[llvm] [MemCpyOpt] fix incorrect handling of lifetime markers (PR #143782)

[Jameson Nash via llvm-commits]

================
@@ -1366,56 +1366,68 @@ bool MemCpyOptPass::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
 
 /// Determine whether the pointer V had only undefined content (due to Def) up
 /// to the given Size, either because it was freshly alloca'd or started its
-/// lifetime.
-static bool hasUndefContents(MemorySSA *MSSA, BatchAAResults &AA, Value *V,
-                             MemoryDef *Def, Value *Size) {
-  if (MSSA->isLiveOnEntryDef(Def))
-    return isa<AllocaInst>(getUnderlyingObject(V));
-
-  if (auto *II = dyn_cast_or_null<IntrinsicInst>(Def->getMemoryInst())) {
-    if (II->getIntrinsicID() == Intrinsic::lifetime_start) {
-      auto *LTSize = cast<ConstantInt>(II->getArgOperand(0));
-
-      if (auto *CSize = dyn_cast<ConstantInt>(Size)) {
-        if (AA.isMustAlias(V, II->getArgOperand(1)) &&
-            LTSize->getZExtValue() >= CSize->getZExtValue())
-          return true;
-      }
+/// lifetime by walking the MSSA graph.
+static bool hadUndefContentsBefore(MemorySSA *MSSA, BatchAAResults &BAA,
+                                   Value *V, MemoryAccess *Clobber,
+                                   MemoryLocation Loc, Value *Size) {
+  while (1) {
+    Clobber = MSSA->getWalker()->getClobberingMemoryAccess(Clobber, Loc, BAA);
+    MemoryDef *Def = dyn_cast<MemoryDef>(Clobber);
+    if (!Def)
+      return false;
+
+    if (MSSA->isLiveOnEntryDef(Def))
+      return isa<AllocaInst>(getUnderlyingObject(V));
+
+    if (auto *II = dyn_cast_or_null<IntrinsicInst>(Def->getMemoryInst())) {
+      if (II->getIntrinsicID() == Intrinsic::lifetime_start) {
+        auto *LTSize = cast<ConstantInt>(II->getArgOperand(0));
 
-      // If the lifetime.start covers a whole alloca (as it almost always
-      // does) and we're querying a pointer based on that alloca, then we know
-      // the memory is definitely undef, regardless of how exactly we alias.
-      // The size also doesn't matter, as an out-of-bounds access would be UB.
-      if (auto *Alloca = dyn_cast<AllocaInst>(getUnderlyingObject(V))) {
-        if (getUnderlyingObject(II->getArgOperand(1)) == Alloca) {
-          const DataLayout &DL = Alloca->getDataLayout();
-          if (std::optional<TypeSize> AllocaSize =
-                  Alloca->getAllocationSize(DL))
-            if (*AllocaSize == LTSize->getValue())
+        if (Size)
+          if (auto CSize = dyn_cast<ConstantInt>(Size))
+            if (BAA.isMustAlias(V, II->getArgOperand(1)) &&
+                LTSize->getZExtValue() >= CSize->getZExtValue())
               return true;
+
+        // If the lifetime.start covers a whole alloca (as it almost always
+        // does) and we're querying a pointer based on that alloca, then we know
+        // the memory is definitely undef, regardless of how exactly we alias.
+        // The size also doesn't matter, as an out-of-bounds access would be UB.
+        if (auto *Alloca = dyn_cast<AllocaInst>(getUnderlyingObject(V))) {
+          if (getUnderlyingObject(II->getArgOperand(1)) == Alloca) {
+            const DataLayout &DL = Alloca->getDataLayout();
+            if (std::optional<TypeSize> AllocaSize =
+                    Alloca->getAllocationSize(DL))
+              if (*AllocaSize == LTSize->getValue())
+                return true;
+          }
         }
+        Clobber = Def->getDefiningAccess();
+        continue;
+      } else if (II->getIntrinsicID() == Intrinsic::lifetime_end) {
+        Clobber = Def->getDefiningAccess();
+        continue;
----------------
vtjnash wrote:

I've trimmed this PR down to just the part I needed for https://github.com/vtjnash/llvm-project/commit/645f2472085420b9e9a4e4c1b59c4c1768a31e28, pending separate discussion of whether it is even worthwhile optimizing those tests for which lifetime.start does not cover the whole alloca.

Additionally, I made a separate PR as you suggested (https://github.com/llvm/llvm-project/pull/143958) to handle those simple cases in InstCombine instead, and to see how that affects the benchmarks.

https://github.com/llvm/llvm-project/pull/143782