[PATCH] D101440: [DSE] Eliminate store after calloc (PR50143)

[Nikita Popov via Phabricator via llvm-commits]

nikic requested changes to this revision.
nikic added inline comments.
This revision now requires changes to proceed.


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Comment at: llvm/lib/Analysis/AliasAnalysis.cpp:241
   if (onlyAccessesInaccessibleMem(MRB))
-    return ModRefInfo::NoModRef;
+    return isAllocLikeFn(Call, &TLI) ? ModRefInfo::Mod : ModRefInfo::NoModRef;
 
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xbolva00 wrote:
> fhahn wrote:
> > From this workaround, it seems like how `inaccessiblememonly` is used for `calloc` does not completely match AA's interpretation. Are there other cases in the code base? I'm not sure if special casing `calloc` here is ideal. 
> > 
> > I think we should probably treat the `AA` change as a separate patch, with an AA only test as well, perhaps `llvm/test/Analysis/BasicAA/libfuncs.ll`?
> Not only calloc, all alloc (calloc, malloc, aligned_alloc) fns have inaccessiblememonly. inaccessiblememonly was introduced long time ago but this attribute was basically dead, no libfunc used it until recently.
> 
> 
> Sadly, inaccessiblememonly is somehow broken nowdays as the code assumes that such call does not write or read memory, but of course it returns memory.
> 
> This AA change can be separated patch (but this DSE needs it as a prerequisite)
> 
> 
> Another solution is removal of inaccessiblememonly from alloc fns, as it caused more harm then improvements.
Your new implementation claims that inaccessiblememonly alloc-like fns mod all locations -- if that's what you want, just drop the attribute from the functions entirely. That will have about the same effect.

I think inaccessiblememonly is fine for modeling malloc behavior (or do you see any issues there as well?), the problem is with calloc which combines malloc (inaccessiblememonly) with a write to the location (accessible). As @jdoerfert already pointed out on the PR, properly modelling this would require "inaccessible_or_returned_memonly".

While I'm somewhat open to DSE hacks, I don't think making this change on the AA layer is acceptable. I think dropping inaccessiblememonly inference for calloc() specifically would be the right thing to do in the meantime.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D101440/new/

https://reviews.llvm.org/D101440