[LLVMdev] Proposal: Improved regression test support for RuntimeDyld/MCJIT.

Lang Hames
2014-06-24T17:57:16Z

Hi All,

Dropping in a relevant discussion that Dave and I had off list:

>From Dave: "In my mind it's mostly that we have a fairly general
purpose
tool for checking things and we do that by separating output from
verification that has proved pretty valuable in other areas (by having
general purpose output we can use it for better debugging/investigation,
rather than just for writing test cases, for example - I can run
llvm-rtdyld with the disassembly mode to see what the state in-memory looks
like, with a -verify mode I can't do that)"

My argument against testing RuntimeDyld via examining textual output is
that sensibly rendering RuntimeDyld's memory would require considerable
work (see discussion below), and I don't think anybody actually wants to
look at it. People have requested the ability to examine the output of the
JIT, and I'm happy to add that feature if it hasn't been added already
(I'll have check), but the appropriate way to do that is to dump the object
files produced by the JIT before they're passed to RuntimeDyld, not to try
to render RuntimeDyld's memory.

As for why dumping RuntimeDyld's memory is a lot of work: RuntimeDyld
really only knows about blobs of bytes (one blob per section in the object
file) and addresses of symbols. Consider, for example, how the following C
struct looks in assembly and to RuntimeDyld:

In C:

struct {
  unsigned a;
  void* b;
  unsigned c;
} x = {1, &foo, 2};

In pseudo-asm:

.section __data:
x:
  .long 1
  .quad foo
  .long 2

To RuntimeDyld:

x: 0x01 0x00 0x00 0x00 0xEF 0xBE 0xAD 0xDE 0xCE 0xFA 0xED 0xFE 0x02 0x00
0x00 0x00


RuntimeDyld doesn't know where the fields of x start or end, or indeed
where x itself ends. We could add extra labels to the assembly code
to delineate the fields and teach RuntimeDyld to render the bytes between
each symbol, and how to recognize common integer sizes and render them
appropriately (so that you don't have to write expressions to reassemble
ints from the individual bytes). But all of that complicates both the test
cases themselves  and RuntimeDyld. None of those features would be
available to a client who just wants to inspect the JIT's memory, since the
code-generator doesn't (and IMHO shouldn't) pepper its output with
extraneous labels on the off-chance that someone wants to pretty-print
RuntimeDyld's memory.

>From Dave: "in many other cases we've invented canonical textual
representations to check against (objdump, dwarfdump, readelf, etc)."

In each of those cases there was a desire for people to be able to visually
inspect the information for understanding/correctness. Once you have that
functionality, inspecting dumps with FileCheck makes much more sense. In
RuntimeDyld's case all the interesting information is contained in the
object file, which can already be rendered with existing tools. The only
thing RuntimeDyld changes is some byte values in memory. Being able to
inspect them when something goes wrong tells you nothing interesting about
why they were wrong, so textually rendering isn't helpful.

I think directly checking the invariants makes more sense here, rather than
going via text.

Cheers,
Lang.




On Mon, Jun 23, 2014 at 8:54 PM, Lang Hames <lhames at gmail.com> wrote:

