most of the weakness resolved.  The difference
with
>>> > Solution-1 is
>>> >       1. Function name symbols are emitted into the symbol
table as weak
>>> > externs. They don't occupy any space at runtime and can be
easily
>>> > stripped.
>>> >       2. -fcoverage-mapping does not need special handling --
it
>>> > automatically benefit from the same size saving.
>>> >       3. llvm-cov is changed to read symbol info from the
symtab instead
>>> > of
>>> > reading them from the name section data
>>> >       4. llvm-profdata is enhanced to take a binary as input
and dump
>>> > profile with names attached. Function filtering is fully
supported
>>> > (option
>>> > can also be introduced to force dumping names into binary and
profile
>>> > data,
>>> > so that llvm-profdata use case is not changed at all).
>>> >
>>> > Pros:
>>> >        1. All the pros from Solution-1
>>> >        2. Size savings for coverage-mapping case
>>> > Cons:
>>> >        Format change is required for profile data and coverage
mapping.
>>> >
>>> > The initial patch is here: http://reviews.llvm.org/D13251
>>> >
>>> > With this patch, the size of a release clang binary with
coverage
>>> > mapping is
>>> > reduced from 986M to 569M.
>>> >
>>> > If there are no major concerns,  I will carve out the patch
into smaller
>>> > ones for review.
>>> >
>>> > thanks,
>>> >
>>> > David
>>> >
>>> >
>>> >
>>> >
>>> >
>>> >
>>> >
>>> > On Tue, Sep 8, 2015 at 3:47 PM, Xinliang David Li <davidxl
at google.com>
>>> > wrote:
>>> >>
>>> >> >> >>
>>> >> >> >> yes -- it is fixed length (8byte) blob
which may include null
>>> >> >> >> byte
>>> >> >> >> in
>>> >> >> >> the middle.
>>> >> >> >
>>> >> >> >
>>> >> >> > For reference, MD5 sum is 16 bytes
(128-bit):
>>> >> >> > https://en.wikipedia.org/wiki/MD5
>>> >> >>
>>> >> >> yes, LLVM's MD5 hash only takes the lower
64bit.
>>> >> >>
>>> >> >>
>>> >> >> >
>>> >> >> >>
>>> >> >> >>
>>> >> >> >> >
>>> >> >> >> > Or to say it another way, suppose
that Itanium mangling
>>> >> >> >> > required
>>> >> >> >> > as a
>>> >> >> >> > final
>>> >> >> >> > step to replace the string with its
md5 sum in hex. Therefore
>>> >> >> >> > all
>>> >> >> >> > symbol
>>> >> >> >> > names are "small". My
understanding is that this is effectively
>>> >> >> >> > all
>>> >> >> >> > your
>>> >> >> >> > patch is doing.
>>> >> >> >>
>>> >> >> >> The key type before the change is
StringRef, while the after the
>>> >> >> >> key
>>> >> >> >> type is uint64_t. Are you suggesting
treating uint64_t md5 sum
>>> >> >> >> key
>>> >> >> >> as
>>> >> >> >> a string of 8 bytes or storing md5 has
in text form which will
>>> >> >> >> double
>>> >> >> >> the size?
>>> >> >> >
>>> >> >> >
>>> >> >> > How much does this change the benefit? If
most of the benefit is
>>> >> >> > avoiding
>>> >> >> > extraordinarily long mangled names then it
may be sufficient.
>>> >> >> >
>>> >> >> > With IR-level instrumentation like Rong is
pursuing the size may
>>> >> >> > be
>>> >> >> > reduced
>>> >> >> > sufficiently that we do not need the
optimization proposed in this
>>> >> >> > thread.
>>> >> >> > For example, Rong found >2x size
reduction on Google's C++
>>> >> >> > benchmarks,
>>> >> >> > which
>>> >> >> > I assume are representative of the extremely
large Google binaries
>>> >> >> > that
>>> >> >> > are
>>> >> >> > causing the problems addressed by your
proposal in this thread.
>>> >> >> > The
>>> >> >> > measurements you mention for Clang in this
thread provide similar
>>> >> >> > size
>>> >> >> > reductions, so Rong's approach may be
sufficient (especially
>>> >> >> > because
>>> >> >> > functions with extremely large mangled names
tend to be small
>>> >> >> > inline
>>> >> >> > functions in header-only template
libraries).
>>> >> >>
>>> >> >> Late instrumentation helps many cases. In some
cases (as shown in
>>> >> >> SPEC), the reduction in size is not as large.
Reducing PGO overhead
>>> >> >> will lower the bar for its adoption.
>>> >> >>
>>> >> >> >
>>> >> >> > Of the points you mention in "Large
size of overhead can limit the
>>> >> >> > usability
>>> >> >> > of PGO greatly", many of the issues are
hard limits that prevent
>>> >> >> > the
>>> >> >> > use
>>> >> >> > of
>>> >> >> > PGO. Do you have a lower bound on how much
the size of the PGO
>>> >> >> > data
>>> >> >> > must
>>> >> >> > be
>>> >> >> > reduced in order to overcome the hard
limits?
>>> >> >>
>>> >> >> This is a static view:  Think about the situation
where application
>>> >> >> size is ever increasing; also think about
situation where we want to
>>> >> >>
>>> >> >> collect more types of profile data. Think about
situation where user
>>> >> >> want to run pgo binaries on small devices with
tiny memory/storage
>>> >> >> ..
>>> >> >
>>> >> >
>>> >> > If we want to reduce memory overhead at runtime and
reduce the size
>>> >> > of
>>> >> > the
>>> >> > raw profile data extracted from the target, there are
clear
>>> >> > solutions.
>>> >> > Consider that debug info does not need to be loaded
into the memory
>>> >> > image of
>>> >> > the target; why should information identifying each
counter need to
>>> >> > be?
>>> >> > A file containing raw profile counters is a subset of
a core dump; in
>>> >> > most
>>> >> > environments, a core dump does not need to have debug
info or symbol
>>> >> > names
>>> >> > in it, but can be still be read in full detail in
conjunction with
>>> >> > the
>>> >> > original binary.
