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[BranchFolding] Merge debug locs on common hoisted code (#140063)
branch-folder hoists common instructions from TBB and FBB into their pred. Without this patch it achieves this by splicing the instructions from TBB and deleting the common ones in FBB. That moves the debug locations and debug instructions from TBB into the pred without modification, which is not ideal. The merged instructions should get merged debug locations for debugging and PGO purposes, which is handled in this patch. Debug instructions also need to be handled differently. That'll come in another patch. This issue was found by @omern1.
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‎llvm/lib/CodeGen/BranchFolding.cpp

Lines changed: 32 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -2071,7 +2071,38 @@ bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
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if (!HasDups)
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return false;
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2074-
MBB->splice(Loc, TBB, TBB->begin(), TIB);
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// Hoist the instructions from [T.begin, TIB) and then delete [F.begin, FIB).
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// Merge the debug locations. FIXME: We should do something with the
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// debug instructions too (from BOTH branches).
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{
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// TIB and FIB point to the end of the regions to hoist/merge in TBB and
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// FBB.
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MachineBasicBlock::iterator FE = FIB;
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MachineBasicBlock::iterator FI = FBB->begin();
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for (MachineBasicBlock::iterator TI :
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make_early_inc_range(make_range(TBB->begin(), TIB))) {
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// Move debug instructions and pseudo probes without modifying them.
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// FIXME: This is the wrong thing to do for debug locations, which
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// should at least be killed.
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if (TI->isDebugOrPseudoInstr()) {
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TI->moveBefore(&*Loc);
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continue;
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}
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// Get the next non-meta instruction in FBB.
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FI = skipDebugInstructionsForward(FI, FE, false);
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// NOTE: The loop above checks CheckKillDead but we can't do that here as
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// it modifies some kill markers after the check.
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assert(TI->isIdenticalTo(*FI, MachineInstr::CheckDefs) &&
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"Expected non-debug lockstep");
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// Merge debug locs on hoisted instructions.
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TI->setDebugLoc(
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DILocation::getMergedLocation(TI->getDebugLoc(), FI->getDebugLoc()));
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TI->moveBefore(&*Loc);
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++FI;
2104+
}
2105+
}
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FBB->erase(FBB->begin(), FIB);
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20772108
if (UpdateLiveIns)

