Fix out-of-order stepping behavior in programs with sunk instructions.

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 is no longer consistent with the
original source code. In order to ensure correct single-stepping and
profiling, the debug location for sunk instructions is either merged
with the insertion point or erased if the target successor block is
empty.

Patch by Matthew Voss!

Differential Revision: https://reviews.llvm.org/D39933

llvm-svn: 318679

llvm/lib/CodeGen/MachineSink.cpp

@@ -38,6 +38,7 @@
 #include "llvm/CodeGen/TargetSubtargetInfo.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 @@ bool MachineSinking::SinkInstruction(MachineInstr &MI, bool &SawStore,
   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));

llvm/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)