1 files changed, 21 insertions, 0 deletions
diff --git a/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
index bdbcc658b88..4eac0316876 100644
--- a/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
+++ b/llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
@@ -1349,6 +1349,8 @@ bool SIFoldOperands::runOnMachineFunction(MachineFunction &MF) {
 
   for (MachineBasicBlock *MBB : depth_first(&MF)) {
     MachineBasicBlock::iterator I, Next;
+
+    MachineOperand *CurrentKnownM0Val = nullptr;
     for (I = MBB->begin(); I != MBB->end(); I = Next) {
       Next = std::next(I);
       MachineInstr &MI = *I;
@@ -1361,6 +1363,25 @@ bool SIFoldOperands::runOnMachineFunction(MachineFunction &MF) {
         if (IsIEEEMode || (!HasNSZ && !MI.getFlag(MachineInstr::FmNsz)) ||
             !tryFoldOMod(MI))
           tryFoldClamp(MI);
+
+        // Saw an unknown clobber of m0, so we no longer know what it is.
+        if (CurrentKnownM0Val && MI.modifiesRegister(AMDGPU::M0, TRI))
+          CurrentKnownM0Val = nullptr;
+        continue;
+      }
+
+      // Specially track simple redefs of m0 to the same value in a block, so we
+      // can erase the later ones.
+      if (MI.getOperand(0).getReg() == AMDGPU::M0) {
+        MachineOperand &NewM0Val = MI.getOperand(1);
+        if (CurrentKnownM0Val && CurrentKnownM0Val->isIdenticalTo(NewM0Val)) {
+          MI.eraseFromParent();
+          continue;
+        }
+
+        // We aren't tracking other physical registers
+        CurrentKnownM0Val = (NewM0Val.isReg() && NewM0Val.getReg().isPhysical()) ?
+          nullptr : &NewM0Val;
         continue;
       }