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[AMDGPU] Lower S_ABSDIFF_I32 to VALU instructions #167691

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Nov 13, 2025
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[AMDGPU] Lower S_ABSDIFF_I32 to VALU instructions #167691

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2769800
[AMDGPU] Lower S_ABSDIFF_I32 to VALU instructions
mariusz-sikora-at-amd Nov 12, 2025
0cbff2f
Use liveins in test
mariusz-sikora-at-amd Nov 12, 2025
8c5c6dc
Changes after review (add const and remove verify-machineinstrs)
mariusz-sikora-at-amd Nov 13, 2025
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36 changes: 36 additions & 0 deletions llvm/lib/Target/AMDGPU/SIInstrInfo.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7775,6 +7775,11 @@ void SIInstrInfo::moveToVALUImpl(SIInstrWorklist &Worklist,
Inst.eraseFromParent();
return;

case AMDGPU::S_ABSDIFF_I32:
lowerScalarAbsDiff(Worklist, Inst);
Inst.eraseFromParent();
return;

case AMDGPU::S_CBRANCH_SCC0:
case AMDGPU::S_CBRANCH_SCC1: {
// Clear unused bits of vcc
Expand Down Expand Up @@ -8424,6 +8429,37 @@ void SIInstrInfo::lowerScalarAbs(SIInstrWorklist &Worklist,
addUsersToMoveToVALUWorklist(ResultReg, MRI, Worklist);
}

void SIInstrInfo::lowerScalarAbsDiff(SIInstrWorklist &Worklist,
MachineInstr &Inst) const {
MachineBasicBlock &MBB = *Inst.getParent();
MachineRegisterInfo &MRI = MBB.getParent()->getRegInfo();
MachineBasicBlock::iterator MII = Inst;
const DebugLoc &DL = Inst.getDebugLoc();

MachineOperand &Dest = Inst.getOperand(0);
MachineOperand &Src1 = Inst.getOperand(1);
MachineOperand &Src2 = Inst.getOperand(2);
Register SubResultReg = MRI.createVirtualRegister(&AMDGPU::VGPR_32RegClass);
Register TmpReg = MRI.createVirtualRegister(&AMDGPU::VGPR_32RegClass);
Register ResultReg = MRI.createVirtualRegister(&AMDGPU::VGPR_32RegClass);

unsigned SubOp =
ST.hasAddNoCarry() ? AMDGPU::V_SUB_U32_e32 : AMDGPU::V_SUB_CO_U32_e32;

BuildMI(MBB, MII, DL, get(SubOp), SubResultReg)
.addReg(Src1.getReg())
.addReg(Src2.getReg());

BuildMI(MBB, MII, DL, get(SubOp), TmpReg).addImm(0).addReg(SubResultReg);

BuildMI(MBB, MII, DL, get(AMDGPU::V_MAX_I32_e64), ResultReg)
.addReg(SubResultReg)
.addReg(TmpReg);

MRI.replaceRegWith(Dest.getReg(), ResultReg);
addUsersToMoveToVALUWorklist(ResultReg, MRI, Worklist);
}

void SIInstrInfo::lowerScalarXnor(SIInstrWorklist &Worklist,
MachineInstr &Inst) const {
MachineBasicBlock &MBB = *Inst.getParent();
Expand Down
2 changes: 2 additions & 0 deletions llvm/lib/Target/AMDGPU/SIInstrInfo.h
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Original file line number Diff line number Diff line change
Expand Up @@ -136,6 +136,8 @@ class SIInstrInfo final : public AMDGPUGenInstrInfo {

void lowerScalarAbs(SIInstrWorklist &Worklist, MachineInstr &Inst) const;

void lowerScalarAbsDiff(SIInstrWorklist &Worklist, MachineInstr &Inst) const;

void lowerScalarXnor(SIInstrWorklist &Worklist, MachineInstr &Inst) const;

void splitScalarNotBinop(SIInstrWorklist &Worklist, MachineInstr &Inst,
Expand Down
38 changes: 38 additions & 0 deletions llvm/test/CodeGen/AMDGPU/absdiff.ll
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@@ -1,6 +1,44 @@
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
; RUN: llc -mtriple=amdgcn-amd-amdpal -mcpu=gfx900 < %s | FileCheck %s


define amdgpu_gs float @absdiff_valu_input_regression() {
; CHECK-LABEL: absdiff_valu_input_regression:
; CHECK: ; %bb.0: ; %bb
; CHECK-NEXT: s_mov_b32 s0, 0
; CHECK-NEXT: .LBB0_1: ; %bb1
; CHECK-NEXT: ; =>This Inner Loop Header: Depth=1
; CHECK-NEXT: s_mov_b32 s1, s0
; CHECK-NEXT: s_or_b32 s0, s0, 1
; CHECK-NEXT: s_cmp_gt_i32 s1, 0
; CHECK-NEXT: s_cbranch_scc1 .LBB0_1
; CHECK-NEXT: ; %bb.2: ; %bb11
; CHECK-NEXT: v_med3_i32 v0, s1, 0, 1
; CHECK-NEXT: v_sub_u32_e32 v0, 0, v0
; CHECK-NEXT: v_sub_u32_e32 v1, 0, v0
; CHECK-NEXT: v_max_i32_e32 v0, v0, v1
Comment on lines +17 to +19
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@jayfoad jayfoad Nov 12, 2025

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This is a confusing example because I think it is calculating absdiff(0, x) which should really have been simplified to abs(x) already. But I think the code is correct.

