AMDGPU: Don't delete instructions if S_ENDPGM has implicit uses

[arsenm]

Summary

This can leave behind the uses with the defs removed.
Since this should only really happen in tests, it's not worth the
effort of trying to handle this.

[rampitec]

LGTM

[arsenm]

r340866

lib/Target/AMDGPU/SIOptimizeExecMaskingPreRA.cpp

bool SIOptimizeExecMaskingPreRA::runOnMachineFunction(MachineFunction &MF) {
   LiveIntervals *LIS = &getAnalysis<LiveIntervals>();
   DenseSet<unsigned> RecalcRegs({AMDGPU::EXEC_LO, AMDGPU::EXEC_HI});
   bool Changed = false;
 
   for (MachineBasicBlock &MBB : MF) {
 
     // Try to remove unneeded instructions before s_endpgm.
     if (MBB.succ_empty()) {
-      if (MBB.empty() || MBB.back().getOpcode() != AMDGPU::S_ENDPGM)
+      if (MBB.empty())
+        continue;
+
+      // Skip this if the endpgm has any implicit uses, otherwise we would need
+      // to be careful to update / remove them.
+      MachineInstr &Term = MBB.back();
+      if (Term.getOpcode() != AMDGPU::S_ENDPGM ||
+          Term.getNumOperands() != 0)
         continue;
 
       SmallVector<MachineBasicBlock*, 4> Blocks({&MBB});
 
       while (!Blocks.empty()) {
         auto CurBB = Blocks.pop_back_val();
         auto I = CurBB->rbegin(), E = CurBB->rend();
         if (I != E) {

test/CodeGen/AMDGPU/endpgm-dce.mir

body:             |
   bb.1:
     $vcc = IMPLICIT_DEF
     $sgpr0_sgpr1 = S_AND_B64 $exec, killed $vcc, implicit-def $scc
     S_BRANCH %bb.2
 
   bb.2:
     S_ENDPGM
 ...
+
+# GCN-LABEL: name: implicit_use_on_s_endpgm
+# GCN: V_ADD_I32
+# GCN: COPY
+# GCN: V_ADDC_U32
+# GCN: S_ENDPGM implicit %3
+name: implicit_use_on_s_endpgm
+tracksRegLiveness: true
+
+body:             |
+  bb.0:
+    dead %0:vgpr_32 = V_ADD_I32_e32 12345, undef %1:vgpr_32, implicit-def $vcc, implicit $exec
+    %2:sreg_64_xexec = COPY $vcc
+    %3:vgpr_32, dead %4:sreg_64_xexec = V_ADDC_U32_e64 undef %5:vgpr_32, undef %6:vgpr_32, %2, implicit $exec
+    S_ENDPGM implicit %3
+
+...