[llvm-branch-commits] [llvm] [AMDGPU] Only scale the frame register in place for the last frame index (PR #224079)
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Wed Sep 16 09:58:08 PDT 2026
llvmorg-github-actions[bot] wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-backend-amdgpu
Author: Domenic Nutile (saxlungs)
<details>
<summary>Changes</summary>
When nothing can be scavenged, eliminateFrameIndex falls back to scaling
FrameReg in place and restoring it after MI. With two frame indices on
one instruction that fallback fires while lowering the first one, and
FrameReg is left scaled for everything that follows: the second frame
index is then scaled a second time from the already-scaled value, and any
emergency spill emitted in between uses it as its scratch offset.
---
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---
Full diff: https://github.com/llvm/llvm-project/pull/224079.diff
2 Files Affected:
- (modified) llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp (+9)
- (modified) llvm/test/CodeGen/AMDGPU/sgpr-spill-to-vmem-scc-clobber-reserved-exec-copy.mir (+78)
``````````diff
diff --git a/llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp b/llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp
index 86c23558e8fb3..f33c099b845bf 100644
--- a/llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp
+++ b/llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp
@@ -3429,7 +3429,16 @@ bool SIRegisterInfo::eliminateFrameIndex(MachineBasicBlock::iterator MI,
// Scaling FrameReg in place is the last resort when there is nothing to
// scavenge. It has to be undone after MI, which is only possible while MI
// does not use FrameReg for anything besides the frame index.
+ // Lowering a second frame index on the same MI would read FrameReg after
+ // it has been scaled in place, and any emergency spill in between would
+ // use the scaled value as its scratch offset, so require this to be the
+ // only frame index left.
+ bool IsOnlyFrameIndex =
+ llvm::count_if(MI->operands(), [](const MachineOperand &MO) {
+ return MO.isFI();
+ }) == 1;
bool CanUseFrameRegAsScratch = IsSALU && !LiveSCC && FrameReg &&
+ IsOnlyFrameIndex &&
!MI->readsRegister(FrameReg, this) &&
!MI->modifiesRegister(FrameReg, this);
diff --git a/llvm/test/CodeGen/AMDGPU/sgpr-spill-to-vmem-scc-clobber-reserved-exec-copy.mir b/llvm/test/CodeGen/AMDGPU/sgpr-spill-to-vmem-scc-clobber-reserved-exec-copy.mir
index 5ea0cbfbebab7..1e232ac8f8a25 100644
--- a/llvm/test/CodeGen/AMDGPU/sgpr-spill-to-vmem-scc-clobber-reserved-exec-copy.mir
+++ b/llvm/test/CodeGen/AMDGPU/sgpr-spill-to-vmem-scc-clobber-reserved-exec-copy.mir
@@ -166,3 +166,81 @@ body: |
S_ENDPGM 0, implicit $vcc
...
+
+---
+# Two frame indices on one instruction. Scaling FrameReg in place for the first
+# one would leave the second to be scaled from an already-scaled FrameReg, and
+# any emergency spill in between would use the scaled value as its scratch
+# offset. Only the last remaining frame index may use FrameReg, so the first is
+# lowered through the ordinary scavenge and every BUFFER_* here keeps reading an
+# unscaled $sgpr32.
+name: two_frame_indices_scc_dead
+tracksRegLiveness: true
+stack:
+ - { id: 0, size: 64, alignment: 4 }
+ - { id: 1, size: 64, alignment: 4 }
+machineFunctionInfo:
+ isEntryFunction: false
+ scratchRSrcReg: '$sgpr0_sgpr1_sgpr2_sgpr3'
