[LLVMdev] Upstreaming PNaCl's IR simplification passes

[Mark Seaborn]

On 4 March 2014 17:15, Chris Lattner <clattner at apple.com> wrote:

> On Mar 4, 2014, at 3:17 PM, Chandler Carruth <chandlerc at google.com> wrote:
>
> On Tue, Mar 4, 2014 at 1:04 PM, Mark Seaborn <mseaborn at chromium.org>wrote:
>
>> The PNaCl project has implemented various IR simplification passes that
>> simplify LLVM IR by lowering complex features to simpler features.  We'd
>> like to upstream some of these IR passes to LLVM.  We'd like to explore if
>> this acceptable, and if so, how we should go about doing this.
>>
>
> My question is somewhat different. I'm not questioning whether these are
> acceptable, I'm questioning why these are interesting and important for the
> LLVM project.
>
>
> I share Chandler's concern.  If these aren't actively used by something in
> tree, they will bit rot.  The way to counter the bit rot would be to add
> extensive testcases... but that would just add an even larger burden on
> core LLVM developers to keep them up to date.
>

Here's a data point on the size of the maintenance burden of PNaCl's IR
simplification passes:

I recently updated PNaCl's branch of LLVM from being based on LLVM 3.3 to
3.4.  I made 8 changes to the passes and their tests in order to keep the
tests passing.  They were:

 1) Compile fix:  Missing #includes of "llvm/IR/Constants.h" (
https://chromium.googlesource.com/native_client/pnacl-llvm/+/72676e03d8956af100029b42355d25b8ba102785
)
 2) Compile fix:  Removed code for handling case ranges in the input, since
case ranges were removed in 3.4 (
https://chromium.googlesource.com/native_client/pnacl-llvm/+/be43d266f0919675062d5b875efb370e264a07da
)
 3) Compile fix:  Changed a use of PassManager to PassManagerBase to fix an
ambiguity (
https://chromium.googlesource.com/native_client/pnacl-llvm/+/6a38d188d29270fc40fa6af6c6c7d36ba4a34637
)
 4) Test fix:  Fix test expectations after "readonly" attr was added to
memcpy intrinsic (
https://chromium.googlesource.com/native_client/pnacl-llvm/+/7f634ce6f622188cd551dbf283131b03d019583d
)

Some of the changes happened as a result of 'lit' becoming stricter in 3.4:
 5) Fixed a mistake that led to a CHECK being ignored (
https://chromium.googlesource.com/native_client/pnacl-llvm/+/051a09c5e14e6dd00352483127db0d13f38d2359
)
 6) Added "not" to a test where an error is expected (
https://chromium.googlesource.com/native_client/pnacl-llvm/+/803421ad03f4ac4db30581a8cff10b3d52ac4362
)
 7) Fixed a bug in stripping llvm.invariant.end (
https://chromium.googlesource.com/native_client/pnacl-llvm/+/fc31cc7b91935b59b2011b06c5ed49e82e49bf9b
)

The last change was made based on a test failure in end-to-end compiler
tests rather than a test failure in "make test":
 8) Changed StripAttributes to strip unrecognised attrs by default
https://chromium.googlesource.com/native_client/pnacl-llvm/+/d970bf995e01e96c3c9597f333d7d1f251eee71a

This is based on looking through the changes listed by "gitk
lib/Transforms/NaCl/ test/Transforms/NaCl/" in the PNaCl branch.

So if PNaCl's simplification passes had been upstream in LLVM 3.3,
developers committing changes to trunk would have had to make 7 additional
changes in the six months between branching 3.3 and branching 3.4 (about
1.2 changes per month, on average).  The changes are fairly minor.

Cheers,
Mark
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