> Hi Dave,
>
> Jim Grosbach asked the same question, so you're in good company. With
> hindsight I think it was a mistake to say "FileCheck workflow".
What I
> really meant was that this system plays well with lit. Not that your
> question about using FileCheck would have been any less valid.
>
> I did consider using FileCheck for this, but decided it was the wrong
> approach. The fundamental reason is that there's no demand for
textually
> rendering RuntimeDyld's memory, and developing a textual renderer so
that
> we could output text just to pattern match and re-assemble ints in
> FileCheck would be a lot of pain for (as far as I can see) no gain.
>
> If there's a desire for FileCheck to support expression evaluation we
> could flesh out this evaluator and make it available as a support library
> that both FileCheck and RuntimeDyld could use.
>
> You (and independently Nick Kledzik) do raise the really useful idea of
> leveraging the disassembler though. I like the idea of adding some special
> syntax to disassemble an instruction at a label and use one of its
> immediates. That would eliminate a lot of the bit bashing that would have
> been required on instruction sets with tricky immediate encodings (E.g.
> ARM). Something like:
>
> # rtdyld-check: @test_inst[0] = foo - (test_inst + 5)
> test_inst:
>   callq foo
>
> Cheers,
> Lang.
>
> > On Jun 23, 2014, at 5:01 PM, David Blaikie <dblaikie at
gmail.com> wrote:
> >
> >> On Mon, Jun 23, 2014 at 3:01 PM, Lang Hames <lhames at
gmail.com> wrote:
> >> Hi Everyone,
> >>
> >> For your consideration: A proposal to improve regression test
support
> for
> >> RuntimeDyld.
> >
> > Thanks for working on this, Lang. It's great to see.
> >
> >> Short version: We can make RuntimeDyld far more testable by adding
a
> trivial
> >> pointer-expression language that allows us to describe how memory
should
> >> look post-relocation. Jump down to "The Proposal" for
details.
> >
> > I've been trying to puzzle over what this would look like with a
> > possibly more general feature*.
> >
> > What would testing look like if we had a rtdyld dumping mode that
> > printed the disassembly of the relocated machine code, and a symbol
> > table (or just inserted the labels for the symbols into the
> > disassembly?).
> >
> > I understand we'd need to beef up FileCheck with slightly more
> > arithmetic operations - but is it really so much (& would they be
so
> > useless for other tests) that it's not worth putting it there?
> >
> > To take your example, here's my vague idea of what it might look
like
> > to use a dump+FileCheck. The dump would look something like:
> >
> > (obviously I don't know, nor for this purpose care, how big the
> > instructions are, just that they have distinct addresses, etc)
> >
> >  0x42: bar:
> >  0x42:   retq
> >  0x43: foo:
> >  0x43:   callq 0x42
> >  0x44: inst1:
> >  0x44:   retq
> >
> > And the FileCheck equivalent of
> >
> >  # rtdyld-check: *{4}(inst1 - 4) = (bar - inst1) & 0xffffffff
> >
> > would be something like:
> >
> >  CHECK: [[CALL_ADDR:.*]]: bar:
> >  CHECK: callq [[CALL_ADDR]]
> >
> > Which, I suppose, depends on disassembler working correctly, not sure
> > if that's high risk/complicated.
> >
> > Alternatively - could llvm-rtdyld just print a simple description of
> > relocations its applied and the location of symbols? (similar to a
> > static display of relocations like llvm-objdump -r) then FileCheck
> > that.
> >
> > * all that said, a feature like you've proposed/implemented
isn't
> > without precedent - clang's -verify is very similar to what
you've got
> > here
> >
> >
> >> Long version:
> >>
> >> Background:
> >>
> >> For those unfamiliar with it, RuntimeDyld a component of MCJIT,
LLVM's
> JIT
> >> compiler infrastructure. MCJIT produces an object file in memory
for
> each
> >> module that is JIT'd. RuntimeDyld's job is to apply all
the relocations
> >> necessary to make the code in the object file runnable. In other
words,
> >> RuntimeDyld is acting as both the static and dynamic linker for
the JIT.
> >>
> >> The Problem:
> >>
> >> We can't directly test RuntimeDyld at the moment. We currently
infer the
> >> correctness of RuntimeDyld indirectly from the success of the
MCJIT
> >> regression tests - if they pass, we assume RuntimeDyld must have
done
> its
> >> job right. That's far from an ideal. The biggest issues with
it are:
> >>
> >> (1) Each platform is testing only its own relocations and no
others.
> I.e.
> >> X86 testers are testing X86 relocations only. ARM testers are
testing
> ARM
> >> relocations only. If someone running on X86 breaks a relocation
for ARM
> they
> >> won't see the error in their regression test run - they'll
have to wait
> >> until an ARM buildbot breaks before they realize anything is
wrong.
> Fixes
> >> for platforms that you don't have access to are difficult to
test - all
> you
> >> can do is eyeball disassembled memory and see if everything looks
sane.
> This
> >> is not much fun.
> >>
> >> (2) Relocations are produced by CodeGen from IR, rather than
described
> >> directly. That's a lot of machinery to have between the