+ frameOffsetReg: '$sgpr33'
+ stackPtrOffsetReg: '$sgpr32'
+ sgprForEXECCopy: '$sgpr100_sgpr101'
+ occupancy: 2
+body: |
+ ; CHECK-LABEL: name: two_frame_indices_scc_dead
+ ; CHECK: bb.0:
+ ; CHECK-NEXT: successors: %bb.1(0x80000000)
+ ; CHECK-NEXT: {{ $}}
+ ; CHECK-NEXT: frame-setup CFI_INSTRUCTION llvm_def_aspace_cfa $sgpr32, 0, 6
+ ; CHECK-NEXT: frame-setup CFI_INSTRUCTION llvm_register_pair $pc_reg, $sgpr30, 32, $sgpr31, 32
+ ; CHECK-NEXT: S_BRANCH %bb.1
+ ; CHECK-NEXT: {{ $}}
+ ; CHECK-NEXT: bb.1:
+ ; CHECK-NEXT: successors: %bb.2(0x80000000)
+ ; CHECK-NEXT: liveins: $sgpr4_sgpr5_sgpr6_sgpr7_sgpr8_sgpr9_sgpr10_sgpr11_sgpr12_sgpr13_sgpr14_sgpr15_sgpr16_sgpr17_sgpr18_sgpr19, $sgpr20_sgpr21_sgpr22_sgpr23_sgpr24_sgpr25_sgpr26_sgpr27_sgpr28_sgpr29_sgpr30_sgpr31_sgpr32_sgpr33_sgpr34_sgpr35, $sgpr36_sgpr37_sgpr38_sgpr39_sgpr40_sgpr41_sgpr42_sgpr43_sgpr44_sgpr45_sgpr46_sgpr47_sgpr48_sgpr49_sgpr50_sgpr51, $sgpr52_sgpr53_sgpr54_sgpr55_sgpr56_sgpr57_sgpr58_sgpr59_sgpr60_sgpr61_sgpr62_sgpr63_sgpr64_sgpr65_sgpr66_sgpr67, $sgpr68_sgpr69_sgpr70_sgpr71_sgpr72_sgpr73_sgpr74_sgpr75_sgpr76_sgpr77_sgpr78_sgpr79_sgpr80_sgpr81_sgpr82_sgpr83, $sgpr84_sgpr85_sgpr86_sgpr87_sgpr88_sgpr89_sgpr90_sgpr91_sgpr92_sgpr93_sgpr94_sgpr95, $sgpr96_sgpr97_sgpr98_sgpr99, $vcc
+ ; CHECK-NEXT: {{ $}}
+ ; CHECK-NEXT: $exec = S_NOT_B64 $exec, implicit-def dead $scc, implicit-def $vgpr0
+ ; CHECK-NEXT: BUFFER_STORE_DWORD_OFFSET killed $vgpr0, $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr32, 128, 0, 0, implicit $exec :: ("amdgpu-thread-private" store (s32) into %stack.2, addrspace 5)
+ ; CHECK-NEXT: $vgpr0 = SI_SPILL_S32_TO_VGPR killed $sgpr4, 0, undef $vgpr0
+ ; CHECK-NEXT: BUFFER_STORE_DWORD_OFFSET $vgpr0, $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr32, 128, 0, 0, implicit $exec :: ("amdgpu-thread-private" store (s32) into %stack.2, addrspace 5)
+ ; CHECK-NEXT: $exec = S_NOT_B64 $exec, implicit-def dead $scc
+ ; CHECK-NEXT: BUFFER_STORE_DWORD_OFFSET killed $vgpr0, $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr32, 128, 0, 0, implicit $exec :: ("amdgpu-thread-private" store (s32) into %stack.2, addrspace 5)
+ ; CHECK-NEXT: $exec = S_NOT_B64 $exec, implicit-def dead $scc
+ ; CHECK-NEXT: $vgpr0 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr32, 128, 0, 0, implicit $exec :: ("amdgpu-thread-private" load (s32) from %stack.2, addrspace 5)
+ ; CHECK-NEXT: $exec = S_NOT_B64 $exec, implicit-def dead $scc, implicit killed $vgpr0
+ ; CHECK-NEXT: $sgpr4 = S_LSHR_B32 $sgpr32, 6, implicit-def $scc
+ ; CHECK-NEXT: $sgpr4 = S_ADD_I32 killed $sgpr4, 64, implicit-def $scc
+ ; CHECK-NEXT: $sgpr32 = S_LSHR_B32 $sgpr32, 6, implicit-def dead $scc
+ ; CHECK-NEXT: renamable $vcc_lo = S_AND_B32 killed $sgpr4, $sgpr32, implicit-def dead $scc
+ ; CHECK-NEXT: $sgpr32 = S_MUL_I32 $sgpr32, 64
+ ; CHECK-NEXT: $exec = S_NOT_B64 $exec, implicit-def dead $scc, implicit-def $vgpr1
+ ; CHECK-NEXT: BUFFER_STORE_DWORD_OFFSET killed $vgpr1, $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr32, 128, 0, 0, implicit $exec :: ("amdgpu-thread-private" store (s32) into %stack.2, addrspace 5)
+ ; CHECK-NEXT: $vgpr1 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr32, 128, 0, 0, implicit $exec :: ("amdgpu-thread-private" load (s32) from %stack.2, addrspace 5)
+ ; CHECK-NEXT: $exec = S_NOT_B64 $exec, implicit-def dead $scc