>>> >>
>>> >> Yes -- there are many alternatives:
>>> >> 1) emit the name key mapping as a side data at compile
time, or
>>> >> 2) emit them into nonloadable sections of the object file.
>>> >>
>>> >> Compared with the above, LLVM's existing design does
have its own
>>> >> advantage -- making it easier for tool to access
'debug' info for
>>> >> counters.
>>> >>
>>> >> LLVM's coverage testing, on the other hand, take a
hybrid approach: It
>>> >> emits the coverage map as rodata, but does not pass it to
the profile
>>> >> dumper. I think it is better to emit covmap as a side data
not
>>> >> attached to target binary.
>>> >>
>>> >>
>>> >> >
>>> >> > Thus, as we require that the binary be passed to
llvm-profdata, there
>>> >> > is
>>> >> > no
>>> >> > fundamental reason that the memory image of the
program, or the raw
>>> >> > data
>>> >> > extracted from the program, must have any size
overhead besides the
>>> >> > raw
>>> >> > values of the counters themselves and any text size
increase for
>>> >> > incrementing them. If we are willing to impose this
requirement on
>>> >> > users,
>>> >> > then as far as reducing memory overhead at runtime
and reducing the
>>> >> > size
>>> >> > of
>>> >> > the raw profile data extracted from the target, using
hashed function
>>> >> > names
>>> >> > is clearly the wrong direction.
>>> >> >
>>> >> > *Without* imposing the requirement of passing the
binary to
>>> >> > llvm-profdata, I
>>> >> > do like the ability to use hashed function names like
you are
>>> >> > proposing.
>>> >> > It
>>> >> > is a simple solution for reducing size overhead of
function name
>>> >> > strings
>>> >> > with little complexity, as it is just swapping one
string for
>>> >> > another.
>>> >>
>>> >> Agree. The good news is that the overhead of hashed
function names is
>>> >> small enough that makes this approach attractive.
>>> >>
>>> >> thanks,
>>> >>
>>> >> David
>>> >> >
>>> >> >>
>>> >> >>
>>> >> >> >
>>> >> >> > Obviously LLVM must be able to support the
extremely large
>>> >> >> > binaries
>>> >> >> > in
>>> >> >> > your
>>> >> >> > configuration (otherwise what use is LLVM as
a compiler ;) My
>>> >> >> > questions
>>> >> >> > are
>>> >> >> > primarily aimed at establishing which
tradeoffs are acceptable for
>>> >> >> > supporting this (both for LLVM and for you
guys).
>>> >> >>
>>> >> >> As I said, with the modified proposal (after
getting your feedback),
>>> >> >> no PGO users in LLVM land is going to lose
anything/functionality.
>>> >> >> The
>>> >> >> end result will be net win for general users of
LLVM (even though
>>> >> >> your
>>> >> >> customers don't care about it), not just
'us' as you have mentioned
>>> >> >> many times.
>>> >> >>
>>> >> >> >
>>> >> >> > Btw, for us, the issue of PGO data size is
not completely
>>> >> >> > immaterial
>>> >> >> > but
>>> >> >> > is
>>> >> >> > very different from your use case. For us,
the primary issue is
>>> >> >> > the
>>> >> >> > additional memory use at run time, since PS4
games usually use
>>> >> >> > "all"
>>> >> >> > available memory. We had a problem with
UBSan where the large
>>> >> >> > amount
>>> >> >> > of
>>> >> >> > memory required for storing the UBSan
diagnostic data at runtime
>>> >> >> > required
>>> >> >> > the game programmers to manually change
their memory map to make
>>> >> >> > room.
>>> >> >> > +Filipe, do you remember how much memory
UBSan was using that
>>> >> >> > caused
>>> >> >> > a
>>> >> >> > problem?
>>> >> >> >
>>> >> >>
>>> >> >> My proposal does help reducing rodata size
significantly.
>>> >> >
>>> >> >
>>> >> > Yes, that is why I think that this is a useful thing
to do. I just
>>> >> > want
>>> >> > to
>>> >> > be careful about existing use cases and the relevant
workflow issues.
>>> >> >
>>> >> > -- Sean Silva
>>> >> >
>>> >> >>
>>> >> >>
>>> >> >> David
>>> >> >>
>>> >> >>
>>> >> >> > -- Sean Silva
>>> >> >> >
>>> >> >> >>
>>> >> >> >>
>>> >> >> >> In the raw format, md5 sum key can be an
embedded field in the
>>> >> >> >> prf_data variable instead of as
different var referenced by
>>> >> >> >> prf_data.
>>> >> >> >>
>>> >> >> >> >
>>> >> >> >> > If this is not the case, you should
show your current patch so
>>> >> >> >> > that
>>> >> >> >> > we
>>> >> >> >> > can
>>> >> >> >> > discuss things concretely.
>>> >> >> >>
>>> >> >> >> It is not. See above about the
difference.
>>> >> >> >>
>>> >> >> >> >
>>> >> >> >> >>
>>> >> >> >> >> >
>>> >> >> >> >> >
>>> >> >> >> >> >>
>>> >> >> >> >> >> It will be
>>> >> >> >> >> >> very messy to support
multiple formats in instr-codegen and
>>> >> >> >> >> >> instr-runtime.  For
compatibility concerns, the reader is
>>> >> >> >> >> >> taught
>>> >> >> >> >> >> to
>>> >> >> >> >> >> support previous
format, but the changes there are isolated
>>> >> >> >> >> >> (also
>>> >> >> >> >> >> expected to be removed
in the future).
>>> >> >> >> >> >>
>>> >> >> >> >> >> >
>>> >> >> >> >> >> > My primary
concern is that if the function name are not
>>> >> >> >> >> >> > kept
>>> >> >> >> >> >> > at
>>> >> >> >> >> >> > all
>>> >> >> >> >> >> > stages,
>>> >> >> >> >> >> > then it becomes
difficult to analyze the profile data in
>>> >> >> >> >> >> > a
>>> >> >> >> >> >> > standalone
>>> >> >> >> >> >> > way.
