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

[Jameson Nash via llvm-commits]

================
@@ -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;
----------------
vtjnash 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). 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`)?

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