[RegisterCoalescer] Do not call LiveIntervals::getInstructionIndex with a DBG_VALUE

[bcahoon]

Summary

An assert occurs due to calling getInstructionIndex with a DBG_VALUE instruction because the DBG_VALUE instruction does not have a Slot Index. This occurs in updateRegDefUses.

I'm not sure if this is the correct fix, but there are other places where the call to getInstructionIndex is guarded by a check for DBG_VALUE.

I don't have a reproducible test case because the code that causes the crash is not in-tree. Also, I haven't had any luck reproducing the crash with in-tree code. I tried to generate a test case for ARM because it has composed SubRegs, but I was unsuccessful.

The error occurs, on Hexagon, when introducing a new register class that represents 4 vector registers. The MIR prior to joining the intervals looks like the code below. In the example, the wsub_lo SubReg contains a vsub_hi and vsub_lo register.

%vreg2<def> = load
DBG_VALUE %vreg2, %noreg, !"a",
%vreg4:vsub_lo<def,read-undef> = COPY %vreg2
%vreg6:wsub_lo<def,read-undef> = COPY %vreg4
%vreg6:wsub_hi<def> = COPY %vreg4

After %vreg2 and %vreg4:vsub_lo are coalesced:
...

%vreg4:vsub_lo<def,read-undef> = load
DBG_VALUE %vreg4:vsub_lo<undef>, %noreg, !"a",
%vreg6:wsub_lo<def,read-undef> = COPY %vreg4
%vreg6:wsub_hi<def> = COPY %vreg4

...

The DBG_VALUE that has a sub register results in the call to getInstructionIndex and the assert.

[MatzeB]

At a first glance this looks like it fixes symptoms but not the root cause. It would probably be good if we could get a public reproducer.

[bcahoon]

In D29048#653998, @MatzeB wrote:

At a first glance this looks like it fixes symptoms but not the root cause. It would probably be good if we could get a public reproducer.

I agree with the symptom vs. root cause comment. I also am not sure if I was doing something else that might be problematic. Especially after I was unable to generate a public test case that reproduced the problem. I tried a bunch of different things with ARM since that target has a composable SubRegs without any luck.

[MatzeB]

In D29048#654084, @bcahoon wrote:

In D29048#653998, @MatzeB wrote:

At a first glance this looks like it fixes symptoms but not the root cause. It would probably be good if we could get a public reproducer.

I agree with the symptom vs. root cause comment. I also am not sure if I was doing something else that might be problematic. Especially after I was unable to generate a public test case that reproduced the problem. I tried a bunch of different things with ARM since that target has a composable SubRegs without any luck.

Sometimes you can take an internal/private benchmark and reduce it with bugpoint to reduce it something simple that doesn't resemble the original code anymore but still fails.

As for other targets: hexagon and amdgpu are the only public targets with subregister liveness enabled which I suspect is necessary to trigger this.

I can take a look at this later whether I can spot something obvious, but a reproducer would obviously help...

[MatzeB]

It would also help if you could put up a backtrace with file+linenumbers (debug build).

[bcahoon]

I've created a test case using the AMDGPU target that shows the error. This required a couple of steps. I took an existing AMDGPU lit test, and added debug metadata manually. Then, created MIR for the test by stopping after the TwoAddressInstruction pass. Then, I needed to insert a DBG_VALUE instruction in the correct location. I'm not really sure if it's possible for a DBG_VALUE instruction to appear in the specific place that I put it. But, with these changes, the test case causes an assert because there is a call to getInstructionIndex with a DBG_VALUE insruction.

[kparzysz]

I think the fix is reasonable. This is the dump of the problematic moment (with the fix), including the dump of LIS right before the crashing copy. The crash (assertion: "instruction not found in map") occurred when updating the register in the DBG_VALUE instruction, which indeed does not have a slot index.