>>> >> >> >> >> >> > Many times, I
have used `llvm-profdata show
>>> >> >> >> >> >> > -all-functions
>>> >> >> >> >> >> > foo.profdata`
>>> >> >> >> >> >> > on
>>> >> >> >> >> >> > the resulting
profile data and then imported that data
>>> >> >> >> >> >> > into
>>> >> >> >> >> >> > Mathematica
>>> >> >> >> >> >> > for
>>> >> >> >> >> >> > analysis.
>>> >> >> >> >> >>
>>> >> >> >> >> >> This is certainly a
very valid use case.
>>> >> >> >> >> >>
>>> >> >> >> >> >> >My understanding
of your proposal is that `llvm-profdata
>>> >> >> >> >> >> > show
>>> >> >> >> >> >> > -all-functions
foo.profdata` will not show the actual
>>> >> >> >> >> >> > function
>>> >> >> >> >> >> > names
>>> >> >> >> >> >> > but
>>> >> >> >> >> >> > instead MD5
hashes,
>>> >> >> >> >> >>
>>> >> >> >> >> >> Yes.
>>> >> >> >> >> >>
>>> >> >> >> >> >> To support your use
case, there are two solutions:
>>> >> >> >> >> >>
>>> >> >> >> >> >> 1) user can add
-fcoverage-mapping option in the build
>>> >> >> >> >> >> 2) introduce a new
option -fprofile-instr-names that force
>>> >> >> >> >> >> the
>>> >> >> >> >> >> emission of the name
sections in the .o file. This is
>>> >> >> >> >> >> similar
>>> >> >> >> >> >> to
>>> >> >> >> >> >> 1),
>>> >> >> >> >> >> but no covmap section
is needed.
>>> >> >> >> >> >>
>>> >> >> >> >> >> llvm-profdata tool 
will be taught to read the name section
>>> >> >> >> >> >> and
>>> >> >> >> >> >> attach
>>> >> >> >> >> >> function names to the
profile records.
>>> >> >> >> >> >
>>> >> >> >> >> >
>>> >> >> >> >> > Needing to pass the
executable to llvm-profdata would cause
>>> >> >> >> >> > deployment
>>> >> >> >> >> > issues for my customers in
practice.
>>> >> >> >> >>
>>> >> >> >> >> Why? The deployment needs to
pass the profile data anyway
>>> >> >> >> >> right?
>>> >> >> >> >
>>> >> >> >> >
>>> >> >> >> > Yes, but not the executable.
>>> >> >> >> >
>>> >> >> >> > The PGO training run is likely
being run by a gameplay tester
>>> >> >> >> > (non-programmer). In general the
binary will not be lying
>>> >> >> >> > around
>>> >> >> >> > as a
>>> >> >> >> > loose
>>> >> >> >> > file anywhere, it will be part of a
full package of the
>>> >> >> >> > binary+assets
>>> >> >> >> > (think
>>> >> >> >> > like what will end up on a bluray
disc). A game's binary
>>> >> >> >> > *completely
>>> >> >> >> > useless* without the assets, so
except locally on a
>>> >> >> >> > programmer's
>>> >> >> >> > machine
>>> >> >> >> > while they iterate/debug, there is
no reason for a binary to
>>> >> >> >> > ever
>>> >> >> >> > exist
>>> >> >> >> > as a
>>> >> >> >> > standalone file.
>>> >> >> >> >
>>> >> >> >> > I'm not saying that needing the
binary is insurmountable in any
>>> >> >> >> > particular
>>> >> >> >> > scenario. Just that it will cause a
strict increase in the
>>> >> >> >> > number
>>> >> >> >> > of
>>> >> >> >> > issues
>>> >> >> >> > to deploying PGO.
>>> >> >> >>
>>> >> >> >>  Your concern is acknowledged.
>>> >> >> >>
>>> >> >> >> >
>>> >> >> >> > These are much bigger
"compatibility concerns" for me than for
>>> >> >> >> > newer
>>> >> >> >> > toolchains to accept the old
format. For a change in format I
>>> >> >> >> > can
>>> >> >> >> > easily
>>> >> >> >> > tell my users to replace an exe
with a newer one and that is
>>> >> >> >> > all
>>> >> >> >> > they
>>> >> >> >> > need
>>> >> >> >> > to do and it takes 10 seconds,
guaranteed. A workflow change is
>>> >> >> >> > potentially
>>> >> >> >> > a massive disruption and guaranteed
to take more than 10
>>> >> >> >> > seconds
>>> >> >> >> > to
>>> >> >> >> > fix
>>> >> >> >> > (perhaps hours or days).
>>> >> >> >>
>>> >> >> >> ok.
>>> >> >> >>
>>> >> >> >> >
>>> >> >> >> >
>>> >> >> >> >>
>>> >> >> >> >>   This
>>> >> >> >> >> is no different from llvm-cov
usage model.
>>> >> >> >> >
>>> >> >> >> >
>>> >> >> >> > In practice, getting the
performance of PGO is a higher
>>> >> >> >> > priority
>>> >> >> >> > for
>>> >> >> >> > my
>>> >> >> >> > users, so we should not assume that
llvm-cov is being used.
>>> >> >> >>
>>> >> >> >> Glad to hear that :)
>>> >> >> >>
>>> >> >> >> thanks,
>>> >> >> >>
>>> >> >> >> David
>>> >> >> >>
>>> >> >> >> >
>>> >> >> >> > -- Sean Silva
>>> >> >> >> >
>>> >> >> >> >>
>>> >> >> >> >>
>>> >> >> >> >> David
>>> >> >> >> >>
>>> >> >> >> >>
>>> >> >> >> >>
>>> >> >> >> >> >
>>> >> >> >> >> >>
>>> >> >> >> >> >>
>>> >> >> >> >> >> Note that with 1) or
2), the user can still benefit from
>>> >> >> >> >> >> the
>>> >> >> >> >> >> reduced
>>> >> >> >> >> >> profile size.
