[llvm] [AMDGPU] Ensure non-reserved CSR spilled regs are live-in (PR #146427)
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Mon Jun 30 14:57:42 PDT 2025
https://github.com/macurtis-amd created https://github.com/llvm/llvm-project/pull/146427
Fixes:
```
*** Bad machine code: Using an undefined physical register ***
- function: widget
- basic block: %bb.0 bb (0x564092cbe140)
- instruction: $vgpr63 = V_ACCVGPR_READ_B32_e64 killed $agpr13, implicit $exec
- operand 1: killed $agpr13
LLVM ERROR: Found 1 machine code errors.
```
The detailed sequence of events that led to this assert:
1. MachineVerifier fails because `$agpr13` is not defined on line 19 below:
``` 1: bb.0.bb:
2: successors: %bb.1(0x80000000); %bb.1(100.00%)
3: liveins: $agpr14, $agpr15, $sgpr12, $sgpr13, $sgpr14, \
4: $sgpr15, $sgpr30, $sgpr31, $sgpr34, $sgpr35, \
5: $sgpr36, $sgpr37, $sgpr38, $sgpr39, $sgpr48, \
6: $sgpr49, $sgpr50, $sgpr51, $sgpr52, $sgpr53, \
7: $sgpr54, $sgpr55, $sgpr64, $sgpr65, $sgpr66, \
8: $sgpr67, $sgpr68, $sgpr69, $sgpr70, $sgpr71, \
9: $sgpr80, $sgpr81, $sgpr82, $sgpr83, $sgpr84, \
10: $sgpr85, $sgpr86, $sgpr87, $sgpr96, $sgpr97, \
11: $sgpr98, $sgpr99, $vgpr0, $vgpr31, $vgpr40, $vgpr41, \
12: $sgpr4_sgpr5, $sgpr6_sgpr7, $sgpr8_sgpr9, \
13: $sgpr10_sgpr11
14: $sgpr16 = COPY $sgpr33
15: $sgpr33 = frame-setup COPY $sgpr32
16: $sgpr18_sgpr19 = S_XOR_SAVEEXEC_B64 -1, \
17: implicit-def $exec, implicit-def dead $scc, \
18: implicit $exec
19: $vgpr63 = V_ACCVGPR_READ_B32_e64 killed $agpr13, \
20: implicit $exec
21: BUFFER_STORE_DWORD_OFFSET killed $vgpr63, \
22: $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr33, 0, 0, 0, \
23: implicit $exec :: (store (s32) into %stack.38, \
24: addrspace 5)
25: ...
26: $vgpr43 = IMPLICIT_DEF
27: $vgpr43 = SI_SPILL_S32_TO_VGPR $sgpr15, 0, \
28: killed $vgpr43(tied-def 0)
29: $vgpr43 = SI_SPILL_S32_TO_VGPR $sgpr14, 1, \
30: killed $vgpr43(tied-def 0)
31: $sgpr100_sgpr101 = S_OR_SAVEEXEC_B64 -1, \
32: implicit-def $exec, implicit-def dead $scc, \
33: implicit $exec
34: renamable $agpr13 = COPY killed $vgpr43, implicit $exec
```
2. That instruction is created by [`emitCSRSpillStores`](https://github.com/llvm/llvm-project/blob/d599bdeaa49d7a2b1246328630328d23ddda5a47/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp#L977) (called by [`SIFrameLowering::emitPrologue`](https://github.com/llvm/llvm-project/blob/d599bdeaa49d7a2b1246328630328d23ddda5a47/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp#L1122)) because `$agpr13` is in `WWMSpills`.
See lines 982, 998, and 993 below.
```
977: // Spill Whole-Wave Mode VGPRs. Save only the inactive lanes of the scratch
978: // registers. However, save all lanes of callee-saved VGPRs. Due to this, we
979: // might end up flipping the EXEC bits twice.
