Remove debug info when hoisting instruction from then/else branch.

Summary: The hoisted instruction is executed speculatively. It could affect the debugging experience as user would see gdb go into code that may not be expected to execute. It will also affect sample profile accuracy by assigning incorrect frequency to source within then/else branch.

Reviewers: davidxl, dblaikie, chandlerc, kcc, echristo

Subscribers: mehdi_amini, probinson, eric_niebler, andreadb, llvm-commits

Differential Revision: https://reviews.llvm.org/D24164

llvm-svn: 280995

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -1242,6 +1242,14 @@ static bool HoistThenElseCodeToIf(BranchInst *BI,
                            LLVMContext::MD_dereferenceable_or_null,
                            LLVMContext::MD_mem_parallel_loop_access};
     combineMetadata(I1, I2, KnownIDs);
+
+    // If the debug loc for I1 and I2 are different, as we are combining them
+    // into one instruction, we do not want to select debug loc randomly from 
+    // I1 or I2. Instead, we set the 0-line DebugLoc to note that we do not
+    // know the debug loc of the hoisted instruction.
+    if (!isa<CallInst>(I1) &&  I1->getDebugLoc() != I2->getDebugLoc())
+      I1->setDebugLoc(DebugLoc());
+
     I2->eraseFromParent();
     Changed = true;
 

llvm/test/Transforms/SimplifyCFG/remove-debug.ll

@@ -0,0 +1,83 @@
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+
+; TODO: Track the acutal DebugLoc of the hoisted instruction when no-line
+; DebugLoc is supported (https://reviews.llvm.org/D24180)
+; CHECK: line: 6
+; CHECK-NOT: line: 7
+; CHECK: line: 8
+; CHECK: line: 9
+; CHECK-NOT: line: 10
+; CHECK: line: 11
+
+; Checks if the debug info for hoisted "x = i" is removed
+; int x;
+; void bar();
+; void baz();
+;
+; void foo(int i) {
+;   if (i == 0) {
+;     x = i;
+;     bar();
+;   } else {
+;     x = i;
+;     baz();
+;   }
+; }
+
+target triple = "x86_64-unknown-linux-gnu"
+
+@x = global i32 0, align 4
+
+; Function Attrs: uwtable
+define void @_Z3fooi(i32) #0 !dbg !6 {
+  %2 = alloca i32, align 4
+  store i32 %0, i32* %2, align 4, !tbaa !8
+  %3 = load i32, i32* %2, align 4, !dbg !12, !tbaa !8
+  %4 = icmp eq i32 %3, 0, !dbg !13
+  br i1 %4, label %5, label %7, !dbg !12
+
+; <label>:5:
+  %6 = load i32, i32* %2, align 4, !dbg !14, !tbaa !8
+  store i32 %6, i32* @x, align 4, !dbg !15, !tbaa !8
+  call void @_Z3barv(), !dbg !16
+  br label %9, !dbg !17
+
+; <label>:7:
+  %8 = load i32, i32* %2, align 4, !dbg !18, !tbaa !8
+  store i32 %8, i32* @x, align 4, !dbg !19, !tbaa !8
+  call void @_Z3bazv(), !dbg !20
+  br label %9
+
+; <label>:9:
+  ret void, !dbg !21
+}
+
+declare void @_Z3barv() #1
+
+declare void @_Z3bazv() #1
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1)
+!1 = !DIFile(filename: "a", directory: "b/")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{}
+!6 = distinct !DISubprogram(unit: !0)
+!7 = !DISubroutineType(types: !2)
+!8 = !{!9, !9, i64 0}
+!9 = !{!"int", !10, i64 0}
+!10 = !{!"omnipotent char", !11, i64 0}
+!11 = !{!"Simple C++ TBAA"}
+!12 = !DILocation(line: 6, column: 7, scope: !6)
+!13 = !DILocation(line: 6, column: 9, scope: !6)
+!14 = !DILocation(line: 7, column: 9, scope: !6)
+!15 = !DILocation(line: 7, column: 7, scope: !6)
+!16 = !DILocation(line: 8, column: 5, scope: !6)
+!17 = !DILocation(line: 9, column: 3, scope: !6)
+!18 = !DILocation(line: 10, column: 9, scope: !6)
+!19 = !DILocation(line: 10, column: 7, scope: !6)
+!20 = !DILocation(line: 11, column: 5, scope: !6)
+!21 = !DILocation(line: 13, column: 1, scope: !6)