[X86] Avoid codegen changes when DBG_VALUE appears between lowered selects

X86TargetLowering::EmitLoweredSelect presently detects sequences of CMOV pseudo
instructions without accounting for debug intrinsics. This leads to different
codegen with and without option -g, if a DBG_VALUE instruction lands in the
middle of several lowered selects.

Work around this by skipping over debug instructions when looking for CMOV
sequences, and sinking those debug insts into the EmitLoweredSelect sunk block.
This might slightly shift where variables appear in the instruction sequence,
but won't re-order assignments.

Differential Revision: https://reviews.llvm.org/D58672

llvm-svn: 355307

1 files changed, 15 insertions, 4 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 7e6ec0ea5d0..fa18985e4f8 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -28814,20 +28814,21 @@ X86TargetLowering::EmitLoweredSelect(MachineInstr &MI,
   X86::CondCode CC = X86::CondCode(MI.getOperand(3).getImm());
   X86::CondCode OppCC = X86::GetOppositeBranchCondition(CC);
   MachineInstr *LastCMOV = &MI;
-  MachineBasicBlock::iterator NextMIIt =
-      std::next(MachineBasicBlock::iterator(MI));
+  MachineBasicBlock::iterator NextMIIt = MachineBasicBlock::iterator(MI);
 
   // Check for case 1, where there are multiple CMOVs with the same condition
   // first.  Of the two cases of multiple CMOV lowerings, case 1 reduces the
   // number of jumps the most.
 
   if (isCMOVPseudo(MI)) {
-    // See if we have a string of CMOVS with the same condition.
+    // See if we have a string of CMOVS with the same condition. Skip over
+    // intervening debug insts.
     while (NextMIIt != ThisMBB->end() && isCMOVPseudo(*NextMIIt) &&
            (NextMIIt->getOperand(3).getImm() == CC ||
             NextMIIt->getOperand(3).getImm() == OppCC)) {
       LastCMOV = &*NextMIIt;
       ++NextMIIt;
+      NextMIIt = skipDebugInstructionsForward(NextMIIt, ThisMBB->end());
     }
   }
 
@@ -28859,8 +28860,18 @@ X86TargetLowering::EmitLoweredSelect(MachineInstr &MI,
     SinkMBB->addLiveIn(X86::EFLAGS);
   }
 
+  // Transfer any debug instructions inside the CMOV sequence to the sunk block.
+  auto DbgEnd = MachineBasicBlock::iterator(LastCMOV);
+  auto DbgIt = MachineBasicBlock::iterator(MI);
+  while (DbgIt != DbgEnd) {
+    auto Next = std::next(DbgIt);
+    if (DbgIt->isDebugInstr())
+      SinkMBB->push_back(DbgIt->removeFromParent());
+    DbgIt = Next;
+  }
+
   // Transfer the remainder of ThisMBB and its successor edges to SinkMBB.
-  SinkMBB->splice(SinkMBB->begin(), ThisMBB,
+  SinkMBB->splice(SinkMBB->end(), ThisMBB,
                   std::next(MachineBasicBlock::iterator(LastCMOV)),
                   ThisMBB->end());
   SinkMBB->transferSuccessorsAndUpdatePHIs(ThisMBB);