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[AMDGPU] Don't optimize agpr phis if the operand doesn't have subreg …
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…use (#91267)

If the operand doesn't have any subreg use, the optimization could
potentially
generate `V_ACCVGPR_READ_B32_e64` with wrong register class. The
following example demonstrates the issue.

Input MIR:

```
bb.0:
  %0:sgpr_32 = S_MOV_B32 0
  %1:sgpr_128 = REG_SEQUENCE %0:sgpr_32, %subreg.sub0, %0:sgpr_32, %subreg.sub1, %0:sgpr_32, %subreg.sub2, %0:sgpr_32, %subreg.sub3
  %2:vreg_128 = COPY %1:sgpr_128
  %3:areg_128 = COPY %2:vreg_128, implicit $exec

bb.1:
  %4:areg_128 = PHI %3:areg_128, %bb.0, %6:areg_128, %bb.1
  %5:areg_128 = PHI %3:areg_128, %bb.0, %7:areg_128, %bb.1
  ...
```

Output of current implementation:

```
bb.0:
  %0:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
  %1:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
  %2:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
  %3:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
  %4:areg_128 = REG_SEQUENCE %0:agpr_32, %subreg.sub0, %1:agpr_32, %subreg.sub1, %2:agpr_32, %subreg.sub2, %3:agpr_32, %subreg.sub3
  %5:vreg_128 = V_ACCVGPR_READ_B32_e64 %4:areg_128, implicit $exec
  %6:areg_128 = COPY %46:vreg_128

bb.1:
  %7:areg_128 = PHI %6:areg_128, %bb.0, %9:areg_128, %bb.1
  %8:areg_128 = PHI %6:areg_128, %bb.0, %10:areg_128, %bb.1
  ...
```

The problem is the generated `V_ACCVGPR_READ_B32_e64` instruction.
Apparently the operand `%4:areg_128` is not valid for this.

In this patch, we don't count the none-subreg use because
`V_ACCVGPR_READ_B32_e64` can't handle none-32-bit operand.

Fixes: SWDEV-459556
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@shiltian
shiltian authored May 7, 2024
1 parent 082c81a commit 0b50d09
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Showing 2 changed files with 84 additions and 1 deletion.
2 changes: 2 additions & 0 deletions llvm/lib/Target/AMDGPU/SIFoldOperands.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2106,6 +2106,8 @@ bool SIFoldOperands::tryOptimizeAGPRPhis(MachineBasicBlock &MBB) {

for (unsigned K = 1; K < MI.getNumOperands(); K += 2) {
MachineOperand &PhiMO = MI.getOperand(K);
if (!PhiMO.getSubReg())
continue;
RegToMO[{PhiMO.getReg(), PhiMO.getSubReg()}].push_back(&PhiMO);
}
}
Expand Down
83 changes: 82 additions & 1 deletion llvm/test/CodeGen/AMDGPU/fold-agpr-phis.mir
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Original file line number Diff line number Diff line change
Expand Up @@ -465,7 +465,6 @@ body: |
; GFX90A-NEXT: bb.2:
; GFX90A-NEXT: S_ENDPGM 0
bb.0:
; Tests that tryOptimizeAGPRPhis kicks in for GFX908.
liveins: $sgpr0, $scc
successors: %bb.1
Expand Down Expand Up @@ -715,3 +714,85 @@ body: |
bb.3:
S_ENDPGM 0
...

