[InstCombine] Salvage debug info during initial DCE

InstCombine salvages debug info for every instruction it erases from its
worklist, but it wasn't doing it during its initial DCE when populating
its worklist. This fixes that.

This should help improve availability of 'this' in optimized debug info
when casts are necessary.

llvm-svn: 318320

1 files changed, 70 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/debuginfo-dce2.ll b/llvm/test/Transforms/InstCombine/debuginfo-dce2.ll
new file mode 100644
index 00000000000..398f3c6d155
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/debuginfo-dce2.ll
@@ -0,0 +1,70 @@
+; RUN: opt -instcombine -S %s -o - | FileCheck %s
+
+; In this example, the cast from i8* to i32* becomes trivially dead. We should
+; salvage its debug info.
+
+; C source:
+; void use_as_void(void *);
+; void f(void *p) {
+;   int *q = (int *)p;
+;   use_as_void(q);
+; }
+
+; ModuleID = '<stdin>'
+source_filename = "t.c"
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc19.11.25508"
+
+; Function Attrs: nounwind uwtable
+define void @f(i8* %p) !dbg !11 {
+entry:
+  call void @llvm.dbg.value(metadata i8* %p, metadata !16, metadata !DIExpression()), !dbg !18
+  %0 = bitcast i8* %p to i32*, !dbg !19
+  call void @llvm.dbg.value(metadata i32* %0, metadata !17, metadata !DIExpression()), !dbg !20
+  %1 = bitcast i32* %0 to i8*, !dbg !21
+  call void @use_as_void(i8* %1), !dbg !22
+  ret void, !dbg !23
+}
+
+; CHECK-LABEL: define void @f(i8* %p)
+; CHECK: call void @llvm.dbg.value(metadata i8* %p, metadata ![[P_VAR:[0-9]+]], metadata !DIExpression())
+; CHECK-NOT: bitcast
+; CHECK: call void @llvm.dbg.value(metadata i8* %p, metadata ![[Q_VAR:[0-9]+]], metadata !DIExpression())
+; CHECK-NOT: bitcast
+; CHECK ret void
+
+; CHECK: ![[P_VAR]] = !DILocalVariable(name: "p", {{.*}})
+; CHECK: ![[Q_VAR]] = !DILocalVariable(name: "q", {{.*}})
+
+declare void @use_as_void(i8*)
+
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!6, !7, !8, !9}
+!llvm.ident = !{!10}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 6.0.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, retainedTypes: !3)
+!1 = !DIFile(filename: "t.c", directory: "C:\5Csrc\5Cllvm-project\5Cbuild", checksumkind: CSK_MD5, checksum: "56c40617ada23a8cccbd9a16bcec57af")
+!2 = !{}
+!3 = !{!4}
+!4 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !5, size: 64)
+!5 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!6 = !{i32 2, !"CodeView", i32 1}
+!7 = !{i32 2, !"Debug Info Version", i32 3}
+!8 = !{i32 1, !"wchar_size", i32 2}
+!9 = !{i32 7, !"PIC Level", i32 2}
+!10 = !{!"clang version 6.0.0 "}
+!11 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 2, type: !12, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !15)
+!12 = !DISubroutineType(types: !13)
+!13 = !{null, !14}
+!14 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: null, size: 64)
+!15 = !{!16, !17}
+!16 = !DILocalVariable(name: "p", arg: 1, scope: !11, file: !1, line: 2, type: !14)
+!17 = !DILocalVariable(name: "q", scope: !11, file: !1, line: 3, type: !4)
+!18 = !DILocation(line: 2, column: 14, scope: !11)
+!19 = !DILocation(line: 3, column: 12, scope: !11)
+!20 = !DILocation(line: 3, column: 8, scope: !11)
+!21 = !DILocation(line: 4, column: 15, scope: !11)
+!22 = !DILocation(line: 4, column: 3, scope: !11)
+!23 = !DILocation(line: 5, column: 1, scope: !11)