980: Register ScratchExecCopy;
981: SmallVector<std::pair<Register, int>, 2> WWMCalleeSavedRegs, WWMScratchRegs;
982: FuncInfo->splitWWMSpillRegisters(MF, WWMCalleeSavedRegs, WWMScratchRegs);
983: if (!WWMScratchRegs.empty())
984: ScratchExecCopy =
985: buildScratchExecCopy(LiveUnits, MF, MBB, MBBI, DL,
986: /*IsProlog*/ true, /*EnableInactiveLanes*/ true);
987:
988: auto StoreWWMRegisters =
989: [&](SmallVectorImpl<std::pair<Register, int>> &WWMRegs) {
990: for (const auto &Reg : WWMRegs) {
991: Register VGPR = Reg.first;
992: int FI = Reg.second;
993: buildPrologSpill(ST, TRI, *FuncInfo, LiveUnits, MF, MBB, MBBI, DL,
994: VGPR, FI, FrameReg);
995: }
996: };
997:
998: StoreWWMRegisters(WWMScratchRegs);
```
3. `$agpr13` got added to `WWMSpills` by [`SILowerWWMCopies::run`](https://github.com/llvm/llvm-project/blob/59a7185dd9d69cbf737a98f5c2d1cf3d456bee03/llvm/lib/Target/AMDGPU/SILowerWWMCopies.cpp#L137) as it processed the `WWM_COPY` on line 3 below (corresponds to line 34 above in point #_1_):
```
1: %45:vgpr_32 = SI_SPILL_S32_TO_VGPR $sgpr15, 0, %45:vgpr_32(tied-def 0)
2: %45:vgpr_32 = SI_SPILL_S32_TO_VGPR $sgpr14, 1, %45:vgpr_32(tied-def 0)
3: %44:av_32 = WWM_COPY %45:vgpr_32
```
>From bb8aab70f83761523e17037c60eae31084a8e300 Mon Sep 17 00:00:00 2001
From: Matthew Curtis <macurtis at amd.com>
Date: Mon, 30 Jun 2025 16:23:55 -0500
Subject: [PATCH] [AMDGPU] Ensure non-reserved CSR spilled regs are live-in
Fixes:
```
*** Bad machine code: Using an undefined physical register ***
- function: widget
- basic block: %bb.0 bb (0x564092cbe140)
- instruction: $vgpr63 = V_ACCVGPR_READ_B32_e64 killed $agpr13, implicit $exec
- operand 1: killed $agpr13
LLVM ERROR: Found 1 machine code errors.
```
The detailed sequence of events that led to this change:
1. MachineVerifier fails because $agpr13 is not defined on line 19 below:
```
1: bb.0.bb:
2: successors: %bb.1(0x80000000); %bb.1(100.00%)
3: liveins: $agpr14, $agpr15, $sgpr12, $sgpr13, $sgpr14, \
4: $sgpr15, $sgpr30, $sgpr31, $sgpr34, $sgpr35, \
5: $sgpr36, $sgpr37, $sgpr38, $sgpr39, $sgpr48, \
6: $sgpr49, $sgpr50, $sgpr51, $sgpr52, $sgpr53, \
7: $sgpr54, $sgpr55, $sgpr64, $sgpr65, $sgpr66, \
8: $sgpr67, $sgpr68, $sgpr69, $sgpr70, $sgpr71, \
9: $sgpr80, $sgpr81, $sgpr82, $sgpr83, $sgpr84, \
10: $sgpr85, $sgpr86, $sgpr87, $sgpr96, $sgpr97, \
11: $sgpr98, $sgpr99, $vgpr0, $vgpr31, $vgpr40, $vgpr41, \
12: $sgpr4_sgpr5, $sgpr6_sgpr7, $sgpr8_sgpr9, \
13: $sgpr10_sgpr11
14: $sgpr16 = COPY $sgpr33
15: $sgpr33 = frame-setup COPY $sgpr32
16: $sgpr18_sgpr19 = S_XOR_SAVEEXEC_B64 -1, \
17: implicit-def $exec, implicit-def dead $scc, \
18: implicit $exec
19: $vgpr63 = V_ACCVGPR_READ_B32_e64 killed $agpr13, \
20: implicit $exec
21: BUFFER_STORE_DWORD_OFFSET killed $vgpr63, \
22: $sgpr0_sgpr1_sgpr2_sgpr3, $sgpr33, 0, 0, 0, \
23: implicit $exec :: (store (s32) into %stack.38, \
24: addrspace 5)
25: ...
26: $vgpr43 = IMPLICIT_DEF
27: $vgpr43 = SI_SPILL_S32_TO_VGPR $sgpr15, 0, \
28: killed $vgpr43(tied-def 0)
29: $vgpr43 = SI_SPILL_S32_TO_VGPR $sgpr14, 1, \
30: killed $vgpr43(tied-def 0)
31: $sgpr100_sgpr101 = S_OR_SAVEEXEC_B64 -1, \
32: implicit-def $exec, implicit-def dead $scc, \
33: implicit $exec
34: renamable $agpr13 = COPY killed $vgpr43, implicit $exec
```
2. That instruction is created by emitCSRSpillStores (called by
SIFrameLowering::emitPrologue) because $agpr13 is in WWMSpills.
See lines 982, 998, and 993 below.
```
977: // Spill Whole-Wave Mode VGPRs. Save only the inactive lanes of the scratch
978: // registers. However, save all lanes of callee-saved VGPRs. Due to this, we
979: // might end up flipping the EXEC bits twice.
