[PATCH] D39933: Summary: Fix out-of-order stepping behavior in programs with sunk instructions

[David Blaikie via llvm-commits]

Should this code have more test coverage? Looks like it's only looking for
the absence of a location. Should there be testing for the merge case as
well?

On Fri, Nov 10, 2017 at 5:11 PM Matthew Voss via Phabricator <
reviews at reviews.llvm.org> wrote:

> ormris created this revision.
>
> MachineSink attempts to place instructions near the basic blocks where
> they are needed.
>
> Once an instruction has been sunk, its location relative to other
> instructions no longer is consistent with the original source code. In
> order to ensure correct stepping in the debugger, the debug location for
> sunk instructions is either merged with the insertion point or erased if
> the
> target successor block is empty.
>
>
> https://reviews.llvm.org/D39933
>
> Files:
>   lib/CodeGen/MachineSink.cpp
>   test/CodeGen/X86/machinesink-debuginfo.ll
>
>
> Index: test/CodeGen/X86/machinesink-debuginfo.ll
> ===================================================================
> --- /dev/null
> +++ test/CodeGen/X86/machinesink-debuginfo.ll
> @@ -0,0 +1,49 @@
> +; RUN: llc -O2 -o - < %s | FileCheck %s
> +
> +; ModuleID = 'test-sink-debug.cpp'
> +source_filename = "test-sink-debug.cpp"
> +target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> +target triple = "x86_64-unknown-linux-gnu"
> +
> +; Function Attrs: nounwind readnone uwtable
> +define double @_Z3fooddb(double %x, double %y, i1 zeroext %c)
> local_unnamed_addr !dbg !7 {
> +  tail call void @llvm.dbg.value(metadata double %x, metadata !13,
> metadata !DIExpression()), !dbg !16
> +  tail call void @llvm.dbg.value(metadata double %y, metadata !14,
> metadata !DIExpression()), !dbg !17
> +  tail call void @llvm.dbg.value(metadata i1 %c, metadata !15, metadata
> !DIExpression()), !dbg !18
> +  %a = fdiv double %x, 3.000000e+00
> +  %b = fdiv double %y, 5.000000e+00, !dbg !21
> +  %cond = select i1 %c,  double %a, double %b
> +; CHECK-NOT: .loc 1 2 26
> +  ret double %cond, !dbg !22
> +}
> +
> +; Function Attrs: nounwind readnone speculatable
> +declare void @llvm.dbg.value(metadata, metadata, metadata) #1
> +
> +attributes #1 = { nounwind readnone speculatable }
> +
> +!llvm.dbg.cu = !{!0}
> +!llvm.module.flags = !{!3, !4, !5}
> +!llvm.ident = !{!6}
> +
> +!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1,
> producer: "clang version 6.0.0 (trunk 313291)", isOptimized: true,
> runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
> +!1 = !DIFile(filename: "test-sink-debug.cpp", directory: "/tmp")
> +!2 = !{}
> +!3 = !{i32 2, !"Dwarf Version", i32 4}
> +!4 = !{i32 2, !"Debug Info Version", i32 3}
> +!5 = !{i32 1, !"wchar_size", i32 4}
> +!6 = !{!"clang version 6.0.0 (trunk 313291)"}
> +!7 = distinct !DISubprogram(name: "foo", linkageName: "_Z3fooddb", scope:
> !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true,
> scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0,
> variables: !12)
> +!8 = !DISubroutineType(types: !9)
> +!9 = !{!10, !10, !10, !11}
> +!10 = !DIBasicType(name: "double", size: 64, encoding: DW_ATE_float)
> +!11 = !DIBasicType(name: "bool", size: 8, encoding: DW_ATE_boolean)
> +!12 = !{!13, !14, !15}
> +!13 = !DILocalVariable(name: "x", arg: 1, scope: !7, file: !1, line: 1,
> type: !10)
> +!14 = !DILocalVariable(name: "y", arg: 2, scope: !7, file: !1, line: 1,
> type: !10)
> +!15 = !DILocalVariable(name: "c", arg: 3, scope: !7, file: !1, line: 1,
> type: !11)
> +!16 = !DILocation(line: 1, column: 19, scope: !7)
> +!17 = !DILocation(line: 1, column: 29, scope: !7)
> +!18 = !DILocation(line: 1, column: 37, scope: !7)
> +!21 = !DILocation(line: 2, column: 26, scope: !7)
> +!22 = !DILocation(line: 2, column: 3, scope: !7)
> Index: lib/CodeGen/MachineSink.cpp
> ===================================================================
> --- lib/CodeGen/MachineSink.cpp
> +++ lib/CodeGen/MachineSink.cpp
> @@ -36,6 +36,7 @@
>  #include "llvm/CodeGen/TargetInstrInfo.h"
>  #include "llvm/IR/BasicBlock.h"
>  #include "llvm/IR/LLVMContext.h"
> +#include "llvm/IR/DebugInfoMetadata.h"
>  #include "llvm/Pass.h"
>  #include "llvm/Support/BranchProbability.h"
>  #include "llvm/Support/CommandLine.h"
> @@ -868,6 +869,16 @@
>    SmallVector<MachineInstr *, 2> DbgValuesToSink;
>    collectDebugValues(MI, DbgValuesToSink);
>
> +  // Merge or erase debug location to ensure consistent stepping in
> profilers
> +  // and debuggers.
> +  if (!SuccToSinkTo->empty()) {
> +    MI.setDebugLoc(DILocation::getMergedLocation(MI.getDebugLoc(),
> +
>  InsertPos->getDebugLoc()));
> +  } else {
> +    MI.setDebugLoc(DebugLoc());
> +  }
> +
> +
>    // Move the instruction.
>    SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
>                         ++MachineBasicBlock::iterator(MI));
>
>
>
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