ScheduleDAG: Fix incorrectly killing registers in bundles

[arsenm]

Summary

When looking for uses/defs to add kill flags, the iterator was double
incremented, skipping the first instruction in the bundle. The use
register in the first bundle instruction was then incorrectly killed.
The "First" instruction should be the BUNDLE itself as the proper
reverse iterator endpoint.

[fhahn]

LGTM, thanks!

lib/CodeGen/ScheduleDAGInstrs.cpp

1107

I think this is not well named. It does not point to the first instruction in the bundle, but the BUNDLE. IMO it would be good to adjust that.

1108

nit: no braces required anymore.

[arsenm]

r365216

lib/CodeGen/ScheduleDAGInstrs.cpp

for (ConstMIBundleOperands O(MI); O.isValid(); ++O) {
         LiveRegs.removeRegsInMask(MO);
       }
     }
 
     // If there is a bundle header fix it up first.
     if (!MI.isBundled()) {
       toggleKills(MRI, LiveRegs, MI, true);
     } else {
       MachineBasicBlock::instr_iterator First = MI.getIterator();
       if (MI.isBundle()) {
         toggleKills(MRI, LiveRegs, MI, false);
-        ++First;
       }
       // Some targets make the (questionable) assumtion that the instructions
       // inside the bundle are ordered and consequently only the last use of
       // a register inside the bundle can kill it.
       MachineBasicBlock::instr_iterator I = std::next(First);
       while (I->isBundledWithSucc())
         ++I;
       do {

test/CodeGen/AMDGPU/post-ra-sched-kill-bundle-use-inst.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=amdgcn-mesa-mesa3d -mcpu=gfx900 -run-pass=post-RA-sched -verify-machineinstrs -o -  %s | FileCheck %s
+
+# The scheduler was not inspecting the first instruction in the bundle
+# when adding kill flags, so it would incorrectly mark the first use
+# of $vgpr0 as killed.
+
+---
+name: kill_flag_use_first_bundle_inst
+tracksRegLiveness: true
+machineFunctionInfo:
+  isEntryFunction: true
+body:             |
+  bb.0:
+    liveins: $sgpr4_sgpr5, $sgpr7
+
+    ; CHECK-LABEL: name: kill_flag_use_first_bundle_inst
+    ; CHECK: liveins: $sgpr4_sgpr5, $sgpr7
+    ; CHECK: renamable $sgpr0 = S_LOAD_DWORD_IMM killed renamable $sgpr4_sgpr5, 0, 0, 0
+    ; CHECK: $m0 = S_MOV_B32 -1
+    ; CHECK: $vgpr0 = V_MOV_B32_e32 killed $sgpr0, implicit $exec, implicit $exec
+    ; CHECK: BUNDLE implicit $vgpr0, implicit $m0, implicit $exec {
+    ; CHECK:   DS_GWS_INIT $vgpr0, 8, -1, implicit $m0, implicit $exec
+    ; CHECK:   S_WAITCNT 0
+    ; CHECK: }
+    ; CHECK: BUNDLE implicit killed $vgpr0, implicit $m0, implicit $exec {
+    ; CHECK:   DS_GWS_BARRIER killed $vgpr0, 8, -1, implicit $m0, implicit $exec
+    ; CHECK:   S_WAITCNT 0
+    ; CHECK: }
+    renamable $sgpr0 = S_LOAD_DWORD_IMM killed renamable $sgpr4_sgpr5, 0, 0, 0
+    $m0 = S_MOV_B32 -1
+    $vgpr0 = V_MOV_B32_e32 $sgpr0, implicit $exec, implicit $exec
+    BUNDLE implicit $vgpr0, implicit $m0, implicit $exec {
+      DS_GWS_INIT $vgpr0, 8, -1, implicit $m0, implicit $exec
+      S_WAITCNT 0
+    }
+    BUNDLE implicit killed $vgpr0, implicit $m0, implicit $exec {
+      DS_GWS_BARRIER $vgpr0, 8, -1, implicit $m0, implicit $exec
+      S_WAITCNT 0
+    }
+
+...