[AMDGPU] Iterate LoweredEndCf in the reverse order

The function that optimally inserts the exec mask
restore operations by combining the blocks currently
visits the lowered END_CF pseudos in the forward
direction as it iterates the setvector in the order
the entries are inserted in it.

Due to the absence of BranchFolding at -O0, the
irregularly placed BBs cause the forward traversal
to incorrectly place two unconditional branches in
certain BBs while combining them, especially when
an intervening block later gets optimized away in
subsequent iterations.

It is avoided by reverse iterating the setvector.
The blocks at the bottom of a function will get
optimized first before processing those at the top.

Fixes: SWDEV-315215

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D116273

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp b/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp
index 3168bcd..6ec37b3 100644
--- a/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp
+++ b/llvm/lib/Target/AMDGPU/SILowerControlFlow.cpp
@@ -582,7 +582,7 @@ void SILowerControlFlow::optimizeEndCf() {
   if (!RemoveRedundantEndcf)
     return;
 
-  for (MachineInstr *MI : LoweredEndCf) {
+  for (MachineInstr *MI : reverse(LoweredEndCf)) {
     MachineBasicBlock &MBB = *MI->getParent();
     auto Next =
       skipIgnoreExecInstsTrivialSucc(MBB, std::next(MI->getIterator()));