[DSE] Use alias analysis to identify no-op stores

Author: AlexMaclean

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -2116,7 +2116,9 @@ struct DSEState {
       return true;
 
     if (auto *LoadI = dyn_cast<LoadInst>(Store->getOperand(0))) {
-      if (LoadI->getPointerOperand() == Store->getOperand(1)) {
+      if (LoadI->getPointerOperand() == Store->getOperand(1) ||
+          AA.isMustAlias(MemoryLocation::get(LoadI),
+                         MemoryLocation::get(Store))) {
         // Get the defining access for the load.
         auto *LoadAccess = MSSA.getMemoryAccess(LoadI)->getDefiningAccess();
         // Fast path: the defining accesses are the same.

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

@@ -1152,3 +1152,14 @@ if.else:
 end:
   ret void
 }
+
+define i32 @test_use_alias_analysis(ptr %Q) {
+; CHECK-LABEL: @test_use_alias_analysis(
+; CHECK-NEXT:    [[A:%.*]] = load i32, ptr [[Q:%.*]], align 4
+; CHECK-NEXT:    ret i32 [[A]]
+;
+  %a = load i32, ptr %Q
+  %Q2 = addrspacecast ptr %Q to ptr addrspace(5)
+  store i32 %a, ptr addrspace(5) %Q2
+  ret i32 %a
+}