980: Register ScratchExecCopy;
981: SmallVector<std::pair<Register, int>, 2> WWMCalleeSavedRegs, WWMScratchRegs;
982: FuncInfo->splitWWMSpillRegisters(MF, WWMCalleeSavedRegs, WWMScratchRegs);
983: if (!WWMScratchRegs.empty())
984: ScratchExecCopy =
985: buildScratchExecCopy(LiveUnits, MF, MBB, MBBI, DL,
986: /*IsProlog*/ true, /*EnableInactiveLanes*/ true);
987:
988: auto StoreWWMRegisters =
989: [&](SmallVectorImpl<std::pair<Register, int>> &WWMRegs) {
990: for (const auto &Reg : WWMRegs) {
991: Register VGPR = Reg.first;
992: int FI = Reg.second;
993: buildPrologSpill(ST, TRI, *FuncInfo, LiveUnits, MF, MBB, MBBI, DL,
994: VGPR, FI, FrameReg);
995: }
996: };
997:
998: StoreWWMRegisters(WWMScratchRegs);
```
3. $agpr13 got added to WWMSpills by SILowerWWMCopies::runOnMachineFunction
as it processed the WWM_COPY on line 11 below (corresponds to line 34
above in point #1):
```
1: %45:vgpr_32 = SI_SPILL_S32_TO_VGPR $sgpr15, 0, %45:vgpr_32(tied-def 0)
2: %45:vgpr_32 = SI_SPILL_S32_TO_VGPR $sgpr14, 1, %45:vgpr_32(tied-def 0)
3: %44:av_32 = WWM_COPY %45:vgpr_32
```
---
llvm/lib/Target/AMDGPU/SIFrameLowering.cpp | 7 +
.../CodeGen/AMDGPU/bug-undef-spilled-agpr.ll | 126 ++++++++++++++++++
2 files changed, 133 insertions(+)
create mode 100644 llvm/test/CodeGen/AMDGPU/bug-undef-spilled-agpr.ll
diff --git a/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp b/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp
index 6a3867937d57f..15ad1470036d2 100644
--- a/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp
+++ b/llvm/lib/Target/AMDGPU/SIFrameLowering.cpp
@@ -973,6 +973,7 @@ void SIFrameLowering::emitCSRSpillStores(
const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
const SIInstrInfo *TII = ST.getInstrInfo();
const SIRegisterInfo &TRI = TII->getRegisterInfo();
+ MachineRegisterInfo &MRI = MF.getRegInfo();
// Spill Whole-Wave Mode VGPRs. Save only the inactive lanes of the scratch
// registers. However, save all lanes of callee-saved VGPRs. Due to this, we
@@ -995,6 +996,12 @@ void SIFrameLowering::emitCSRSpillStores(
}
};
+ for (const auto &Reg : WWMScratchRegs) {
+ if (!MRI.isReserved(Reg.first)) {
+ MRI.addLiveIn(Reg.first);
+ MBB.addLiveIn(Reg.first);
+ }
+ }
StoreWWMRegisters(WWMScratchRegs);
if (!WWMCalleeSavedRegs.empty()) {
if (ScratchExecCopy) {
diff --git a/llvm/test/CodeGen/AMDGPU/bug-undef-spilled-agpr.ll b/llvm/test/CodeGen/AMDGPU/bug-undef-spilled-agpr.ll
new file mode 100644
index 0000000000000..ef2ae5ca5169b
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/bug-undef-spilled-agpr.ll
@@ -0,0 +1,126 @@
+; Just ensure that llc -O1 does not error out
+; RUN: llc -O1 -mtriple=amdgcn-amd-amdhsa -mcpu=gfx908 -verify-machineinstrs %s -o - &>/dev/null
+
+define fastcc void @widget(i1 %arg) {
+bb:
+ br label %bb1
+
+bb1: ; preds = %bb3, %bb
+ br i1 %arg, label %bb3, label %bb2
+
+bb2: ; preds = %bb1
+ ret void
+
+bb3: ; preds = %bb1
+ %call = call fastcc i1 @baz(i1 false, float 0.000000e+00, i1 false, float 0.000000e+00, i1 false, i1 false, float 0.000000e+00, float 0.000000e+00, float 0.000000e+00, float 0.000000e+00, ptr addrspace(5) null, i1 false, ptr null, ptr null, ptr null, ptr null, ptr null, ptr addrspace(5) null, ptr null, ptr null, ptr null, ptr null, ptr null, ptr null, ptr null, ptr addrspace(5) null)
+ br label %bb1
+}
+
+define fastcc i1 @baz(i1 %arg, float %arg1, i1 %arg2, float %arg3, i1 %arg4, i1 %arg5, float %arg6, float %arg7, float %arg8, float %arg9, ptr addrspace(5) %arg10, i1 %arg11, ptr %arg12, ptr %arg13, ptr %arg14, ptr %arg15, ptr %arg16, ptr addrspace(5) %arg17, ptr %arg18, ptr %arg19, ptr %arg20, ptr %arg21, ptr %arg22, ptr %arg23, ptr %arg24, ptr addrspace(5) %arg25) #0 {