>>> >> >> >> >> >
>>> >> >> >> >> >
>>> >> >> >> >> > Let me reiterate that the
size of the profile is not a
>>> >> >> >> >> > problem
>>> >> >> >> >> > I
>>> >> >> >> >> > have
>>> >> >> >> >> > observed in practice (nor
have I heard of this being a
>>> >> >> >> >> > problem
>>> >> >> >> >> > in
>>> >> >> >> >> > practice
>>> >> >> >> >> > until this thread).
Therefore I'm skeptical of any changes
>>> >> >> >> >> > to
>>> >> >> >> >> > our
>>> >> >> >> >> > default
>>> >> >> >> >> > behavior or any new
requirements that are not opt-in.
>>> >> >> >> >> >
>>> >> >> >> >> > -- Sean Silva
>>> >> >> >> >> >
>>> >> >> >> >> >>
>>> >> >> >> >> >>
>>> >> >> >> >> >> thanks,
>>> >> >> >> >> >>
>>> >> >> >> >> >> David
>>> >> >> >> >> >>
>>> >> >> >> >> >>
>>> >> >> >> >> >>
>>> >> >> >> >> >>
>>> >> >> >> >> >> >which will make it
more difficult for me to do this kind
>>> >> >> >> >> >> > of analysis
(would require using nm on the original
>>> >> >> >> >> >> > binary,
>>> >> >> >> >> >> > hashing
>>> >> >> >> >> >> > everything,
etc.).
>>> >> >> >> >> >> >
>>> >> >> >> >> >> > btw, feel free to
attach the patch even if it in a rough
>>> >> >> >> >> >> > state.
>>> >> >> >> >> >> > It
>>> >> >> >> >> >> > can
>>> >> >> >> >> >> > still
>>> >> >> >> >> >> > help to clarify
the proposal and be a good talking point.
>>> >> >> >> >> >> > Fine-grained
>>> >> >> >> >> >> > patch
>>> >> >> >> >> >> > review for caring
about the rough parts will happen on
>>> >> >> >> >> >> > llvm-commits;
>>> >> >> >> >> >> > the
>>> >> >> >> >> >> > rough parts will
not distract the discussion here on
>>> >> >> >> >> >> > llvm-dev.
>>> >> >> >> >> >> >
>>> >> >> >> >> >> > -- Sean Silva
>>> >> >> >> >> >> >
>>> >> >> >> >> >> >>
>>> >> >> >> >> >> >>
>>> >> >> >> >> >> >> thanks,
>>> >> >> >> >> >> >>
>>> >> >> >> >> >> >> David
>>> >> >> >> >> >> >>
_______________________________________________
>>> >> >> >> >> >> >> LLVM
Developers mailing list
>>> >> >> >> >> >> >> llvm-dev at
lists.llvm.org
>>> >> >> >> >> >> >>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> >> >> >> >> >> >
>>> >> >> >> >> >> >
>>> >> >> >> >> >
>>> >> >> >> >> >
>>> >> >> >> >
>>> >> >> >> >
>>> >> >> >
>>> >> >> >
>>> >> >
>>> >> >
>>> >
>>> >
>>
>>

We are now very close to push the final stage of the PGO size
reduction task. Here is the updated plan going forward:

1) In this round, the format of the indexed profile data won't be unchanged.
2) there will be *no* changes in user interfaces to all profiling
related tools including llvm-profdata, llvm-cov -- the change will be
transparent in terms of PGO usability.
3) The implementation will be using  compression for the function name
section (the compression ratio is about 5:1). As a result, the linker
input object size, unstripped binary size, stripped binary size,
process image size, and raw profile data size will all be greatly
reduced;
4) The change will also greatly improve usability of coverage-mapping
due to the reduced data size in memory.

Before the final patch, there are a few more small preparation patches
: 1) abstract naming reading into a class (ProfSymtab) (currently the
reader uses/assumes the raw/uncompressed object section. 2) add
compression support in ProfSymtab.

thanks,

David





On Wed, Oct 14, 2015 at 12:30 AM, Xinliang David Li <davidxl at
google.com> wrote:
> I plan to divide the patch into series of small patches. The
> preparation patches will mostly be refactoring changes with NFC. This
> will minimize the size of final patch(es) with functional changes to
> help discussions.
>
> thanks,
>
> David
>
> On Fri, Oct 9, 2015 at 4:06 PM, Xinliang David Li <davidxl at
google.com> wrote:
>> On Fri, Oct 9, 2015 at 3:58 PM, Sean Silva <chisophugis at
gmail.com> wrote:
>>>
>>>
>>> On Wed, Oct 7, 2015 at 11:12 PM, Xinliang David Li <davidxl at
google.com>
>>> wrote:
>>>>
>>>> There is no further response to this, so I will assume general
>>>> direction of solution-3 is acceptable ;)
>>
>>> No response does not mean "LGTM".
>>>
>>
>> What I meant is that the discussion can be moved on to the formal code
>> review. I have not yet submitted the final patch for review yet.
>> Before that is ready, continue using this thread to voice your
>> concerns.
>>
>> David
>>
>>
>>
>>
>>> -- Sean Silva
>>>
>>>>
>>>>
>>>> Solution-3 can be further improved. Instead of using static
symbol
>>>> table (with zero size __llvm_prf_nm symbols) to store function
names
>>>> for profile display and coverage mapping, the function names
can be
>>>> stored compressed in a non-allocatable section. The compression
ratio
>>>> for function name strings can be very high (~5x).  The
covmapping data
>>>> can also be made non-allocatable.
