[clang] [llvm] [InstCombine] remove dead loads, such as memcpy from undef (PR #143958)

[Nikita Popov via llvm-commits]

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@@ -3283,10 +3285,15 @@ static bool isAllocSiteRemovable(Instruction *AI,
           case Intrinsic::memcpy:
           case Intrinsic::memset: {
             MemIntrinsic *MI = cast<MemIntrinsic>(II);
-            if (MI->isVolatile() || MI->getRawDest() != PI)
+            if (MI->isVolatile())
               return false;
+            // Note: this could also be ModRef, but we can still interpret that as just Mod in that case.
+            ModRefInfo NewAccess = MI->getRawDest() == PI ? ModRefInfo::Mod : ModRefInfo::Ref;
+            if ((Access & ~NewAccess) != ModRefInfo::NoModRef)
+              return false;
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nikic wrote:

> I originally was going to have it use AA to compute ModRef at the top, but that seemed potentially slower (since we already know PI aliases).

Yeah, we shouldn't use AA here.

> I wasn't sure how else to centralize the test and also preserve exit early unless I just duplicate it at the top and after the loop (`if (Access == ModRefInfo::ModRef) return false`)?

Yeah, that's what I had in mind, but no strong opinion. We should probably at least assert that it's not ModRef after the loop though, to make sure no case was missed.

https://github.com/llvm/llvm-project/pull/143958