********** MACHINEINSTRS **********
# Machine code for function test: NoPHIs, TracksLiveness
Function Live Ins: %SGPR0_SGPR1 in %vreg0, %VGPR0 in %vreg3

0B      BB#0: derived from LLVM BB %0
            Live Ins: %SGPR0_SGPR1 %VGPR0
16B             %vreg19:sub0<def,read-undef> = COPY %VGPR0; VReg_64:%vreg19
32B             %vreg0<def> = COPY %SGPR0_SGPR1; SGPR_64:%vreg0
48B             %vreg13:sub0_sub1<def,read-undef> = S_LOAD_DWORDX2_IMM %vreg0, 9, 0; mem:LD8[undef(addrspace=2)](nontemporal)(dereferenceable)(invariant) SGPR_128:%vreg13 SGPR_64:%vreg0
64B             %vreg5<def> = S_LOAD_DWORD_IMM %vreg0, 13, 0; mem:LD4[undef(addrspace=2)](nontemporal)(dereferenceable)(invariant) SReg_32_XM0_XEXEC:%vreg5 SGPR_64:%vreg0
80B             %vreg19:sub1<def> = V_ASHRREV_I32_e32 31, %vreg19:sub0, %EXEC<imp-use>; VReg_64:%vreg19
128B            %vreg12:sub1<def,read-undef> = S_MOV_B32 61440; SGPR_64:%vreg12
144B            %vreg12:sub0<def> = S_MOV_B32 0; SGPR_64:%vreg12
                DBG_VALUE %vreg12:sub0<undef>, %noreg, !"a", <!11>; SGPR_64:%vreg12 line no:126
208B            %vreg13:sub2_sub3<def> = COPY %vreg12; SGPR_128:%vreg13 SGPR_64:%vreg12
224B            %vreg20<def> = V_LSHL_B64 %vreg19, 2, %EXEC<imp-use>; VReg_64:%vreg20,%vreg19
240B            %vreg16<def> = COPY %vreg5; VGPR_32:%vreg16 SReg_32_XM0_XEXEC:%vreg5
256B            BUFFER_STORE_DWORD_ADDR64 %vreg16, %vreg20, %vreg13, 0, 0, 0, 0, 0, %EXEC<imp-use>; mem:ST4[%gep.out(addrspace=1)] VGPR_32:%vreg16 VReg_64:%vreg20 SGPR_128:%vreg13
272B            S_ENDPGM

# End machine code for function test.

208B    %vreg13:sub2_sub3<def> = COPY %vreg12; SGPR_128:%vreg13 SGPR_64:%vreg12
        Considering merging to SGPR_128 with %vreg12 in %vreg13:sub2_sub3
                RHS = %vreg12 [128r,144r:0)[144r,208r:1)  0@128r 1@144r L00000001 [144r,208r:0)  0@144r L00000002 [128r,208r:0)  0@128r
                LHS = %vreg13 [48r,208r:0)[208r,256r:1)  0@48r 1@208r L00000003 [48r,256r:0)  0@48r L0000000C [208r,256r:0)  0@208r
                merge %vreg13:1@208r into %vreg12:1@144r --> @144r
                LHST = %vreg13 %vreg13 [48r,208r:0)[208r,256r:1)  0@48r 1@208r L00000003 [48r,256r:0)  0@48r L0000000C [208r,256r:0)  0@208r
                Copy+Merge 0000000C into 00000004
                Reduce Lane to 00000008
                merge %vreg13:0@208r into %vreg12:0@144r --> @144r
                joined lanes: [144r,256r:0)  0@144r
                Copy+Merge 00000008 into 00000008
                merge %vreg13:0@208r into %vreg12:0@128r --> @128r
                joined lanes: [128r,256r:0)  0@128r
        Joined SubRanges %vreg13 [48r,208r:0)[208r,256r:1)  0@48r 1@208r L00000004 [144r,256r:0)  0@144r L00000003 [48r,256r:0)  0@48r L00000008 [128r,256r:0)  0@128r
                pruned %vreg13 at 128r: [48r,128r:0)[208r,256r:1)  0@48r 1@208r
                pruned %vreg13 at 144r: [48r,128r:0)[208r,256r:1)  0@48r 1@208r
                erased: 208r    %vreg13:sub2_sub3<def> = COPY %vreg12; SGPR_128:%vreg13 SGPR_64:%vreg12
                restoring liveness to 3 points: 208r,128r,144r:  %vreg13 [48r,128r:0)[128r,144r:1)[144r,256r:2)  0@48r 1@128r 2@144r L00000004 [144r,256r:0)  0@144r L00000003 [48r,256r:0)  0@48r L00000008 [128r,256r:0)  0@12
                updated: 128B   %vreg13:sub3<def> = S_MOV_B32 61440; SGPR_128:%vreg13
                updated: 144B   %vreg13:sub2<def> = S_MOV_B32 0; SGPR_128:%vreg13
[*CRASH*]       updated: DBG_VALUE %vreg13:sub2<undef>, %noreg, !"a", <!11>; SGPR_128:%vreg13 line no:126
        Success: %vreg12:sub2_sub3 -> %vreg13
        Result = %vreg13 [48r,128r:0)[128r,144r:1)[144r,256r:2)  0@48r 1@128r 2@144r L00000004 [144r,256r:0)  0@144r L00000003 [48r,256r:0)  0@48r L00000008 [128r,256r:0)  0@128r