>>>>
>>>> thanks,
>>>>
>>>> David
>>>>
>>>> On Tue, Sep 29, 2015 at 9:52 AM, Xinliang David Li <davidxl
at google.com>
>>>> wrote:
>>>> > Sorry for the late update. I finally found time to
implement a solution
>>>> > (Solution-3) that has the best size savings (for both PGO
and coverage
>>>> > testing) with symbolic information available.  Here is a
re-cap of what
>>>> > we
>>>> > have discussed so far:
>>>> >
>>>> > Solution-1:
>>>> >
>>>> > This is the original solution proposed. In this solution,
A function
>>>> > name's
>>>> > MD5 hash is used as the primary key (combined with IR
hash) for function
>>>> > lookup. __llvm_prf_names section won't be emitted into
the binary nor
>>>> > dumped
>>>> > into the profile data unless -fcoverage-mapping is also
specified.
>>>> >
>>>> >     Pros:
>>>> >         1. maximal reduction of instrumentation binary
process image
>>>> > size
>>>> >         2. maximal reduction of object and unstripped
binary size
>>>> >         3. maximal reduction of raw profile data size
>>>> >         4. maximal reduction of indexed profile data size
>>>> >
>>>> >    Cons:
>>>> >         1. -fcoverage-mapping use case is not handled --
the size
>>>> > problem
>>>> > still exist
>>>> >         2. profile dump with llvm-profdata no longer have
function names
>>>> > associated -- user needs to use postprocessing tool to get
the
>>>> > functionality
>>>> >         3. function filtering with partial function name
is not
>>>> > supported
>>>> >         4. Requires incompatible format change
>>>> >
>>>> >
>>>> > Solution-2: (http://reviews.llvm.org/D12715)
>>>> >
>>>> > In this solution, the MD5 hash string is used to replace
the raw name
>>>> > string
>>>> >       Pros:
>>>> >           1. Very simple to implement
>>>> >           2. have good reduction of all sizes for typical
large C++
>>>> > applications
>>>> >           3. No profile data format change is required.
>>>> >
>>>> >       Cons:
>>>> >           1. Still requires 16byte overhead per-function
-- can actually
>>>> > hurt C programs
>>>> >           2. -fcoverage-mapping use case is still not
handled
>>>> >           3. The problem with llvm-profdata still exists
(no symbolic
>>>> > info,
>>>> > partial filtering support)
>>>> >
>>>> >
>>>> > Solution-3:
>>>> >
>>>> > This is the new solution I am proposing. It is basically
an enhancement
>>>> > of
>>>> > Solution-1 with most of the weakness resolved.  The
difference with
>>>> > Solution-1 is
>>>> >       1. Function name symbols are emitted into the symbol
table as weak
>>>> > externs. They don't occupy any space at runtime and
can be easily
>>>> > stripped.
>>>> >       2. -fcoverage-mapping does not need special handling
-- it
>>>> > automatically benefit from the same size saving.
>>>> >       3. llvm-cov is changed to read symbol info from the
symtab instead
>>>> > of
>>>> > reading them from the name section data
>>>> >       4. llvm-profdata is enhanced to take a binary as
input and dump
>>>> > profile with names attached. Function filtering is fully
supported
>>>> > (option
>>>> > can also be introduced to force dumping names into binary
and profile
>>>> > data,
>>>> > so that llvm-profdata use case is not changed at all).
>>>> >
>>>> > Pros:
>>>> >        1. All the pros from Solution-1
>>>> >        2. Size savings for coverage-mapping case
>>>> > Cons:
>>>> >        Format change is required for profile data and
coverage mapping.
>>>> >
>>>> > The initial patch is here: http://reviews.llvm.org/D13251
>>>> >
>>>> > With this patch, the size of a release clang binary with
coverage
>>>> > mapping is
>>>> > reduced from 986M to 569M.
>>>> >
>>>> > If there are no major concerns,  I will carve out the
patch into smaller
>>>> > ones for review.
>>>> >
>>>> > thanks,
>>>> >
>>>> > David
>>>> >
>>>> >
>>>> >
>>>> >
>>>> >
>>>> >
>>>> >
>>>> > On Tue, Sep 8, 2015 at 3:47 PM, Xinliang David Li
<davidxl at google.com>
>>>> > wrote:
>>>> >>
>>>> >> >> >>
>>>> >> >> >> yes -- it is fixed length (8byte)
blob which may include null
>>>> >> >> >> byte
>>>> >> >> >> in
>>>> >> >> >> the middle.
>>>> >> >> >
>>>> >> >> >
>>>> >> >> > For reference, MD5 sum is 16 bytes
(128-bit):
>>>> >> >> > https://en.wikipedia.org/wiki/MD5
>>>> >> >>
>>>> >> >> yes, LLVM's MD5 hash only takes the lower
64bit.
>>>> >> >>
>>>> >> >>
>>>> >> >> >
>>>> >> >> >>
>>>> >> >> >>
>>>> >> >> >> >
>>>> >> >> >> > Or to say it another way,
suppose that Itanium mangling
>>>> >> >> >> > required
>>>> >> >> >> > as a
>>>> >> >> >> > final
>>>> >> >> >> > step to replace the string with
its md5 sum in hex. Therefore
>>>> >> >> >> > all
>>>> >> >> >> > symbol
>>>> >> >> >> > names are "small". My
understanding is that this is effectively
>>>> >> >> >> > all
>>>> >> >> >> > your
>>>> >> >> >> > patch is doing.
>>>> >> >> >>
>>>> >> >> >> The key type before the change is
StringRef, while the after the
>>>> >> >> >> key
>>>> >> >> >> type is uint64_t. Are you suggesting
treating uint64_t md5 sum
>>>> >> >> >> key
>>>> >> >> >> as
>>>> >> >> >> a string of 8 bytes or storing md5
has in text form which will
>>>> >> >> >> double
>>>> >> >> >> the size?
>>>> >> >> >
>>>> >> >> >
>>>> >> >> > How much does this change the benefit?
If most of the benefit is
>>>> >> >> > avoiding
>>>> >> >> > extraordinarily long mangled names then
it may be sufficient.
>>>> >> >> >
>>>> >> >> > With IR-level instrumentation like Rong
is pursuing the size may
>>>> >> >> > be
>>>> >> >> > reduced
>>>> >> >> > sufficiently that we do not need the
optimization proposed in this
>>>> >> >> > thread.
