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

[Nikita Popov via llvm-commits]

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@@ -3339,16 +3340,23 @@ static bool isAllocSiteRemovable(Instruction *AI,
         if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
           switch (II->getIntrinsicID()) {
           default:
-            return false;
+            return std::nullopt;
 
           case Intrinsic::memmove:
           case Intrinsic::memcpy:
           case Intrinsic::memset: {
             MemIntrinsic *MI = cast<MemIntrinsic>(II);
-            if (MI->isVolatile() || MI->getRawDest() != PI)
-              return false;
-            [[fallthrough]];
+            if (MI->isVolatile())
+              return std::nullopt;
+            // 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 std::nullopt;
+            Access |= NewAccess;
           }
+            [[fallthrough]];
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nikic wrote:

Weird formatting. Can keep the fallthrough in the braces?

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