AMDGPU: Don't assume there is only one terminator copy

[arsenm]

Summary

This would stop on the first in reverse order, failing the verifier if
there were more earlier in the block.

[nhaehnle]

Update the comment above removeTerminatorBit. LGTM otherwise.

[arsenm]

Fix comment and matching in one mir test

[arsenm]

b3e63aa8a47078e8ac24ceffa7160cc1af657dcb

llvm/lib/Target/AMDGPU/SIOptimizeExecMasking.cpp

default:
     return false;
   }
 }
 
 static MachineBasicBlock::reverse_iterator fixTerminators(
   const SIInstrInfo &TII,
   MachineBasicBlock &MBB) {
   MachineBasicBlock::reverse_iterator I = MBB.rbegin(), E = MBB.rend();
+
+  bool Seen = false;
+  MachineBasicBlock::reverse_iterator FirstNonTerm = I;
   for (; I != E; ++I) {
     if (!I->isTerminator())
-      return I;
+      return FirstNonTerm;
 
-    if (removeTerminatorBit(TII, *I))
-      return I;
+    if (removeTerminatorBit(TII, *I)) {
+      if (!Seen) {
+        FirstNonTerm = I;
+        Seen = true;
+      }
+    }
   }
 
-  return E;
+  return FirstNonTerm;
 }
 
 static MachineBasicBlock::reverse_iterator findExecCopy(
   const SIInstrInfo &TII,
   const GCNSubtarget &ST,
   MachineBasicBlock &MBB,
   MachineBasicBlock::reverse_iterator I,
   unsigned CopyToExec) {
bool SIOptimizeExecMasking::runOnMachineFunction(MachineFunction &MF) {
   //
 
   for (MachineBasicBlock &MBB : MF) {
     MachineBasicBlock::reverse_iterator I = fixTerminators(*TII, MBB);
     MachineBasicBlock::reverse_iterator E = MBB.rend();
     if (I == E)
       continue;
 
+    // TODO: It's possible to see other terminator copies after the exec copy.
     Register CopyToExec = isCopyToExec(*I, ST);
     if (!CopyToExec.isValid())
       continue;
 
     // Scan backwards to find the def.
     auto CopyToExecInst = &*I;
     auto CopyFromExecInst = findExecCopy(*TII, ST, MBB, I, CopyToExec);
     if (CopyFromExecInst == E) {

llvm/test/CodeGen/AMDGPU/optimize-exec-masking-strip-terminator-bits.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -march=amdgcn -mcpu=fiji -verify-machineinstrs -run-pass=si-optimize-exec-masking -o - %s | FileCheck %s
+
+# There are multiple _term pseudos here. Starting from the end of the
+# block, they all need to be converted to regular copies removed in
+# order to avoid making the verifier unhappy.
+
+---
+name:  multi_term_pseudos
+tracksRegLiveness: true
+body:             |
+  ; CHECK-LABEL: name: multi_term_pseudos
+  ; CHECK: bb.0:
+  ; CHECK:   successors: %bb.2(0x40000000), %bb.1(0x40000000)
+  ; CHECK:   liveins: $sgpr4_sgpr5, $sgpr6_sgpr7, $sgpr8_sgpr9
+  ; CHECK:   $exec = COPY killed renamable $sgpr4_sgpr5
+  ; CHECK:   renamable $sgpr10_sgpr11 = COPY killed renamable $sgpr6_sgpr7, implicit $exec
+  ; CHECK:   renamable $sgpr12_sgpr13 = COPY killed renamable $sgpr8_sgpr9, implicit $exec
+  ; CHECK:   S_CBRANCH_EXECZ %bb.1, implicit $exec
+  ; CHECK:   S_BRANCH %bb.2
+  ; CHECK: bb.1:
+  ; CHECK:   liveins: $sgpr12_sgpr13
+  ; CHECK:   S_ENDPGM 0, implicit $sgpr12_sgpr13
+  ; CHECK: bb.2:
+  ; CHECK:   liveins: $sgpr12_sgpr13
+  ; CHECK:   S_ENDPGM 0, implicit $sgpr12_sgpr13
+  bb.0:
+    successors: %bb.2(0x40000000), %bb.1(0x40000000)
+    liveins: $sgpr4_sgpr5, $sgpr6_sgpr7, $sgpr8_sgpr9
+    $exec = S_MOV_B64_term killed renamable $sgpr4_sgpr5
+    renamable $sgpr10_sgpr11 = S_MOV_B64_term killed renamable $sgpr6_sgpr7, implicit $exec
+    renamable $sgpr12_sgpr13 = S_MOV_B64_term killed renamable $sgpr8_sgpr9, implicit $exec
+    S_CBRANCH_EXECZ %bb.1, implicit $exec
+    S_BRANCH %bb.2
+
+  bb.1:
+    liveins: $sgpr12_sgpr13
+    S_ENDPGM 0, implicit $sgpr12_sgpr13
+
+  bb.2:
+    liveins: $sgpr12_sgpr13
+    S_ENDPGM 0, implicit $sgpr12_sgpr13
+
+...