>>>> >> >> > For example, Rong found >2x size
reduction on Google's C++
>>>> >> >> > benchmarks,
>>>> >> >> > which
>>>> >> >> > I assume are representative of the
extremely large Google binaries
>>>> >> >> > that
>>>> >> >> > are
>>>> >> >> > causing the problems addressed by your
proposal in this thread.
>>>> >> >> > The
>>>> >> >> > measurements you mention for Clang in
this thread provide similar
>>>> >> >> > size
>>>> >> >> > reductions, so Rong's approach may
be sufficient (especially
>>>> >> >> > because
>>>> >> >> > functions with extremely large mangled
names tend to be small
>>>> >> >> > inline
>>>> >> >> > functions in header-only template
libraries).
>>>> >> >>
>>>> >> >> Late instrumentation helps many cases. In
some cases (as shown in
>>>> >> >> SPEC), the reduction in size is not as large.
Reducing PGO overhead
>>>> >> >> will lower the bar for its adoption.
>>>> >> >>
>>>> >> >> >
>>>> >> >> > Of the points you mention in "Large
size of overhead can limit the
>>>> >> >> > usability
>>>> >> >> > of PGO greatly", many of the issues
are hard limits that prevent
>>>> >> >> > the
>>>> >> >> > use
>>>> >> >> > of
>>>> >> >> > PGO. Do you have a lower bound on how
much the size of the PGO
>>>> >> >> > data
>>>> >> >> > must
>>>> >> >> > be
>>>> >> >> > reduced in order to overcome the hard
limits?
>>>> >> >>
>>>> >> >> This is a static view:  Think about the
situation where application
>>>> >> >> size is ever increasing; also think about
situation where we want to
>>>> >> >>
>>>> >> >> collect more types of profile data. Think
about situation where user
>>>> >> >> want to run pgo binaries on small devices
with tiny memory/storage
>>>> >> >> ..
>>>> >> >
>>>> >> >
>>>> >> > If we want to reduce memory overhead at runtime
and reduce the size
>>>> >> > of
>>>> >> > the
>>>> >> > raw profile data extracted from the target, there
are clear
>>>> >> > solutions.
>>>> >> > Consider that debug info does not need to be
loaded into the memory
>>>> >> > image of
>>>> >> > the target; why should information identifying
each counter need to
>>>> >> > be?
>>>> >> > A file containing raw profile counters is a
subset of a core dump; in
>>>> >> > most
>>>> >> > environments, a core dump does not need to have
debug info or symbol
>>>> >> > names
>>>> >> > in it, but can be still be read in full detail in
conjunction with
>>>> >> > the
>>>> >> > original binary.
>>>> >>
>>>> >> Yes -- there are many alternatives:
>>>> >> 1) emit the name key mapping as a side data at compile
time, or
>>>> >> 2) emit them into nonloadable sections of the object
file.
>>>> >>
>>>> >> Compared with the above, LLVM's existing design
does have its own
>>>> >> advantage -- making it easier for tool to access
'debug' info for
>>>> >> counters.
>>>> >>
>>>> >> LLVM's coverage testing, on the other hand, take a
hybrid approach: It
>>>> >> emits the coverage map as rodata, but does not pass it
to the profile
>>>> >> dumper. I think it is better to emit covmap as a side
data not
>>>> >> attached to target binary.
>>>> >>
>>>> >>
>>>> >> >
>>>> >> > Thus, as we require that the binary be passed to
llvm-profdata, there
>>>> >> > is
>>>> >> > no
>>>> >> > fundamental reason that the memory image of the
program, or the raw
>>>> >> > data
>>>> >> > extracted from the program, must have any size
overhead besides the
>>>> >> > raw
>>>> >> > values of the counters themselves and any text
size increase for
>>>> >> > incrementing them. If we are willing to impose
this requirement on
>>>> >> > users,
>>>> >> > then as far as reducing memory overhead at
runtime and reducing the
>>>> >> > size
>>>> >> > of
>>>> >> > the raw profile data extracted from the target,
using hashed function
>>>> >> > names
>>>> >> > is clearly the wrong direction.
>>>> >> >
>>>> >> > *Without* imposing the requirement of passing the
binary to
>>>> >> > llvm-profdata, I
>>>> >> > do like the ability to use hashed function names
like you are
>>>> >> > proposing.
>>>> >> > It
>>>> >> > is a simple solution for reducing size overhead
of function name
>>>> >> > strings
>>>> >> > with little complexity, as it is just swapping
one string for
>>>> >> > another.
>>>> >>
>>>> >> Agree. The good news is that the overhead of hashed
function names is
>>>> >> small enough that makes this approach attractive.
>>>> >>
>>>> >> thanks,
>>>> >>
>>>> >> David
>>>> >> >
>>>> >> >>
>>>> >> >>
>>>> >> >> >
>>>> >> >> > Obviously LLVM must be able to support
the extremely large
>>>> >> >> > binaries
>>>> >> >> > in
>>>> >> >> > your
>>>> >> >> > configuration (otherwise what use is
LLVM as a compiler ;) My
>>>> >> >> > questions
>>>> >> >> > are
>>>> >> >> > primarily aimed at establishing which
tradeoffs are acceptable for
>>>> >> >> > supporting this (both for LLVM and for
you guys).
>>>> >> >>
>>>> >> >> As I said, with the modified proposal (after
getting your feedback),
>>>> >> >> no PGO users in LLVM land is going to lose
anything/functionality.
>>>> >> >> The
>>>> >> >> end result will be net win for general users
of LLVM (even though
>>>> >> >> your
>>>> >> >> customers don't care about it), not just
'us' as you have mentioned
>>>> >> >> many times.