test-case and
> the
> >> final result. It is difficult to know what relocations each IR
> regression
> >> test is testing (and they're often incidental - we don't
have a
> dedicated
> >> relocation test set). This also means that if/when the code
generator
> >> produces different relocation types the existing tests will keep
on
> passing
> >> but will silently stop testing the thing they used to test.
> >>
> >> The Proposal:
> >>
> >> (1) We provide a mechanism for describing how pieces of relocated
memory
> >> should look immediately prior to execution, and then inspect the
memory
> >> rather than executing it. This addresses point (1) above: Tests
for any
> >> platform can be loaded, linked and verified on any platform. If
you're
> >> coding on X86 and you break an ARM relocation you'll know
about it
> >> immediately.
> >>
> >> (2) RuntimeDyld test cases should be written in assembly, rather
than
> IR.
> >> This addresses point (2) above - we can cut the code generators
out and
> >> guarantee that we're testing what we're interested in.
> >>
> >> The way to do this is to introduce a simple pointer expression
language.
> >> This should be able to express things like: "The immediate
for this call
> >> points at symbol foo".
> >>
> >> Symbolically, what I have in mind would look something like:
> >>
> >>        // some asm ...
> >> # assert *(inst1 + 1) = foo
> >> inst1:
> >>        callq   foo
> >>        // some asm...
> >>
> >> Here we add the "inst1" label to give us a address from
which we can
> get at
> >> the immediate for the call. The " + 1" expression skips
the call opcode
> (we
> >> know the size of the opcode ahead of time, since this is assembly
and so
> >> target-specific).
> >>
> >> To verify that constraints expressed in this language hold, we can
add
> an
> >> expression evaluator to the llvm-rtdyld utility, which is a
command-line
> >> interface to RuntimeDyld.
> >>
> >> I find these things are easier to discuss in the concrete, so
I've
> attached
> >> a basic implementation of this idea. The following discussion is
in
> terms of
> >> my patch, but I'm very open to tweaking all this.
> >>
> >> The language I've implemented is:
> >>
> >> test = expr '=' expr
> >>
> >> expr = '*{' number '}' load_addr_expr
> >>     | binary_expr
> >>     | '(' expr ')'
> >>     | symbol
> >>     | number
> >>
> >> load_addr_expr = symbol
> >>               | '(' symbol '+' number ')'
> >>               | '(' symbol '-' number ')'
> >>
> >> binary_expr = expr '+' expr
> >>            | expr '-' expr
> >>            | expr '&' expr
> >>            | expr '|' expr
> >>            | expr '<<' expr
> >>            | expr '>>' expr
> >>
> >> This expression language supports simple pointer arithmetic,
shifting,
> >> masking and loading. All values are internally held as 64-bit
unsigneds,
> >> since RuntimeDlyd is designed to support cross-platform linking,
> including
> >> linking for 64-bit targets from a 32-bit host. I think the only
> stand-out
> >> wart is the *{#size}<addr> syntax for loads. This comes from
the fact
> that
> >> immediates aren't always 64-bits, so it's not safe to do a
64-bit load:
> you
> >> could read past the end of allocated memory. The #size field
indicates
> how
> >> many bytes to read.
> >>
> >> This patch adds a "-verify" option to llvm-rtdyld to
attach the
> expression
> >> evaluator to a RuntimeDyld instance after linking. When -verify is
> passed,
> >> llvm-rtdyld does not execute any code. Files containing rules are
> passed via
> >> "-check=<filename>" arguments, and rules are read
from any line prefixed
> >> with the string "# rtdyld-check: ". The intended
workflow is modeled on
> the
> >> FileCheck regression tests.
> >>
> >> Here's an example of what a test case for a test for an x86-64
> PC-relative
> >> MACHO_VANILLA relocation would look like:
> >>
> >> ; RUN: clang -triple x86_64-apple-macosx10.9.0 -c -o foo.o %s
> >> ; RUN: llvm-rtdyld -verify -check=foo.s foo.o
> >> ; RUN: rm foo.o
> >> ;
> >> ; Test an x86-64 PC-relative MACHO_VANILLA relocation.
> >>
> >>        .text
> >>        .globl  bar
> >>        .align  16, 0x90
> >> bar:
> >>        retq
> >>
> >>        .globl  foo
> >>        .align  16, 0x90
> >> foo:
> >> # rtdyld-check: *{4}(inst1 - 4) = (bar - inst1) & 0xffffffff
> >>        callq   bar
> >> inst1:
> >>        retq
> >>
> >>
> >> With this system, we could write targeted regression tests for
every
> >> relocation type on every platform, and test them on any system.
Failures
> >> would immediately identify which target and relocation type broke.
> >>
> >> I think this system would massively improve the testability of the
> >> RuntimeDyld layer, which is good news in light of the increased
usage
> MCJIT
> >> is getting these days.
> >>
> >> Please let me know what you think. Comments and critiques are very
> welcome,
> >> both of the language and the proposed workflow.
> >>
> >> Cheers,
> >> Lang.
> >>
> >> TL;DR: lhames responds to dblaikie's incessant demand for test
cases. ;)
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >>
>
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