Stack trace from the crash:

llc: /w/src/llvm.org/include/llvm/CodeGen/SlotIndexes.h:410: llvm::SlotIndex llvm::SlotIndexes::getInstructionIndex(const llvm::MachineInstr &) const: Assertion `itr != mi2iMap.end() && "Instruction not found in maps."' failed.
#0 0x0000000001b80dc8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/w/bld/org/bin/llc+0x1b80dc8)
#1 0x0000000001b81426 SignalHandler(int) (/w/bld/org/bin/llc+0x1b81426)
#2 0x00007f63432aa340 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x10340)
#3 0x00007f63421a1cc9 gsignal /build/eglibc-3GlaMS/eglibc-2.19/signal/../nptl/sysdeps/unix/sysv/linux/raise.c:56:0
#4 0x00007f63421a50d8 abort /build/eglibc-3GlaMS/eglibc-2.19/stdlib/abort.c:91:0
#5 0x00007f634219ab86 __assert_fail_base /build/eglibc-3GlaMS/eglibc-2.19/assert/assert.c:92:0
#6 0x00007f634219ac32 (/lib/x86_64-linux-gnu/libc.so.6+0x2fc32)
#7 0x000000000074eb40 llvm::SlotIndexes::getInstructionIndex(llvm::MachineInstr const&) const (/w/bld/org/bin/llc+0x74eb40)
#8 0x000000000145db6d (anonymous namespace)::RegisterCoalescer::updateRegDefsUses(unsigned int, unsigned int, unsigned int) (/w/bld/org/bin/llc+0x145db6d)
#9 0x0000000001458ecc (anonymous namespace)::RegisterCoalescer::joinCopy(llvm::MachineInstr*, bool&) (/w/bld/org/bin/llc+0x1458ecc)
#10 0x00000000014557c6 (anonymous namespace)::RegisterCoalescer::copyCoalesceWorkList(llvm::MutableArrayRef<llvm::MachineInstr*>) (/w/bld/org/bin/llc+0x14557c6)
#11 0x0000000001455548 (anonymous namespace)::RegisterCoalescer::coalesceLocals() (/w/bld/org/bin/llc+0x1455548)
#12 0x0000000001454b3c (anonymous namespace)::RegisterCoalescer::runOnMachineFunction(llvm::MachineFunction&) (/w/bld/org/bin/llc+0x1454b3c)
#13 0x0000000001369c35 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (/w/bld/org/bin/llc+0x1369c35)
#14 0x000000000162f184 llvm::FPPassManager::runOnFunction(llvm::Function&) (/w/bld/org/bin/llc+0x162f184)
#15 0x000000000162f3d3 llvm::FPPassManager::runOnModule(llvm::Module&) (/w/bld/org/bin/llc+0x162f3d3)
#16 0x000000000162f913 llvm::legacy::PassManagerImpl::run(llvm::Module&) (/w/bld/org/bin/llc+0x162f913)
#17 0x0000000000679cca compileModule(char**, llvm::LLVMContext&) (/w/bld/org/bin/llc+0x679cca)
#18 0x0000000000676c5b main (/w/bld/org/bin/llc+0x676c5b)
#19 0x00007f634218cec5 __libc_start_main /build/eglibc-3GlaMS/eglibc-2.19/csu/libc-start.c:321:0
#20 0x0000000000675048 _start (/w/bld/org/bin/llc+0x675048)
Stack dump:
0.      Program arguments: /w/bld/org/bin/llc -march=amdgcn -run-pass simple-register-coalescing -o - x.mir -print-after-all -debug-only=regalloc
1.      Running pass 'Function Pass Manager' on module 'x.mir'.
2.      Running pass 'Simple Register Coalescing' on function '@test'