>>>> >> >>
>>>> >> >> >
>>>> >> >> > Btw, for us, the issue of PGO data size
is not completely
>>>> >> >> > immaterial
>>>> >> >> > but
>>>> >> >> > is
>>>> >> >> > very different from your use case. For
us, the primary issue is
>>>> >> >> > the
>>>> >> >> > additional memory use at run time, since
PS4 games usually use
>>>> >> >> > "all"
>>>> >> >> > available memory. We had a problem with
UBSan where the large
>>>> >> >> > amount
>>>> >> >> > of
>>>> >> >> > memory required for storing the UBSan
diagnostic data at runtime
>>>> >> >> > required
>>>> >> >> > the game programmers to manually change
their memory map to make
>>>> >> >> > room.
>>>> >> >> > +Filipe, do you remember how much memory
UBSan was using that
>>>> >> >> > caused
>>>> >> >> > a
>>>> >> >> > problem?
>>>> >> >> >
>>>> >> >>
>>>> >> >> My proposal does help reducing rodata size
significantly.
>>>> >> >
>>>> >> >
>>>> >> > Yes, that is why I think that this is a useful
thing to do. I just
>>>> >> > want
>>>> >> > to
>>>> >> > be careful about existing use cases and the
relevant workflow issues.
>>>> >> >
>>>> >> > -- Sean Silva
>>>> >> >
>>>> >> >>
>>>> >> >>
>>>> >> >> David
>>>> >> >>
>>>> >> >>
>>>> >> >> > -- Sean Silva
>>>> >> >> >
>>>> >> >> >>
>>>> >> >> >>
>>>> >> >> >> In the raw format, md5 sum key can
be an embedded field in the
>>>> >> >> >> prf_data variable instead of as
different var referenced by
>>>> >> >> >> prf_data.
>>>> >> >> >>
>>>> >> >> >> >
>>>> >> >> >> > If this is not the case, you
should show your current patch so
>>>> >> >> >> > that
>>>> >> >> >> > we
>>>> >> >> >> > can
>>>> >> >> >> > discuss things concretely.
>>>> >> >> >>
>>>> >> >> >> It is not. See above about the
difference.
>>>> >> >> >>
>>>> >> >> >> >
>>>> >> >> >> >>
>>>> >> >> >> >> >
>>>> >> >> >> >> >
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> It will be
>>>> >> >> >> >> >> very messy to
support multiple formats in instr-codegen and
>>>> >> >> >> >> >> instr-runtime. 
For compatibility concerns, the reader is
>>>> >> >> >> >> >> taught
>>>> >> >> >> >> >> to
>>>> >> >> >> >> >> support previous
format, but the changes there are isolated
>>>> >> >> >> >> >> (also
>>>> >> >> >> >> >> expected to be
removed in the future).
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> >
>>>> >> >> >> >> >> > My primary
concern is that if the function name are not
>>>> >> >> >> >> >> > kept
>>>> >> >> >> >> >> > at
>>>> >> >> >> >> >> > all
>>>> >> >> >> >> >> > stages,
>>>> >> >> >> >> >> > then it
becomes difficult to analyze the profile data in
>>>> >> >> >> >> >> > a
>>>> >> >> >> >> >> > standalone
>>>> >> >> >> >> >> > way.
>>>> >> >> >> >> >> > Many times, I
have used `llvm-profdata show
>>>> >> >> >> >> >> >
-all-functions
>>>> >> >> >> >> >> > foo.profdata`
>>>> >> >> >> >> >> > on
>>>> >> >> >> >> >> > the resulting
profile data and then imported that data
>>>> >> >> >> >> >> > into
>>>> >> >> >> >> >> > Mathematica
>>>> >> >> >> >> >> > for
>>>> >> >> >> >> >> > analysis.
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> This is certainly
a very valid use case.
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> >My
understanding of your proposal is that `llvm-profdata
>>>> >> >> >> >> >> > show
>>>> >> >> >> >> >> >
-all-functions foo.profdata` will not show the actual
>>>> >> >> >> >> >> > function
>>>> >> >> >> >> >> > names
>>>> >> >> >> >> >> > but
>>>> >> >> >> >> >> > instead MD5
hashes,
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> Yes.
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> To support your
use case, there are two solutions:
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> 1) user can add
-fcoverage-mapping option in the build
>>>> >> >> >> >> >> 2) introduce a new
option -fprofile-instr-names that force
>>>> >> >> >> >> >> the
>>>> >> >> >> >> >> emission of the
name sections in the .o file. This is
>>>> >> >> >> >> >> similar
>>>> >> >> >> >> >> to
>>>> >> >> >> >> >> 1),
>>>> >> >> >> >> >> but no covmap
section is needed.
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> llvm-profdata tool
will be taught to read the name section
>>>> >> >> >> >> >> and
>>>> >> >> >> >> >> attach
>>>> >> >> >> >> >> function names to
the profile records.
>>>> >> >> >> >> >
>>>> >> >> >> >> >
>>>> >> >> >> >> > Needing to pass the
executable to llvm-profdata would cause
>>>> >> >> >> >> > deployment
>>>> >> >> >> >> > issues for my
customers in practice.
>>>> >> >> >> >>
>>>> >> >> >> >> Why? The deployment needs
to pass the profile data anyway
>>>> >> >> >> >> right?
>>>> >> >> >> >
>>>> >> >> >> >
>>>> >> >> >> > Yes, but not the executable.
>>>> >> >> >> >
>>>> >> >> >> > The PGO training run is likely
being run by a gameplay tester
>>>> >> >> >> > (non-programmer). In general
the binary will not be lying
>>>> >> >> >> > around
>>>> >> >> >> > as a
>>>> >> >> >> > loose
>>>> >> >> >> > file anywhere, it will be part
of a full package of the
>>>> >> >> >> > binary+assets
>>>> >> >> >> > (think
>>>> >> >> >> > like what will end up on a
bluray disc). A game's binary
>>>> >> >> >> > *completely
>>>> >> >> >> > useless* without the assets, so
except locally on a
>>>> >> >> >> > programmer's
>>>> >> >> >> > machine
>>>> >> >> >> > while they iterate/debug, there
is no reason for a binary to
>>>> >> >> >> > ever
>>>> >> >> >> > exist
>>>> >> >> >> > as a
>>>> >> >> >> > standalone file.
