[BDCE/DebugInfo] Preserve llvm.dbg.value's argument.

BDCE has two phases:
1. It asks SimplifyDemandedBits if all the bits of an instruction are dead, and if so,
replaces all its uses with the constant zero.
2. Then, it asks SimplifyDemandedBits again if the instruction is really dead
(no side effects etc..) and if so, eliminates it.

Now, in 1) if all the bits of an instruction are dead, we may end up replacing a dbg use:
  %call = tail call i32 (...) @g() #4, !dbg !15
  tail call void @llvm.dbg.value(metadata i32 %call, i64 0, metadata !8, metadata !16), !dbg !17
->
  %call = tail call i32 (...) @g() #4, !dbg !15
  tail call void @llvm.dbg.value(metadata i32 0, i64 0, metadata !8, metadata !16), !dbg !17

but not eliminating the call because it may have arbitrary side effects.
In other words, we lose some debug informations.
This patch fixes the problem making sure that BDCE does nothing with the instruction if
it has side effects and no non-dbg uses.

Differential Revision:  https://reviews.llvm.org/D27471

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288851 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/BDCE.cpp b/lib/Transforms/Scalar/BDCE.cpp
index 4f6225f..1baade4 100644 (file)
--- a/lib/Transforms/Scalar/BDCE.cpp
+++ b/lib/Transforms/Scalar/BDCE.cpp
@@ -39,6 +39,11 @@ static bool bitTrackingDCE(Function &F, DemandedBits &DB) {
   SmallVector<Instruction*, 128> Worklist;
   bool Changed = false;
   for (Instruction &I : instructions(F)) {
+    // If the instruction has side effects and no non-dbg uses,
+    // BDCE should skip it.
+    if (I.mayHaveSideEffects() && I.use_empty())
+      continue;
+
     if (I.getType()->isIntegerTy() &&
         !DB.getDemandedBits(&I).getBoolValue()) {
       // For live instructions that have all dead bits, first make them dead by

diff --git a/test/Transforms/BDCE/pr26587.ll b/test/Transforms/BDCE/pr26587.ll
new file mode 100644 (file)
index 0000000..17837ec
--- /dev/null
+++ b/test/Transforms/BDCE/pr26587.ll
@@ -0,0 +1,46 @@
+; Test that BDCE doesn't destroy llvm.dbg.value's argument.
+; RUN: opt -bdce %s -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK: define void @f()
+; CHECK-NEXT: entry:
+; CHECK-NEXT: tail call void (...) @h()
+; CHECK-NEXT: %[[CALL:.*]] = tail call i32 (...) @g()
+; CHECK-NEXT: tail call void @llvm.dbg.value(metadata i32 %[[CALL:.*]]
+
+define void @f() !dbg !6 {
+entry:
+  tail call void (...) @h(), !dbg !9
+  %call = tail call i32 (...) @g(), !dbg !10
+  tail call void @llvm.dbg.value(metadata i32 %call, i64 0, metadata !11, metadata !13), !dbg !14
+  tail call void (...) @h(), !dbg !15
+  ret void, !dbg !16
+}
+
+declare void @h(...)
+declare i32 @g(...)
+declare void @llvm.dbg.value(metadata, i64, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+!llvm.ident = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 4.0.0 (trunk 288665) (llvm/trunk 288725)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "patatino.c", directory: "/home/davide/work/llvm/build-clang/bin")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{!"clang version 4.0.0 (trunk 288665) (llvm/trunk 288725)"}
+!6 = distinct !DISubprogram(name: "f", scope: !1, file: !1, line: 3, type: !7, isLocal: false, isDefinition: true, scopeLine: 3, isOptimized: false, unit: !0, variables: !2)
+!7 = !DISubroutineType(types: !8)
+!8 = !{null}
+!9 = !DILocation(line: 4, column: 3, scope: !6)
+!10 = !DILocation(line: 5, column: 11, scope: !6)
+!11 = !DILocalVariable(name: "a", scope: !6, file: !1, line: 5, type: !12)
+!12 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!13 = !DIExpression()
+!14 = !DILocation(line: 5, column: 7, scope: !6)
+!15 = !DILocation(line: 6, column: 3, scope: !6)
+!16 = !DILocation(line: 7, column: 1, scope: !6)