AMDGPU: Clear kill flags after FoldZeroHighBits (#99582)

Summary:
After folding, all uses of the result register are going to be replaced
by the operand register. The kill flags on the uses of the result and
operand registers are no longer valid after the replacement, and need to
be cleared.
The only exception is, however, if the kill flag is set for the operand
register, we are sure the last use of the result register is the new
last use of the operand register, and thus we are safe to keep the kill
flags.

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Differential Revision: https://phabricator.intern.facebook.com/D60251304

Author: changpeng

llvm/lib/Target/AMDGPU/SIFoldOperands.cpp

@@ -1361,7 +1361,9 @@ bool SIFoldOperands::tryFoldZeroHighBits(MachineInstr &MI) const {
     return false;
 
   Register Dst = MI.getOperand(0).getReg();
-  MRI->replaceRegWith(Dst, SrcDef->getOperand(0).getReg());
+  MRI->replaceRegWith(Dst, Src1);
+  if (!MI.getOperand(2).isKill())
+    MRI->clearKillFlags(Src1);
   MI.eraseFromParent();
   return true;
 }

llvm/test/CodeGen/AMDGPU/fold-zero-high-bits-clear-kill-flags.mir

@@ -0,0 +1,54 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 5
+# RUN: llc -mtriple=amdgcn--amdhsa -mcpu=gfx940 -verify-machineinstrs -run-pass si-fold-operands -o - %s | FileCheck -enable-var-scope -check-prefix=GCN %s
+
+---
+name:            fold_zero_high_bits_src1_alive
+tracksRegLiveness: true
+
+body:             |
+  bb.0:
+    liveins: $vgpr0, $vgpr1
+    ; GCN-LABEL: name: fold_zero_high_bits_src1_alive
+    ; GCN: liveins: $vgpr0, $vgpr1
+    ; GCN-NEXT: {{  $}}
+    ; GCN-NEXT: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN-NEXT: [[V_ADD_U16_e64_:%[0-9]+]]:vgpr_32 = V_ADD_U16_e64 [[COPY]], 1, 0, implicit $exec
+    ; GCN-NEXT: [[S_MOV_B32_:%[0-9]+]]:sreg_32 = S_MOV_B32 65535
+    ; GCN-NEXT: [[V_MUL_U32_U24_e64_:%[0-9]+]]:vgpr_32 = V_MUL_U32_U24_e64 [[V_ADD_U16_e64_]], 1, 0, implicit $exec
+    ; GCN-NEXT: [[COPY1:%[0-9]+]]:vgpr_32 = COPY $vgpr1
+    ; GCN-NEXT: [[V_SUB_U16_e64_:%[0-9]+]]:vgpr_32 = V_SUB_U16_e64 [[V_ADD_U16_e64_]], [[COPY1]], 0, implicit $exec
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sreg_32 = S_MOV_B32 1
+    %2:vgpr_32 = V_ADD_U16_e64 %0:vgpr_32, %1:sreg_32, 0, implicit $exec
+    %3:sreg_32 = S_MOV_B32 65535
+    %4:vgpr_32 = V_AND_B32_e64 %3:sreg_32, %2:vgpr_32, implicit $exec
+    %5:vgpr_32 = V_MUL_U32_U24_e64 killed %4:vgpr_32, %1:sreg_32, 0, implicit $exec
+    %6:vgpr_32 = COPY $vgpr1
+    %7:vgpr_32 = V_SUB_U16_e64 %2:vgpr_32, %6:vgpr_32, 0, implicit $exec
+...
+
+---
+name:            fold_zero_high_bits_src1_killed
+tracksRegLiveness: true
+
+body:             |
+  bb.0:
+    liveins: $vgpr0, $vgpr1
+    ; GCN-LABEL: name: fold_zero_high_bits_src1_killed
+    ; GCN: liveins: $vgpr0, $vgpr1
+    ; GCN-NEXT: {{  $}}
+    ; GCN-NEXT: [[COPY:%[0-9]+]]:vgpr_32 = COPY $vgpr0
+    ; GCN-NEXT: [[V_ADD_U16_e64_:%[0-9]+]]:vgpr_32 = V_ADD_U16_e64 [[COPY]], 1, 0, implicit $exec
+    ; GCN-NEXT: [[COPY1:%[0-9]+]]:vgpr_32 = COPY $vgpr1
+    ; GCN-NEXT: [[V_SUB_U16_e64_:%[0-9]+]]:vgpr_32 = V_SUB_U16_e64 [[V_ADD_U16_e64_]], [[COPY1]], 0, implicit $exec
+    ; GCN-NEXT: [[S_MOV_B32_:%[0-9]+]]:sreg_32 = S_MOV_B32 65535
+    ; GCN-NEXT: [[V_MUL_U32_U24_e64_:%[0-9]+]]:vgpr_32 = V_MUL_U32_U24_e64 killed [[V_ADD_U16_e64_]], 1, 0, implicit $exec
+    %0:vgpr_32 = COPY $vgpr0
+    %1:sreg_32 = S_MOV_B32 1
+    %2:vgpr_32 = V_ADD_U16_e64 %0:vgpr_32, %1:sreg_32, 0, implicit $exec
+    %6:vgpr_32 = COPY $vgpr1
+    %7:vgpr_32 = V_SUB_U16_e64 %2:vgpr_32, %6:vgpr_32, 0, implicit $exec
+    %3:sreg_32 = S_MOV_B32 65535
+    %4:vgpr_32 = V_AND_B32_e64 %3:sreg_32, killed %2:vgpr_32, implicit $exec
+    %5:vgpr_32 = V_MUL_U32_U24_e64 killed %4:vgpr_32, %1:sreg_32, 0, implicit $exec
+...