---
name: skip_optimize_agpr_phi_without_subreg_use
tracksRegLiveness: true
body: |
; GFX908-LABEL: name: skip_optimize_agpr_phi_without_subreg_use
; GFX908: bb.0:
; GFX908-NEXT: successors: %bb.1(0x80000000)
; GFX908-NEXT: liveins: $scc
; GFX908-NEXT: {{ $}}
; GFX908-NEXT: [[V_MOV_B32_e32_:%[0-9]+]]:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
; GFX908-NEXT: [[S_MOV_B32_:%[0-9]+]]:sgpr_32 = S_MOV_B32 0
; GFX908-NEXT: [[V_ACCVGPR_WRITE_B32_e64_:%[0-9]+]]:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
; GFX908-NEXT: [[V_ACCVGPR_WRITE_B32_e64_1:%[0-9]+]]:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
; GFX908-NEXT: [[V_ACCVGPR_WRITE_B32_e64_2:%[0-9]+]]:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
; GFX908-NEXT: [[V_ACCVGPR_WRITE_B32_e64_3:%[0-9]+]]:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
; GFX908-NEXT: [[REG_SEQUENCE:%[0-9]+]]:areg_128_align2 = REG_SEQUENCE [[V_ACCVGPR_WRITE_B32_e64_]], %subreg.sub0, [[V_ACCVGPR_WRITE_B32_e64_1]], %subreg.sub1, [[V_ACCVGPR_WRITE_B32_e64_2]], %subreg.sub2, [[V_ACCVGPR_WRITE_B32_e64_3]], %subreg.sub3
; GFX908-NEXT: {{ $}}
; GFX908-NEXT: bb.1:
; GFX908-NEXT: successors: %bb.1(0x40000000), %bb.2(0x40000000)
; GFX908-NEXT: liveins: $scc
; GFX908-NEXT: {{ $}}
; GFX908-NEXT: [[PHI:%[0-9]+]]:areg_128_align2 = PHI [[REG_SEQUENCE]], %bb.0, %7, %bb.1
; GFX908-NEXT: [[V_MFMA_F32_16X16X4F32_e64_:%[0-9]+]]:areg_128_align2 = V_MFMA_F32_16X16X4F32_e64 [[V_MOV_B32_e32_]], [[V_MOV_B32_e32_]], [[PHI]], 0, 0, 0, implicit $mode, implicit $exec
; GFX908-NEXT: [[COPY:%[0-9]+]]:areg_128_align2 = COPY [[V_MFMA_F32_16X16X4F32_e64_]], implicit $exec
; GFX908-NEXT: S_CBRANCH_SCC1 %bb.1, implicit $scc
; GFX908-NEXT: {{ $}}
; GFX908-NEXT: bb.2:
; GFX908-NEXT: S_ENDPGM 0
;
; GFX90A-LABEL: name: skip_optimize_agpr_phi_without_subreg_use
; GFX90A: bb.0:
; GFX90A-NEXT: successors: %bb.1(0x80000000)
; GFX90A-NEXT: liveins: $scc
; GFX90A-NEXT: {{ $}}
; GFX90A-NEXT: [[V_MOV_B32_e32_:%[0-9]+]]:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
; GFX90A-NEXT: [[S_MOV_B32_:%[0-9]+]]:sgpr_32 = S_MOV_B32 0
; GFX90A-NEXT: [[V_ACCVGPR_WRITE_B32_e64_:%[0-9]+]]:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
; GFX90A-NEXT: [[V_ACCVGPR_WRITE_B32_e64_1:%[0-9]+]]:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
; GFX90A-NEXT: [[V_ACCVGPR_WRITE_B32_e64_2:%[0-9]+]]:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
; GFX90A-NEXT: [[V_ACCVGPR_WRITE_B32_e64_3:%[0-9]+]]:agpr_32 = V_ACCVGPR_WRITE_B32_e64 0, implicit $exec
; GFX90A-NEXT: [[REG_SEQUENCE:%[0-9]+]]:areg_128_align2 = REG_SEQUENCE [[V_ACCVGPR_WRITE_B32_e64_]], %subreg.sub0, [[V_ACCVGPR_WRITE_B32_e64_1]], %subreg.sub1, [[V_ACCVGPR_WRITE_B32_e64_2]], %subreg.sub2, [[V_ACCVGPR_WRITE_B32_e64_3]], %subreg.sub3
; GFX90A-NEXT: {{ $}}
; GFX90A-NEXT: bb.1:
; GFX90A-NEXT: successors: %bb.1(0x40000000), %bb.2(0x40000000)
; GFX90A-NEXT: liveins: $scc
; GFX90A-NEXT: {{ $}}
; GFX90A-NEXT: [[PHI:%[0-9]+]]:areg_128_align2 = PHI [[REG_SEQUENCE]], %bb.0, %7, %bb.1
; GFX90A-NEXT: [[V_MFMA_F32_16X16X4F32_e64_:%[0-9]+]]:areg_128_align2 = V_MFMA_F32_16X16X4F32_e64 [[V_MOV_B32_e32_]], [[V_MOV_B32_e32_]], [[PHI]], 0, 0, 0, implicit $mode, implicit $exec
; GFX90A-NEXT: [[COPY:%[0-9]+]]:areg_128_align2 = COPY [[V_MFMA_F32_16X16X4F32_e64_]], implicit $exec
; GFX90A-NEXT: S_CBRANCH_SCC1 %bb.1, implicit $scc
; GFX90A-NEXT: {{ $}}
; GFX90A-NEXT: bb.2:
; GFX90A-NEXT: S_ENDPGM 0
bb.0:
liveins: $scc
successors: %bb.1
%0:vgpr_32 = V_MOV_B32_e32 0, implicit $exec
%1:sgpr_32 = S_MOV_B32 0
%2:sgpr_128 = REG_SEQUENCE %1, %subreg.sub0, %1, %subreg.sub1, %1, %subreg.sub2, %1, %subreg.sub3
%3:vreg_128 = COPY %2
%4:sreg_64 = S_MOV_B64 0
%5:areg_128_align2 = COPY %3, implicit $exec
bb.1:
liveins: $scc
successors: %bb.1, %bb.2
%9:areg_128_align2 = PHI %5, %bb.0, %10, %bb.1
%11:areg_128_align2 = V_MFMA_F32_16X16X4F32_e64 %0:vgpr_32, %0:vgpr_32, %9:areg_128_align2, 0, 0, 0, implicit $mode, implicit $exec
%12:vgpr_32 = COPY %11.sub3
%13:vgpr_32 = COPY %11.sub2
%14:vgpr_32 = COPY %11.sub1
%15:vgpr_32 = COPY %11.sub0
%10:areg_128_align2 = COPY %11, implicit $exec
S_CBRANCH_SCC1 %bb.1, implicit $scc
bb.2:
S_ENDPGM 0
...

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