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

[Dávid Bolvanský via Phabricator via llvm-commits]

xbolva00 added inline comments.


================
Comment at: llvm/lib/Analysis/AliasAnalysis.cpp:241
   if (onlyAccessesInaccessibleMem(MRB))
-    return ModRefInfo::NoModRef;
+    return isAllocLikeFn(Call, &TLI) ? ModRefInfo::Mod : ModRefInfo::NoModRef;
 
----------------
xbolva00 wrote:
> nikic wrote:
> > 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.
> Yeah, I will just drop it from calloc.
>> I think inaccessiblememonly is fine for modeling malloc behavior (or do you see any issues there as well?)

Just found one I think.

Imagine you want to change malloc+memset combo to calloc 

         auto *UnderlyingDef =
              cast<MemoryDef>(MSSA.getMemoryAccess(MallocInst));
          // If UnderlyingDef is the clobbering access of Def, no instructions
          // between them can modify the memory location.
          auto *ClobberDef =
              MSSA.getSkipSelfWalker()->getClobberingMemoryAccess(MemsetDef);
          return UnderlyingDef == ClobberDef;
        
This will fail.


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https://reviews.llvm.org/D101440