Improve the accuracy of variable ranges .debug_loc location lists.

For the following motivating example
  bool c();
  void f();
  bool start() {
    bool result = c();
    if (!c()) {
      result = false;
      goto exit;
    }
    f();
    result = true;
  exit:
    return result;
  }

we would previously generate a single DW_AT_const_value(1) because
only the DBG_VALUE in the second-to-last basic block survived
codegen. This patch improves the heuristic used to determine when a
DBG_VALUE is available at the beginning of its variable's enclosing
lexical scope:

- Stop giving singular constants blanket permission to take over the
  entire scope. There is still a special case for constants in the
  function prologue that we also miight want to retire later.

- Use the lexical scope information to determine available-at-entry
  instead of proximity to the function prologue.

After this patch we generate a location list with a more accurate
narrower availability for the constant true value. As a pleasant side
effect, we also generate inline locations instead of location lists
where a loacation covers the entire range of the enclosing lexical
scope.

Measured on compiling llc with four targets this doesn't have an
effect on compile time and reduces the size of the debug info for llc
by ~600K.

rdar://problem/30286912

llvm-svn: 305599

1 files changed, 82 insertions, 0 deletions

diff --git a/llvm/test/DebugInfo/Generic/partial-constant.ll b/llvm/test/DebugInfo/Generic/partial-constant.ll
new file mode 100644
index 00000000000..c3abccbc45c
--- /dev/null
+++ b/llvm/test/DebugInfo/Generic/partial-constant.ll
@@ -0,0 +1,82 @@
+; RUN: %llc_dwarf -filetype=obj < %s | llvm-dwarfdump - | FileCheck %s
+; Generated at -O2 from:
+; bool c();
+; void f();
+; bool start() {
+;   bool result = c();
+;   if (!c()) {
+;     result = false;
+;     goto exit;
+;   }
+;   f();
+;   result = true;
+; exit:
+;   return result;
+; }
+;
+; The constant should NOT be available for the entire function.
+; CHECK-NOT: DW_AT_const_value
+; CHECK: .debug_loc
+; CHECK: Location description: 10 01 9f
+;                              constu 0x00000001, stack-value
+source_filename = "test.ii"
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.12.0"
+
+; Function Attrs: noimplicitfloat noredzone nounwind optsize
+define zeroext i1 @_Z5startv() local_unnamed_addr #0 !dbg !7 {
+entry:
+  %call = tail call zeroext i1 @_Z1cv() #3, !dbg !13
+  %call1 = tail call zeroext i1 @_Z1cv() #3, !dbg !14
+  br i1 %call1, label %if.end, label %exit, !dbg !16
+
+if.end:                                           ; preds = %entry
+  tail call void @_Z1fv() #3, !dbg !17
+  tail call void @llvm.dbg.value(metadata i8 1, i64 0, metadata !12, metadata !18), !dbg !19
+  br label %exit, !dbg !20
+
+exit:                                             ; preds = %entry, %if.end
+  %result.0 = phi i1 [ true, %if.end ], [ false, %entry ]
+  ret i1 %result.0, !dbg !21
+}
+
+; Function Attrs: noimplicitfloat noredzone optsize
+declare zeroext i1 @_Z1cv() local_unnamed_addr #1
+
+; Function Attrs: noimplicitfloat noredzone optsize
+declare void @_Z1fv() local_unnamed_addr #1
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, i64, metadata, metadata) #2
+
+attributes #0 = { noimplicitfloat noredzone nounwind optsize }
+attributes #1 = { noimplicitfloat noredzone optsize }
+attributes #2 = { nounwind readnone speculatable }
+attributes #3 = { nobuiltin noimplicitfloat noredzone nounwind optsize }
+
+!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 5.0.0 (trunk 303873) (llvm/trunk 303897)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "test.ii", directory: "/")
+!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 5.0.0 (trunk 303873) (llvm/trunk 303897)"}
+!7 = distinct !DISubprogram(name: "start", linkageName: "_Z5startv", scope: !1, file: !1, line: 3, type: !8, isLocal: false, isDefinition: true, scopeLine: 3, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "bool", size: 8, encoding: DW_ATE_boolean)
+!11 = !{!12}
+!12 = !DILocalVariable(name: "result", scope: !7, file: !1, line: 4, type: !10)
+!13 = !DILocation(line: 4, column: 17, scope: !7)
+!14 = !DILocation(line: 5, column: 8, scope: !15)
+!15 = distinct !DILexicalBlock(scope: !7, file: !1, line: 5, column: 7)
+!16 = !DILocation(line: 5, column: 7, scope: !7)
+!17 = !DILocation(line: 9, column: 3, scope: !7)
+!18 = !DIExpression()
+!19 = !DILocation(line: 4, column: 8, scope: !7)
+!20 = !DILocation(line: 10, column: 3, scope: !7)
+!21 = !DILocation(line: 12, column: 3, scope: !7)