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@mariusz-sikora-at-amd mariusz-sikora-at-amd Nov 12, 2025

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This example is a reduce version of CTS shader that started to fail after #164835

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@arsenm arsenm Nov 12, 2025

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I think instcombine is missing a lot of patterns for these newer intrinsics

; CHECK-NEXT: ; return to shader part epilog
bb:
br label %bb1

bb1: ; preds = %bb1, %bb
%i = phi i32 [ 0, %bb ], [ %i9, %bb1 ]
%i2 = phi i32 [ 0, %bb ], [ %i5, %bb1 ]
%i3 = or i32 %i2, 1
%i4 = or i32 %i3, 0
%i5 = call i32 @llvm.smax.i32(i32 %i, i32 0)
%i6 = call i32 @llvm.umin.i32(i32 %i5, i32 1)
%i7 = sub i32 0, %i6
%i8 = call i32 @llvm.abs.i32(i32 %i7, i1 false)
%i9 = or i32 %i, 1
%i10 = icmp sgt i32 %i, 0
br i1 %i10, label %bb1, label %bb11

bb11: ; preds = %bb1
%i12 = bitcast i32 %i8 to float
ret float %i12
}

define amdgpu_ps i16 @absdiff_i16_false(i16 inreg %arg0, i16 inreg %arg1) {
; CHECK-LABEL: absdiff_i16_false:
; CHECK: ; %bb.0:
Expand Down
31 changes: 31 additions & 0 deletions llvm/test/CodeGen/AMDGPU/move-to-valu-absdiff.mir
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@@ -0,0 +1,31 @@
# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
# RUN: llc -mtriple=amdgcn -mcpu=fiji -run-pass=si-fix-sgpr-copies -o - %s | FileCheck --check-prefix=GFX8 %s
# RUN: llc -mtriple=amdgcn -mcpu=gfx1200 -run-pass=si-fix-sgpr-copies -o - %s | FileCheck --check-prefix=GFX12 %s

---
name: absdiff_i32
body: |
bb.0:
liveins: $vgpr0, $vgpr1, $vgpr2
; GFX8-LABEL: name: absdiff_i32
; GFX8: liveins: $vgpr0, $vgpr1, $vgpr2
; GFX8-NEXT: {{ $}}
; GFX8-NEXT: [[V_LSHL_ADD_U32_e64_:%[0-9]+]]:vgpr_32 = V_LSHL_ADD_U32_e64 $vgpr0, $vgpr1, $vgpr2, implicit $exec
; GFX8-NEXT: [[S_MOV_B32_:%[0-9]+]]:sreg_32 = S_MOV_B32 10
; GFX8-NEXT: [[V_SUB_CO_U32_e32_:%[0-9]+]]:vgpr_32 = V_SUB_CO_U32_e32 [[S_MOV_B32_]], [[V_LSHL_ADD_U32_e64_]], implicit-def $vcc, implicit $exec
; GFX8-NEXT: [[V_SUB_CO_U32_e32_1:%[0-9]+]]:vgpr_32 = V_SUB_CO_U32_e32 0, [[V_SUB_CO_U32_e32_]], implicit-def $vcc, implicit $exec
; GFX8-NEXT: [[V_MAX_I32_e64_:%[0-9]+]]:vgpr_32 = V_MAX_I32_e64 [[V_SUB_CO_U32_e32_]], [[V_SUB_CO_U32_e32_1]], implicit $exec
;
; GFX12-LABEL: name: absdiff_i32
; GFX12: liveins: $vgpr0, $vgpr1, $vgpr2
; GFX12-NEXT: {{ $}}
; GFX12-NEXT: [[V_LSHL_ADD_U32_e64_:%[0-9]+]]:vgpr_32 = V_LSHL_ADD_U32_e64 $vgpr0, $vgpr1, $vgpr2, implicit $exec
; GFX12-NEXT: [[S_MOV_B32_:%[0-9]+]]:sreg_32 = S_MOV_B32 10
; GFX12-NEXT: [[V_SUB_U32_e32_:%[0-9]+]]:vgpr_32 = V_SUB_U32_e32 [[S_MOV_B32_]], [[V_LSHL_ADD_U32_e64_]], implicit $exec
; GFX12-NEXT: [[V_SUB_U32_e32_1:%[0-9]+]]:vgpr_32 = V_SUB_U32_e32 0, [[V_SUB_U32_e32_]], implicit $exec
; GFX12-NEXT: [[V_MAX_I32_e64_:%[0-9]+]]:vgpr_32 = V_MAX_I32_e64 [[V_SUB_U32_e32_]], [[V_SUB_U32_e32_1]], implicit $exec
%0:vgpr_32 = V_LSHL_ADD_U32_e64 $vgpr0, $vgpr1, $vgpr2, implicit $exec
%1:sreg_32 = COPY %0:vgpr_32
%2:sreg_32 = S_MOV_B32 10
%3:sreg_32 = S_ABSDIFF_I32 killed %2:sreg_32, %1:sreg_32, implicit-def dead $scc
...
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