[MatzeB]

Thanks for providing the testcase, that made investigating this easier. The fix here seems reasonable to me as well.

However I was surprised to see this problem popping up at all, so I dug a bit deeper:

• This problem only occurs for DBG_VALUE undef %X which doesn't make much sense (but is legal)
• The undef flag there is actually produced by code in the same function a bit further down (// A subreg use of a partially undef...) which right now looks like a dangerous bug to me (in that it adds an undef flag here even though the register seems live to me). I will investigate this further.

I'm fine with the patch going in, if the testcase is cleaned up (see below):

test/CodeGen/AMDGPU/regcoalesce-dbg.mir

1 ↗

(On Diff #85981)

Good tests should check some output.
Or put another way: The test should fail if someone replaces llc with /usr/bin/true.

7 ↗

(On Diff #85981)

You should be able to remove the complete IR part from the test and replace it with a dummy define void @test() { ret void }. The only thing you need to do is remove IR back-reference in the MIR part. Which for this case should just be removing the (%ir-block.0) from the bb.0 label.

75–80 ↗

(On Diff #85981)

you should be able to remove alignemnt, exposesReturnsTwice, legalize, regBankSelected, selected.

106–119 ↗

(On Diff #85981)

You can probably remove the frameInfo completely with the test still working.

[bcahoon]

Thanks for the comments - much appreciated. I'll update the test case and then push.

Brendon

llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp

if (SrcReg == DstReg && !Visited.insert(UseMI).second)
       continue;
 
     SmallVector<unsigned,8> Ops;
     bool Reads, Writes;
     std::tie(Reads, Writes) = UseMI->readsWritesVirtualRegister(SrcReg, &Ops);
 
     // If SrcReg wasn't read, it may still be the case that DstReg is live-in
     // because SrcReg is a sub-register.
-    if (DstInt && !Reads && SubIdx)
+    if (DstInt && !Reads && SubIdx && !UseMI->isDebugValue())
       Reads = DstInt->liveAt(LIS->getInstructionIndex(*UseMI));
 
     // Replace SrcReg with DstReg in all UseMI operands.
     for (unsigned i = 0, e = Ops.size(); i != e; ++i) {
       MachineOperand &MO = UseMI->getOperand(Ops[i]);
 