+ ; CHECK-NEXT: $vgpr1 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr32, 128, 0, 0, implicit $exec :: ("amdgpu-thread-private" load (s32) from %stack.2, addrspace 5)
+ ; CHECK-NEXT: $exec = S_NOT_B64 $exec, implicit-def dead $scc
+ ; CHECK-NEXT: $sgpr4 = SI_RESTORE_S32_FROM_VGPR killed $vgpr1, 0
+ ; CHECK-NEXT: $vgpr1 = BUFFER_LOAD_DWORD_OFFSET $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr32, 128, 0, 0, implicit $exec :: ("amdgpu-thread-private" load (s32) from %stack.2, addrspace 5)
+ ; CHECK-NEXT: $exec = S_NOT_B64 $exec, implicit-def dead $scc, implicit killed $vgpr1
+ ; CHECK-NEXT: S_NOP 0, implicit killed $sgpr4_sgpr5_sgpr6_sgpr7_sgpr8_sgpr9_sgpr10_sgpr11_sgpr12_sgpr13_sgpr14_sgpr15_sgpr16_sgpr17_sgpr18_sgpr19, implicit killed $sgpr20_sgpr21_sgpr22_sgpr23_sgpr24_sgpr25_sgpr26_sgpr27_sgpr28_sgpr29_sgpr30_sgpr31_sgpr32_sgpr33_sgpr34_sgpr35, implicit killed $sgpr36_sgpr37_sgpr38_sgpr39_sgpr40_sgpr41_sgpr42_sgpr43_sgpr44_sgpr45_sgpr46_sgpr47_sgpr48_sgpr49_sgpr50_sgpr51, implicit killed $sgpr52_sgpr53_sgpr54_sgpr55_sgpr56_sgpr57_sgpr58_sgpr59_sgpr60_sgpr61_sgpr62_sgpr63_sgpr64_sgpr65_sgpr66_sgpr67, implicit killed $sgpr68_sgpr69_sgpr70_sgpr71_sgpr72_sgpr73_sgpr74_sgpr75_sgpr76_sgpr77_sgpr78_sgpr79_sgpr80_sgpr81_sgpr82_sgpr83, implicit killed $sgpr84_sgpr85_sgpr86_sgpr87_sgpr88_sgpr89_sgpr90_sgpr91_sgpr92_sgpr93_sgpr94_sgpr95, implicit killed $sgpr96_sgpr97_sgpr98_sgpr99
+ ; CHECK-NEXT: S_BRANCH %bb.2
+ ; CHECK-NEXT: {{ $}}
+ ; CHECK-NEXT: bb.2:
+ ; CHECK-NEXT: liveins: $vcc
+ ; CHECK-NEXT: {{ $}}
+ ; CHECK-NEXT: S_ENDPGM 0, implicit $vcc
+ bb.0:
+ S_BRANCH %bb.1
+
+ bb.1:
+ liveins: $sgpr4_sgpr5_sgpr6_sgpr7_sgpr8_sgpr9_sgpr10_sgpr11_sgpr12_sgpr13_sgpr14_sgpr15_sgpr16_sgpr17_sgpr18_sgpr19, $sgpr20_sgpr21_sgpr22_sgpr23_sgpr24_sgpr25_sgpr26_sgpr27_sgpr28_sgpr29_sgpr30_sgpr31_sgpr32_sgpr33_sgpr34_sgpr35, $sgpr36_sgpr37_sgpr38_sgpr39_sgpr40_sgpr41_sgpr42_sgpr43_sgpr44_sgpr45_sgpr46_sgpr47_sgpr48_sgpr49_sgpr50_sgpr51, $sgpr52_sgpr53_sgpr54_sgpr55_sgpr56_sgpr57_sgpr58_sgpr59_sgpr60_sgpr61_sgpr62_sgpr63_sgpr64_sgpr65_sgpr66_sgpr67, $sgpr68_sgpr69_sgpr70_sgpr71_sgpr72_sgpr73_sgpr74_sgpr75_sgpr76_sgpr77_sgpr78_sgpr79_sgpr80_sgpr81_sgpr82_sgpr83, $sgpr84_sgpr85_sgpr86_sgpr87_sgpr88_sgpr89_sgpr90_sgpr91_sgpr92_sgpr93_sgpr94_sgpr95, $sgpr96_sgpr97_sgpr98_sgpr99, $vcc
+
+ renamable $vcc_lo = S_AND_B32 %stack.1, %stack.0, implicit-def dead $scc
+ S_NOP 0, implicit killed $sgpr4_sgpr5_sgpr6_sgpr7_sgpr8_sgpr9_sgpr10_sgpr11_sgpr12_sgpr13_sgpr14_sgpr15_sgpr16_sgpr17_sgpr18_sgpr19, implicit killed $sgpr20_sgpr21_sgpr22_sgpr23_sgpr24_sgpr25_sgpr26_sgpr27_sgpr28_sgpr29_sgpr30_sgpr31_sgpr32_sgpr33_sgpr34_sgpr35, implicit killed $sgpr36_sgpr37_sgpr38_sgpr39_sgpr40_sgpr41_sgpr42_sgpr43_sgpr44_sgpr45_sgpr46_sgpr47_sgpr48_sgpr49_sgpr50_sgpr51, implicit killed $sgpr52_sgpr53_sgpr54_sgpr55_sgpr56_sgpr57_sgpr58_sgpr59_sgpr60_sgpr61_sgpr62_sgpr63_sgpr64_sgpr65_sgpr66_sgpr67, implicit killed $sgpr68_sgpr69_sgpr70_sgpr71_sgpr72_sgpr73_sgpr74_sgpr75_sgpr76_sgpr77_sgpr78_sgpr79_sgpr80_sgpr81_sgpr82_sgpr83, implicit killed $sgpr84_sgpr85_sgpr86_sgpr87_sgpr88_sgpr89_sgpr90_sgpr91_sgpr92_sgpr93_sgpr94_sgpr95, implicit killed $sgpr96_sgpr97_sgpr98_sgpr99
+ S_BRANCH %bb.2
+
+ bb.2:
+ liveins: $vcc
+
+ S_ENDPGM 0, implicit $vcc
+...
``````````
</details>
https://github.com/llvm/llvm-project/pull/224079
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