+bb:
+ br i1 %arg, label %bb26, label %bb27
+
+bb26: ; preds = %bb
+ ret i1 false
+
+bb27: ; preds = %bb
+ br i1 %arg, label %bb29, label %bb28
+
+bb28: ; preds = %bb27
+ unreachable
+
+bb29: ; preds = %bb49, %bb47, %bb46, %bb39, %bb36, %bb27
+ br i1 %arg4, label %bb55, label %bb30
+
+bb30: ; preds = %bb29
+ br i1 %arg5, label %bb31, label %bb32
+
+bb31: ; preds = %bb30
+ store i1 false, ptr addrspace(5) null, align 2147483648
+ br label %bb55
+
+bb32: ; preds = %bb30
+ store float %arg3, ptr addrspace(5) null, align 2147483648
+ store float %arg7, ptr addrspace(5) %arg10, align 2147483648
+ br i1 %arg2, label %bb34, label %bb33
+
+bb33: ; preds = %bb32
+ %fcmp = fcmp ogt float %arg6, 0.000000e+00
+ br i1 %fcmp, label %bb34, label %bb35
+
+bb34: ; preds = %bb33, %bb32
+ br i1 %arg11, label %bb37, label %bb36
+
+bb35: ; preds = %bb33
+ store float 0.000000e+00, ptr addrspace(5) null, align 2147483648
+ ret i1 false
+
+bb36: ; preds = %bb34
+ store i32 1, ptr addrspace(5) null, align 2147483648
+ br label %bb29
+
+bb37: ; preds = %bb34
+ %load = load i8, ptr %arg12, align 2
+ %trunc = trunc i8 %load to i1
+ br i1 %trunc, label %bb38, label %bb54
+
+bb38: ; preds = %bb37
+ br i1 %arg4, label %bb39, label %bb53
+
+bb39: ; preds = %bb38
+ store float %arg1, ptr addrspace(5) null, align 2147483648
+ %load40 = load float, ptr %arg15, align 8
+ call void @llvm.memcpy.p5.p0.i64(ptr addrspace(5) %arg25, ptr %arg24, i64 12, i1 false)
+ %load41 = load float, ptr %arg16, align 4
+ call void @llvm.memcpy.p5.p0.i64(ptr addrspace(5) %arg17, ptr null, i64 36, i1 false)
+ %load42 = load float, ptr %arg18, align 4
+ %load43 = load float, ptr %arg19, align 4
+ store float 0x7FF8000000000000, ptr addrspace(5) null, align 2147483648
+ %load44 = load float, ptr %arg14, align 16
+ store float %load44, ptr addrspace(5) null, align 2147483648
+ %fcmp45 = fcmp ole float %arg9, 0.000000e+00
+ br i1 %fcmp45, label %bb29, label %bb46
+
+bb46: ; preds = %bb39
+ %fsub = fsub float %arg8, %load40
+ store float %fsub, ptr addrspace(5) null, align 2147483648
+ %fadd = fadd float %load42, %load43
+ br i1 %arg, label %bb29, label %bb47
+
+bb47: ; preds = %bb46
+ br i1 %arg, label %bb29, label %bb48
+
+bb48: ; preds = %bb47
+ br i1 %arg, label %bb49, label %bb52
+
+bb49: ; preds = %bb48
+ store float 0.000000e+00, ptr %arg23, align 4
+ store float 0.000000e+00, ptr %arg22, align 8
+ store float %fadd, ptr addrspace(5) null, align 2147483648
+ %load50 = load float, ptr %arg20, align 4
+ %fdiv = fdiv float %load41, %load50
+ store float %fdiv, ptr addrspace(5) null, align 2147483648
+ %load51 = load float, ptr %arg13, align 16
+ store float %load51, ptr addrspace(5) null, align 2147483648
+ store float 1.000000e+00, ptr %arg21, align 4
+ br label %bb29
+
+bb52: ; preds = %bb48
+ unreachable
+
+bb53: ; preds = %bb38
+ ret i1 false
+
+bb54: ; preds = %bb37
+ ret i1 true
+
+bb55: ; preds = %bb31, %bb29
+ %load56 = load i1, ptr addrspace(5) null, align 2147483648
+ ret i1 %load56
+}
+
+; Function Attrs: nocallback nofree nounwind willreturn memory(argmem: readwrite)
+declare void @llvm.memcpy.p5.p0.i64(ptr addrspace(5) noalias writeonly captures(none), ptr noalias readonly captures(none), i64, i1 immarg) #1
+
+attributes #0 = { "denormal-fp-math-f32"="preserve-sign,preserve-sign" }
+attributes #1 = { nocallback nofree nounwind willreturn memory(argmem: readwrite) }
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