Merging r323759:

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r323759 | spatel | 2018-01-30 14:53:59 +0100 (Tue, 30 Jan 2018) | 10 lines

[DSE] make sure memory is not modified before partial store merging (PR36129)

We missed a critical check in D30703. We must make sure that no intermediate 
store is sitting between the stores that we want to merge.

This should fix:
https://bugs.llvm.org/show_bug.cgi?id=36129

Differential Revision: https://reviews.llvm.org/D42663

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llvm-svn: 324086

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -1176,7 +1176,8 @@ static bool eliminateDeadStores(BasicBlock &BB, AliasAnalysis *AA,
           auto *Earlier = dyn_cast<StoreInst>(DepWrite);
           auto *Later = dyn_cast<StoreInst>(Inst);
           if (Earlier && isa<ConstantInt>(Earlier->getValueOperand()) &&
-              Later && isa<ConstantInt>(Later->getValueOperand())) {
+              Later && isa<ConstantInt>(Later->getValueOperand()) &&
+              memoryIsNotModifiedBetween(Earlier, Later, AA)) {
             // If the store we find is:
             //   a) partially overwritten by the store to 'Loc'
             //   b) the later store is fully contained in the earlier one and

llvm/test/Transforms/DeadStoreElimination/merge-stores.ll

@@ -186,6 +186,23 @@ define void @PR34074(i32* %x, i64* %y) {
   ret void
 }
 
+; We can't eliminate the last store because P and Q may alias.
+
+define void @PR36129(i32* %P, i32* %Q) {
+; CHECK-LABEL: @PR36129(
+; CHECK-NEXT:    store i32 1, i32* [[P:%.*]]
+; CHECK-NEXT:    [[P2:%.*]] = bitcast i32* [[P]] to i8*
+; CHECK-NEXT:    store i32 2, i32* [[Q:%.*]]
+; CHECK-NEXT:    store i8 3, i8* [[P2]]
+; CHECK-NEXT:    ret void
+;
+  store i32 1, i32* %P
+  %P2 = bitcast i32* %P to i8*
+  store i32 2, i32* %Q
+  store i8 3, i8* %P2
+  ret void
+}
+
 !0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 5.0.0 (trunk 306512)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
 !1 = !DIFile(filename: "me.cpp", directory: "/compiler-explorer")
 !2 = !{}