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

[Lang Hames]

Hi Everyone,

For your consideration: A proposal to improve regression test support for
RuntimeDyld.

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.

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. ;)
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