[EarlyCSE] Allow forwarding a non-invariant load into an invariant load.

Reviewers: sanjoy

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D23935

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280265 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/EarlyCSE.cpp

@@ -644,13 +644,13 @@ bool EarlyCSE::processNode(DomTreeNode *Node) {
       // generation or the load is known to be from an invariant location,
       // replace this instruction.
       //
-      // A dominating invariant load implies that the location loaded from is
-      // unchanging beginning at the point of the invariant load, so the load
-      // we're CSE'ing _away_ does not need to be invariant, only the available
-      // load we're CSE'ing _to_ does.
+      // If either the dominating load or the current load are invariant, then
+      // we can assume the current load loads the same value as the dominating
+      // load.
       LoadValue InVal = AvailableLoads.lookup(MemInst.getPointerOperand());
       if (InVal.DefInst != nullptr &&
-          (InVal.Generation == CurrentGeneration || InVal.IsInvariant) &&
+          (InVal.Generation == CurrentGeneration ||
+           InVal.IsInvariant || MemInst.isInvariantLoad()) &&
           InVal.MatchingId == MemInst.getMatchingId() &&
           // We don't yet handle removing loads with ordering of any kind.
           !MemInst.isVolatile() && MemInst.isUnordered() &&

test/Transforms/EarlyCSE/invariant-loads.ll

@@ -20,9 +20,7 @@ define void @f_0(i32* %ptr) {
 }
 
 define void @f_1(i32* %ptr) {
-; We can forward invariant loads to non-invariant loads, since once an
-; invariant load has executed, the location loaded from is known to be
-; unchanging.
+; We can forward invariant loads to non-invariant loads.
 
 ; CHECK-LABEL: @f_1(
 ; CHECK:   %val0 = load i32, i32* %ptr, !invariant.load !0
@@ -37,14 +35,12 @@ define void @f_1(i32* %ptr) {
 }
 
 define void @f_2(i32* %ptr) {
-; Negative test -- we can't forward a non-invariant load into an
-; invariant load.
+; We can forward a non-invariant load into an invariant load.
 
 ; CHECK-LABEL: @f_2(
 ; CHECK:   %val0 = load i32, i32* %ptr
 ; CHECK:   call void @clobber_and_use(i32 %val0)
-; CHECK:   %val1 = load i32, i32* %ptr, !invariant.load !0
-; CHECK:   call void @clobber_and_use(i32 %val1)
+; CHECK:   call void @clobber_and_use(i32 %val0)
 
   %val0 = load i32, i32* %ptr
   call void @clobber_and_use(i32 %val0)