‎llvm/test/DebugInfo/X86/branch-folder-dbg.mir

Lines changed: 111 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,111 @@
1+
# RUN: llc %s --start-before=branch-folder --stop-after=branch-folder -o - | FileCheck %s
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## Check that common instructions hoisted from `if.then` and `if.else` into
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## common pred `entry` get merged debug locations.
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## FIXME: The debug instructions handling here is wrong.
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# CHECK: bb.0
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# CHECK: CALL64pcrel32 @f, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
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## --- Start splice from bb.2.if.else ---
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# CHECK-NEXT: DBG_VALUE 2, $noreg, ![[#]], !DIExpression(), debug-location ![[#]]
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# CHECK-NEXT: $edi = MOV32r0 implicit-def dead $eflags, debug-location !DILocation(line: 0, scope: ![[#]])
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## --- End splice --------------
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# CHECK-NEXT: TEST64rr killed renamable $rax, renamable $rax, implicit-def $eflags
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# CHECK-NEXT: JCC_1 %bb.2, 9, implicit killed $eflags
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# CHECK: bb.1
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--- |
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target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
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target triple = "x86_64-unknown-linux-gnu"
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declare dso_local noundef i64 @f() local_unnamed_addr
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define dso_local noundef i32 @g() local_unnamed_addr !dbg !7 {
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%call = tail call noundef i64 @f()
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%cmp1 = icmp sgt i64 0, %call
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%conv2 = trunc i64 0 to i32
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br i1 %cmp1, label %if.then, label %if.else
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if.then: ; preds = %0
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#dbg_value(i64 0, !11, !DIExpression(), !13)
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tail call void @_Z3fooii(i32 noundef %conv2, i32 noundef 0), !dbg !14
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#dbg_value(i64 1, !11, !DIExpression(), !13)
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br label %if.end, !dbg !15
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if.else: ; preds = %0
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#dbg_value(i64 2, !11, !DIExpression(), !13)
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tail call void @_Z3barii(i32 noundef %conv2, i32 noundef 1), !dbg !16
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#dbg_value(i64 3, !11, !DIExpression(), !13)
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br label %if.end, !dbg !17
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if.end: ; preds = %if.else, %if.then
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ret i32 2
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}
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declare void @_Z3fooii(i32 noundef, i32 noundef) local_unnamed_addr
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declare void @_Z3barii(i32 noundef, i32 noundef) local_unnamed_addr
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!llvm.module.flags = !{!0, !1}
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!llvm.ident = !{!2}
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!llvm.dbg.cu = !{!3}
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!llvm.debugify = !{!5, !6}
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!0 = !{i32 7, !"Dwarf Version", i32 5}
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!1 = !{i32 2, !"Debug Info Version", i32 3}
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!2 = !{!"clang version 21.0.0"}
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!3 = distinct !DICompileUnit(language: DW_LANG_C, file: !4, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
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!4 = !DIFile(filename: "test.nodbg.ll", directory: "/")
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!5 = !{i32 15}
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!6 = !{i32 7}
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!7 = distinct !DISubprogram(name: "g", linkageName: "g", scope: null, file: !4, line: 1, type: !8, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !3, retainedNodes: !10)
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!8 = !DISubroutineType(types: !9)
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!9 = !{}
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!10 = !{!11}
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!11 = !DILocalVariable(name: "1", scope: !7, file: !4, line: 3, type: !12)
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!12 = !DIBasicType(name: "ty64", size: 64, encoding: DW_ATE_unsigned)
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!13 = !DILocation(line: 3, column: 1, scope: !7)
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!14 = !DILocation(line: 9, column: 1, scope: !7)
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!15 = !DILocation(line: 10, column: 1, scope: !7)
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!16 = !DILocation(line: 11, column: 1, scope: !7)
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!17 = !DILocation(line: 12, column: 1, scope: !7)
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...
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---
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name: g
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body: |
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bb.0 (%ir-block.0):
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successors: %bb.1(0x40000000), %bb.2(0x40000000)
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frame-setup PUSH64r undef $rax, implicit-def $rsp, implicit $rsp
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frame-setup CFI_INSTRUCTION def_cfa_offset 16
82+
CALL64pcrel32 @f, csr_64, implicit $rsp, implicit $ssp, implicit-def $rsp, implicit-def $ssp, implicit-def $rax
83+
TEST64rr killed renamable $rax, renamable $rax, implicit-def $eflags
84+
JCC_1 %bb.2, 9, implicit killed $eflags
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JMP_1 %bb.1
86+
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bb.1.if.then:
88+
successors: %bb.3(0x80000000)
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DBG_VALUE 0, $noreg, !11, !DIExpression(), debug-location !13
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$edi = MOV32r0 implicit-def dead $eflags, debug-location !14
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$esi = MOV32r0 implicit-def dead $eflags, debug-location !14
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CALL64pcrel32 target-flags(x86-plt) @_Z3fooii, csr_64, implicit $rsp, implicit $ssp, implicit killed $edi, implicit killed $esi, implicit-def $rsp, implicit-def $ssp, debug-location !14
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DBG_VALUE 1, $noreg, !11, !DIExpression(), debug-location !13
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JMP_1 %bb.3, debug-location !15
96+
97+
bb.2.if.else:
98+
successors: %bb.3(0x80000000)
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DBG_VALUE 2, $noreg, !11, !DIExpression(), debug-location !13
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$edi = MOV32r0 implicit-def dead $eflags, debug-location !16
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$esi = MOV32ri 1, debug-location !16
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CALL64pcrel32 target-flags(x86-plt) @_Z3barii, csr_64, implicit $rsp, implicit $ssp, implicit killed $edi, implicit killed $esi, implicit-def $rsp, implicit-def $ssp, debug-location !16
104+
DBG_VALUE 3, $noreg, !11, !DIExpression(), debug-location !13
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106+
bb.3.if.end:
107+
$eax = MOV32ri 2
108+
$rcx = frame-destroy POP64r implicit-def $rsp, implicit $rsp
109+
frame-destroy CFI_INSTRUCTION def_cfa_offset 8
110+
RET 0, $eax
111+
...

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