>>>> >> >> >> >
>>>> >> >> >> > I'm not saying that needing
the binary is insurmountable in any
>>>> >> >> >> > particular
>>>> >> >> >> > scenario. Just that it will
cause a strict increase in the
>>>> >> >> >> > number
>>>> >> >> >> > of
>>>> >> >> >> > issues
>>>> >> >> >> > to deploying PGO.
>>>> >> >> >>
>>>> >> >> >>  Your concern is acknowledged.
>>>> >> >> >>
>>>> >> >> >> >
>>>> >> >> >> > These are much bigger
"compatibility concerns" for me than for
>>>> >> >> >> > newer
>>>> >> >> >> > toolchains to accept the old
format. For a change in format I
>>>> >> >> >> > can
>>>> >> >> >> > easily
>>>> >> >> >> > tell my users to replace an exe
with a newer one and that is
>>>> >> >> >> > all
>>>> >> >> >> > they
>>>> >> >> >> > need
>>>> >> >> >> > to do and it takes 10 seconds,
guaranteed. A workflow change is
>>>> >> >> >> > potentially
>>>> >> >> >> > a massive disruption and
guaranteed to take more than 10
>>>> >> >> >> > seconds
>>>> >> >> >> > to
>>>> >> >> >> > fix
>>>> >> >> >> > (perhaps hours or days).
>>>> >> >> >>
>>>> >> >> >> ok.
>>>> >> >> >>
>>>> >> >> >> >
>>>> >> >> >> >
>>>> >> >> >> >>
>>>> >> >> >> >>   This
>>>> >> >> >> >> is no different from
llvm-cov usage model.
>>>> >> >> >> >
>>>> >> >> >> >
>>>> >> >> >> > In practice, getting the
performance of PGO is a higher
>>>> >> >> >> > priority
>>>> >> >> >> > for
>>>> >> >> >> > my
>>>> >> >> >> > users, so we should not assume
that llvm-cov is being used.
>>>> >> >> >>
>>>> >> >> >> Glad to hear that :)
>>>> >> >> >>
>>>> >> >> >> thanks,
>>>> >> >> >>
>>>> >> >> >> David
>>>> >> >> >>
>>>> >> >> >> >
>>>> >> >> >> > -- Sean Silva
>>>> >> >> >> >
>>>> >> >> >> >>
>>>> >> >> >> >>
>>>> >> >> >> >> David
>>>> >> >> >> >>
>>>> >> >> >> >>
>>>> >> >> >> >>
>>>> >> >> >> >> >
>>>> >> >> >> >> >>
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> Note that with 1)
or 2), the user can still benefit from
>>>> >> >> >> >> >> the
>>>> >> >> >> >> >> reduced
>>>> >> >> >> >> >> profile size.
>>>> >> >> >> >> >
>>>> >> >> >> >> >
>>>> >> >> >> >> > Let me reiterate that
the size of the profile is not a
>>>> >> >> >> >> > problem
>>>> >> >> >> >> > I
>>>> >> >> >> >> > have
>>>> >> >> >> >> > observed in practice
(nor have I heard of this being a
>>>> >> >> >> >> > problem
>>>> >> >> >> >> > in
>>>> >> >> >> >> > practice
>>>> >> >> >> >> > until this thread).
Therefore I'm skeptical of any changes
>>>> >> >> >> >> > to
>>>> >> >> >> >> > our
>>>> >> >> >> >> > default
>>>> >> >> >> >> > behavior or any new
requirements that are not opt-in.
>>>> >> >> >> >> >
>>>> >> >> >> >> > -- Sean Silva
>>>> >> >> >> >> >
>>>> >> >> >> >> >>
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> thanks,
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> David
>>>> >> >> >> >> >>
>>>> >> >> >> >> >>
>>>> >> >> >> >> >>
>>>> >> >> >> >> >>
>>>> >> >> >> >> >> >which will
make it more difficult for me to do this kind
>>>> >> >> >> >> >> > of analysis
(would require using nm on the original
>>>> >> >> >> >> >> > binary,
>>>> >> >> >> >> >> > hashing
>>>> >> >> >> >> >> > everything,
etc.).
>>>> >> >> >> >> >> >
>>>> >> >> >> >> >> > btw, feel
free to attach the patch even if it in a rough
>>>> >> >> >> >> >> > state.
>>>> >> >> >> >> >> > It
>>>> >> >> >> >> >> > can
>>>> >> >> >> >> >> > still
>>>> >> >> >> >> >> > help to
clarify the proposal and be a good talking point.
>>>> >> >> >> >> >> > Fine-grained
>>>> >> >> >> >> >> > patch
>>>> >> >> >> >> >> > review for
caring about the rough parts will happen on
>>>> >> >> >> >> >> > llvm-commits;
>>>> >> >> >> >> >> > the
>>>> >> >> >> >> >> > rough parts
will not distract the discussion here on
>>>> >> >> >> >> >> > llvm-dev.
>>>> >> >> >> >> >> >
>>>> >> >> >> >> >> > -- Sean Silva
>>>> >> >> >> >> >> >
>>>> >> >> >> >> >> >>
>>>> >> >> >> >> >> >>
>>>> >> >> >> >> >> >> thanks,
>>>> >> >> >> >> >> >>
>>>> >> >> >> >> >> >> David
>>>> >> >> >> >> >> >>
_______________________________________________
>>>> >> >> >> >> >> >> LLVM
Developers mailing list
>>>> >> >> >> >> >> >> llvm-dev
at lists.llvm.org
>>>> >> >> >> >> >> >>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>> >> >> >> >> >> >
>>>> >> >> >> >> >> >
>>>> >> >> >> >> >
>>>> >> >> >> >> >
>>>> >> >> >> >
>>>> >> >> >> >
>>>> >> >> >
>>>> >> >> >
>>>> >> >
>>>> >> >
>>>> >
>>>> >
>>>
>>>