Allow unordered loads to be considered invariant in CodeGen

The actual code change is fairly straight forward, but exercising it isn't. First, it turned out we weren't adding the appropriate flags in SelectionDAG. Second, it turned out that we've got some optimization gaps, so obvious test cases don't work.

My first attempt (in atomic-unordered.ll) points out a deficiency in our peephole-opt folding logic which I plan to fix separately. Instead, I'm exercising this through MachineLICM.

Differential Revision: https://reviews.llvm.org/D59375

llvm-svn: 356494

1 files changed, 5 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/MachineInstr.cpp b/llvm/lib/CodeGen/MachineInstr.cpp
index 17bd0f38964..26d58340b61 100644
--- a/llvm/lib/CodeGen/MachineInstr.cpp
+++ b/llvm/lib/CodeGen/MachineInstr.cpp
@@ -1312,9 +1312,11 @@ bool MachineInstr::isDereferenceableInvariantLoad(AliasAnalysis *AA) const {
   const MachineFrameInfo &MFI = getParent()->getParent()->getFrameInfo();
 
   for (MachineMemOperand *MMO : memoperands()) {
-    if (MMO->isVolatile()) return false;
-    // TODO: Figure out whether isAtomic is really necessary (see D57601).
-    if (MMO->isAtomic()) return false;
+    if (!MMO->isUnordered())
+      // If the memory operand has ordering side effects, we can't move the
+      // instruction.  Such an instruction is technically an invariant load,
+      // but the caller code would need updated to expect that.
+      return false;
     if (MMO->isStore()) return false;
     if (MMO->isInvariant() && MMO->isDereferenceable())
       continue;