       // Adjust <undef> flags in case of sub-register joins. We don't want to
       // turn a full def into a read-modify-write sub-register def and vice

llvm/trunk/test/CodeGen/AMDGPU/regcoalesce-dbg.mir

+# RUN: llc -march=amdgcn -run-pass simple-register-coalescing -o - %s | FileCheck %s
+
+# Test that register coalescing does not allow a call to
+# LIS->getInstructionIndex with a DBG_VALUE instruction, which does not have
+# a slot index.
+
+# CHECK: %13.sub2 = S_MOV_B32 0
+# CHECK: DBG_VALUE{{.*}}debug-use %13.sub2
+
+--- |
+  define void @test(i32 addrspace(1)* %out) { ret void }
+  
+  !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !4, producer: "llvm", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !4, retainedTypes: !4)
+  !1 = !DILocalVariable(name: "a", scope: !2, file: !4, line: 126, type: !6)
+  !2 = distinct !DISubprogram(name: "test", scope: !4, file: !4, line: 1, type: !3, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !5)
+  !3 = !DISubroutineType(types: !4)
+  !4 = !{null}
+  !5 = !{!1}
+  !6 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !7, size: 64, align: 32)
+  !7 = !DIBasicType(name: "int", size: 32, align: 32, encoding: DW_ATE_signed)
+  !8 = !DIExpression()
+  !9 = !DILocation(line: 126, column: 9, scope: !2)
+
+...
+---
+name:            test
+tracksRegLiveness: true
+registers:       
+  - { id: 0, class: sgpr_64 }
+  - { id: 1, class: sreg_32_xm0 }
+  - { id: 2, class: sgpr_32 }
+  - { id: 3, class: vgpr_32 }
+  - { id: 4, class: sreg_64_xexec }
+  - { id: 5, class: sreg_32_xm0_xexec }
+  - { id: 6, class: sreg_32 }
+  - { id: 7, class: sreg_32 }
+  - { id: 8, class: sreg_32_xm0 }
+  - { id: 9, class: sreg_64 }
+  - { id: 10, class: sreg_32_xm0 }
+  - { id: 11, class: sreg_32_xm0 }
+  - { id: 12, class: sgpr_64 }
+  - { id: 13, class: sgpr_128 }
+  - { id: 14, class: sreg_32_xm0 }
+  - { id: 15, class: sreg_64 }
+  - { id: 16, class: vgpr_32 }
+  - { id: 17, class: vreg_64 }
+  - { id: 18, class: vgpr_32 }
+  - { id: 19, class: vreg_64 }
+  - { id: 20, class: vreg_64 }
+liveins:         
+  - { reg: '%sgpr0_sgpr1', virtual-reg: '%0' }
+  - { reg: '%vgpr0', virtual-reg: '%3' }
+body:             |
+  bb.0:
+    liveins: %sgpr0_sgpr1, %vgpr0
+  
+    %3 = COPY killed %vgpr0
+    %0 = COPY killed %sgpr0_sgpr1
+    %4 = S_LOAD_DWORDX2_IMM %0, 9, 0 :: (non-temporal dereferenceable invariant load 8 from `i64 addrspace(2)* undef`)
+    %5 = S_LOAD_DWORD_IMM killed %0, 13, 0 :: (non-temporal dereferenceable invariant load 4 from `i32 addrspace(2)* undef`)
+    %18 = V_ASHRREV_I32_e32 31, %3, implicit %exec
+    undef %19.sub0 = COPY killed %3
+    %19.sub1 = COPY killed %18
+    %10 = S_MOV_B32 61440
+    %11 = S_MOV_B32 0
+    DBG_VALUE debug-use %11, debug-use _, !1, !8, debug-location !9
+    undef %12.sub0 = COPY killed %11
+    %12.sub1 = COPY killed %10
+    undef %13.sub0_sub1 = COPY killed %4
+    %13.sub2_sub3 = COPY killed %12
+    %20 = V_LSHL_B64 killed %19, 2, implicit %exec
+    %16 = COPY killed %5
+    BUFFER_STORE_DWORD_ADDR64 killed %16, killed %20, killed %13, 0, 0, 0, 0, 0, implicit %exec :: (store 4 into %ir.out)
+    S_ENDPGM
+
+...