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

[sanjoy]

lgtm, but please also fix the langref as discussed on llvm-dev.

llvm/trunk/lib/Transforms/Scalar/EarlyCSE.cpp

if (MemInst.isValid() && MemInst.isLoad()) {
         LastStore = nullptr;
         ++CurrentGeneration;
       }
 
       // If we have an available version of this load, and if it is the right
       // 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() &&
           // We can't replace an atomic load with one which isn't also atomic.
           InVal.IsAtomic >= MemInst.isAtomic()) {
         Value *Op = getOrCreateResult(InVal.DefInst, Inst->getType());
         if (Op != nullptr) {
           DEBUG(dbgs() << "EarlyCSE CSE LOAD: " << *Inst

llvm/trunk/test/Transforms/EarlyCSE/invariant-loads.ll

; CHECK:   ret void
   %val1 = load i32, i32* %ptr, !invariant.load !{}
   call void @clobber_and_use(i32 %val1)
   %val2 = load i32, i32* %ptr, !invariant.load !{}
   call void @clobber_and_use(i32 %val2)
   ret void
 }
 
 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
 ; CHECK:   call void @clobber_and_use(i32 %val0)
 ; CHECK:   call void @clobber_and_use(i32 %val0)
 
   %val0 = load i32, i32* %ptr, !invariant.load !{}
   call void @clobber_and_use(i32 %val0)
   %val1 = load i32, i32* %ptr
   call void @clobber_and_use(i32 %val1)
   ret void
 }
 
 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)
   %val1 = load i32, i32* %ptr, !invariant.load !{}
   call void @clobber_and_use(i32 %val1)
   ret void
 }