[codeview] Use the first valid source location at the top of every MBB

If the instructions at the beginning of the block have no location,
we're better off using the location of the first instruction in the
current basic block. At the very least, that instruction post-dominates
this one, whereas if we don't emit a .cv_loc directive, we end up using
the potentially invalid location that falls through from the previous
block.

We could probably do better here by emitting some kind of ".cv_loc end"
directive that stops the line table entry of the previous .cv_loc
directive from bleeding out of its basic block. This would improve the
line table when an entire MBB has no valid location info.

llvm-svn: 306889

2 files changed, 98 insertions, 1 deletions

diff --git a/llvm/test/DebugInfo/COFF/lines-bb-start.ll b/llvm/test/DebugInfo/COFF/lines-bb-start.ll
new file mode 100644
index 00000000000..249b38d3499
--- /dev/null
+++ b/llvm/test/DebugInfo/COFF/lines-bb-start.ll
@@ -0,0 +1,97 @@
+; RUN: llc -O0 < %s | FileCheck %s
+
+source_filename = "t.c"
+target datalayout = "e-m:x-p:32:32-i64:64-f80:32-n8:16:32-a:0:32-S32"
+target triple = "i386-pc-windows-msvc19.0.24215"
+
+@str_const = internal unnamed_addr constant [4 x i8] c"str\00", align 1
+
+declare i32 @puts(i8*)
+
+; We had a line info quality issue where the LEA for the string constant had no
+; location info, so the .cv_loc directive appeared after it. Now we have logic
+; that tries to emit the first valid location to the top of each MBB.
+
+define void @lea_str_loc(i1 %cond) !dbg !8 {
+entry:
+  br i1 %cond, label %if.then, label %if.end, !dbg !17
+
+if.then:                                          ; preds = %entry
+  br label %return, !dbg !18
+
+if.end:                                           ; preds = %entry
+  %call = call i32 @puts(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @str_const, i32 0, i32 0)), !dbg !19
+  br label %return, !dbg !20
+
+return:                                           ; preds = %if.end, %if.then
+  ret void, !dbg !20
+}
+
+; The t.c:5 line marker should appear immediately after the BB label.
+
+; CHECK-LABEL: _lea_str_loc:
+; CHECK:         .cv_loc {{.*}} # t.c:4:5
+; CHECK:         jmp     LBB{{.*}}
+; CHECK: LBB0_{{.*}}:                                 # %if.end
+; CHECK-NEXT:    .cv_loc {{.*}} # t.c:5:3
+; CHECK-NEXT:    leal    _str_const, %[[reg:[^ ]*]]
+; CHECK-NEXT:    movl    %[[reg]], (%esp)
+; CHECK-NEXT:    calll   _puts
+
+define void @instr_no_loc(i1 %cond) !dbg !21 {
+entry:
+  br i1 %cond, label %if.then, label %if.end, !dbg !22
+
+if.then:                                          ; preds = %entry
+  br label %return, !dbg !23
+
+if.end:                                           ; preds = %entry
+  call void asm sideeffect "nop", ""()
+  %call = call i32 @puts(i8* getelementptr inbounds ([4 x i8], [4 x i8]* @str_const, i32 0, i32 0)), !dbg !24
+  br label %return, !dbg !25
+
+return:                                           ; preds = %if.end, %if.then
+  ret void, !dbg !25
+}
+
+; CHECK-LABEL: _instr_no_loc:
+; CHECK:         .cv_loc {{.*}} # t.c:4:5
+; CHECK:         jmp     LBB{{.*}}
+; CHECK: LBB1_{{.*}}:                                 # %if.end
+; CHECK-NEXT:    .cv_loc {{.*}} # t.c:5:3
+; CHECK-NEXT:    #APP
+; CHECK-NEXT:    nop
+; CHECK-NEXT:    #NO_APP
+; CHECK-NEXT:    leal    _str_const, %[[reg:[^ ]*]]
+; CHECK-NEXT:    movl    %[[reg]], (%esp)
+; CHECK-NEXT:    calll   _puts
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 5.0.0 ", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "t.c", directory: "C:\5Csrc\5Cllvm-project\5Cbuild", checksumkind: CSK_MD5, checksum: "b32df088e991f1996b4e4deb3855c14b")
+!2 = !{}
+!3 = !{i32 1, !"NumRegisterParameters", i32 0}
+!4 = !{i32 2, !"CodeView", i32 1}
+!5 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = !{i32 1, !"wchar_size", i32 2}
+!7 = !{!"clang version 5.0.0 "}
+!8 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 2, type: !9, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: false, unit: !0, variables: !2)
+!9 = !DISubroutineType(types: !10)
+!10 = !{null, !11}
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = !DIExpression()
+!14 = !DILocation(line: 2, column: 12, scope: !8)
+!15 = !DILocation(line: 3, column: 7, scope: !16)
+!16 = distinct !DILexicalBlock(scope: !8, file: !1, line: 3, column: 7)
+!17 = !DILocation(line: 3, column: 7, scope: !8)
+!18 = !DILocation(line: 4, column: 5, scope: !16)
+!19 = !DILocation(line: 5, column: 3, scope: !8)
+!20 = !DILocation(line: 6, column: 1, scope: !8)
+!21 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 2, type: !9, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: false, unit: !0, variables: !2)
+!22 = !DILocation(line: 3, column: 7, scope: !21)
+!23 = !DILocation(line: 4, column: 5, scope: !21)
+!24 = !DILocation(line: 5, column: 3, scope: !21)
+!25 = !DILocation(line: 6, column: 1, scope: !21)
diff --git a/llvm/test/DebugInfo/COFF/local-variables.ll b/llvm/test/DebugInfo/COFF/local-variables.ll
index c0bac0d174a..249b6e1103d 100644
--- a/llvm/test/DebugInfo/COFF/local-variables.ll
+++ b/llvm/test/DebugInfo/COFF/local-variables.ll
@@ -193,7 +193,7 @@
 ; OBJ:        ChangeLineOffset: 1
 ; OBJ:        ChangeCodeOffset: 0x35
 ; OBJ:        ChangeCodeOffsetAndLineOffset: {CodeOffset: 0xD, LineOffset: 1}
-; OBJ:        ChangeCodeLength: 0xF
+; OBJ:        ChangeCodeLength: 0xA
 ; OBJ:      ]
 ; OBJ:    }
 ; OBJ:    Local {