[DebugInfo][InstCombine] Prefer to salvage debuginfo over sinking it

When instcombine sinks an instruction between two basic blocks, it sinks any
dbg.value users in the source block with it, to prevent debug use-before-free.
However we can do better by attempting to salvage the debug users, which would
avoid moving where the variable location changes. If we successfully salvage,
still sink a (cloned) dbg.value with the sunk instruction, as the sunk
instruction is more likely to be "live" later in the compilation process.

If we can't salvage dbg.value users of a sunk instruction, mark the dbg.values
in the original block as being undef. This terminates any earlier variable
location range, and represents the fact that we've optimized out the variable
location for a portion of the program.

Differential Revision: https://reviews.llvm.org/D56788

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353936 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstructionCombining.cpp b/lib/Transforms/InstCombine/InstructionCombining.cpp
index 4d04e3ff99e441a360e582e74eb777df2237c894..8cbf57a97ef7e8d86ac26280bf904ab718d8fbdc 100644 (file)
--- a/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3099,13 +3099,35 @@ static bool TryToSinkInstruction(Instruction *I, BasicBlock *DestBlock) {
   ++NumSunkInst;
 
   // Also sink all related debug uses from the source basic block. Otherwise we
-  // get debug use before the def.
-  SmallVector<DbgVariableIntrinsic *, 1> DbgUsers;
+  // get debug use before the def. Attempt to salvage debug uses first, to
+  // maximise the range variables have location for. If we cannot salvage, then
+  // mark the location undef: we know it was supposed to receive a new location
+  // here, but that computation has been sunk.
+  SmallVector<DbgVariableIntrinsic *, 2> DbgUsers;
   findDbgUsers(DbgUsers, I);
-  for (auto *DII : DbgUsers) {
+  for (auto *DII : reverse(DbgUsers)) {
     if (DII->getParent() == SrcBlock) {
-      DII->moveBefore(&*InsertPos);
-      LLVM_DEBUG(dbgs() << "SINK: " << *DII << '\n');
+      // dbg.value is in the same basic block as the sunk inst, see if we can
+      // salvage it. Clone a new copy of the instruction: on success we need
+      // both salvaged and unsalvaged copies.
+      SmallVector<DbgVariableIntrinsic *, 1> TmpUser{
+          cast<DbgVariableIntrinsic>(DII->clone())};
+
+      if (!salvageDebugInfoForDbgValues(*I, TmpUser)) {
+        // We are unable to salvage: sink the cloned dbg.value, and mark the
+        // original as undef, terminating any earlier variable location.
+        LLVM_DEBUG(dbgs() << "SINK: " << *DII << '\n');
+        TmpUser[0]->insertBefore(&*InsertPos);
+        Value *Undef = UndefValue::get(I->getType());
+        DII->setOperand(0, MetadataAsValue::get(DII->getContext(),
+                                                ValueAsMetadata::get(Undef)));
+      } else {
+        // We successfully salvaged: place the salvaged dbg.value in the
+        // original location, and move the unmodified dbg.value to sink with
+        // the sunk inst.
+        TmpUser[0]->insertBefore(DII);
+        DII->moveBefore(&*InsertPos);
+      }
     }
   }
   return true;

diff --git a/test/Transforms/InstCombine/debuginfo-sink.ll b/test/Transforms/InstCombine/debuginfo-sink.ll
new file mode 100644 (file)
index 0000000..1e62777
--- /dev/null
+++ b/test/Transforms/InstCombine/debuginfo-sink.ll
@@ -0,0 +1,78 @@
+; RUN: opt  %s -instcombine -S | FileCheck %s
+
+; Test sinking of dbg.values when instcombine sinks associated instructions.
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+; This GEP is sunk, but can be folded into a DIExpression. Check that it
+; gets folded. The dbg.value should be duplicated in the block its sunk
+; into, to maximise liveness.
+;
+; CHECK-LABEL: define i32 @foo(i32*
+; CHECK:       call void @llvm.dbg.value(metadata i32* %a, metadata !{{[0-9]+}},
+; CHECK-SAME:  metadata !DIExpression(DW_OP_plus_uconst, 4, DW_OP_stack_value))
+; CHECK-NEXT:  br label %sink1
+
+define i32 @foo(i32 *%a) !dbg !7 {
+entry:
+  %gep = getelementptr i32, i32 *%a, i32 1
+  call void @llvm.dbg.value(metadata i32 *%gep, metadata !16, metadata !12), !dbg !15
+  br label %sink1
+
+sink1:
+; CHECK-LABEL: sink1:
+; CHECK:       call void @llvm.dbg.value(metadata i32* %gep,
+; CHECK-SAME:                    metadata !{{[0-9]+}}, metadata !DIExpression())
+; CHECK-NEXT:  load
+  %0 = load i32, i32* %gep, align 4, !dbg !15
+  ret i32 %0, !dbg !15
+}
+
+; In this example the GEP cannot (yet) be salvaged. Check that not only is the
+; dbg.value sunk, but an undef dbg.value is left to terminate any earlier
+; value range.
+
+; CHECK-LABEL: define i32 @bar(
+; CHECK:       call void @llvm.dbg.value(metadata i32* undef,
+; CHECK-NEXT:  br label %sink2
+
+define i32 @bar(i32 *%a, i32 %b) !dbg !70 {
+entry:
+  %gep = getelementptr i32, i32 *%a, i32 %b
+  call void @llvm.dbg.value(metadata i32* %gep, metadata !73, metadata !12), !dbg !74
+  br label %sink2
+
+sink2:
+; CHECK-LABEL: sink2:
+; CHECK:       call void @llvm.dbg.value(metadata i32* %gep,
+; CHECK-SAME:                    metadata !{{[0-9]+}}, metadata !DIExpression())
+; CHECK-NEXT:  load
+; CHECK-NEXT:  ret
+  %0 = load i32, i32* %gep
+  ret i32 %0
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "a.c", directory: ".")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"PIC Level", i32 2}
+!6 = !{!"clang"}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 2, type: !8, isLocal: false, isDefinition: true, scopeLine: 3, flags: DIFlagPrototyped, isOptimized: false, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10, !10}
+!10 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!11 = !DILocalVariable(name: "j", scope: !7, file: !1, line: 2, type: !10)
+!12 = !DIExpression()
+!15 = !DILocation(line: 5, column: 3, scope: !7)
+!16 = !DILocalVariable(name: "h", scope: !7, file: !1, line: 4, type: !10)
+!70 = distinct !DISubprogram(name: "bar", scope: !1, file: !1, line: 2, type: !71, isLocal: false, isDefinition: true, scopeLine: 3, flags: DIFlagPrototyped, isOptimized: false, unit: !0, retainedNodes: !2)
+!71 = !DISubroutineType(types: !72)
+!72 = !{!10, !10, !10}
+!73 = !DILocalVariable(name: "k", scope: !70, file: !1, line: 2, type: !10)
+!74 = !DILocation(line: 5, column: 3, scope: !70)