Remove debug location from common tail when tail-merging

The branch folding pass tail merges blocks into a common-tail.  However, the
tail retains the debug information from one of the original inputs to the
merge (chosen randomly).  This is a problem for sampled-based PGO, as hits
on the common-tail will be attributed to whichever block was chosen,
irrespective of which path was actually taken to the common-tail.

This patch fixes the issue by nulling the debug location for the common-tail.

Differential Revision: https://reviews.llvm.org/D25742

llvm-svn: 285093

llvm/lib/CodeGen/BranchFolding.cpp

@@ -720,8 +720,6 @@ bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
     SameTails[commonTailIndex].getTailStartPos();
   MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
 
-  // If the common tail includes any debug info we will take it pretty
-  // randomly from one of the inputs.  Might be better to remove it?
   DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
                << maxCommonTailLength);
 
@@ -898,6 +896,11 @@ bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
     // Recompute common tail MBB's edge weights and block frequency.
     setCommonTailEdgeWeights(*MBB);
 
+    // Remove the original debug location from the common tail.
+    for (auto &MI : *MBB)
+      if (!MI.isDebugValue())
+        MI.setDebugLoc(DebugLoc());
+
     // MBB is common tail.  Adjust all other BB's to jump to this one.
     // Traversal must be forwards so erases work.
     DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()

llvm/test/DebugInfo/COFF/local-variables.ll

@@ -63,11 +63,9 @@
 ; ASM:         .cv_loc 2 1 5 3                 # t.cpp:5:3
 ; ASM:         callq   capture
 ; ASM:         leaq    36(%rsp), %rcx
-; ASM: [[inline_site2_end:\.Ltmp.*]]:
+; ASM: [[else_end:\.Ltmp.*]]:
 ; ASM: .LBB0_3:                                # %if.end
-; ASM:         .cv_loc 0 1 15 5                # t.cpp:15:5
 ; ASM:         callq   capture
-; ASM: [[else_end:\.Ltmp.*]]:
 ; ASM:         .cv_loc 0 1 17 1                # t.cpp:17:1
 ; ASM:         nop
 ; ASM:         addq    $56, %rsp
@@ -101,7 +99,7 @@
 ; ASM: .long   116                     # TypeIndex
 ; ASM: .short  0                       # Flags
 ; ASM: .asciz  "v"
-; ASM: .cv_def_range    [[inline_site2]] [[inline_site2_end]], "E\021O\001\000\0000\000\000\000"
+; ASM: .cv_def_range    [[inline_site2]] [[else_end]], "E\021O\001\000\0000\000\000\000"
 ; ASM: .short  4430                    # Record kind: S_INLINESITE_END
 
 ; OBJ:  Subsection [
@@ -159,7 +157,7 @@
 ; OBJ:      LocalVariableAddrRange {
 ; OBJ:        OffsetStart: .text+0x2D
 ; OBJ:        ISectStart: 0x0
-; OBJ:        Range: 0x24
+; OBJ:        Range: 0x1F
 ; OBJ:      }
 ; OBJ:    }
 ; OBJ:    InlineSite {
@@ -200,7 +198,7 @@
 ; OBJ:        ChangeLineOffset: 1
 ; OBJ:        ChangeCodeOffset: 0x35
 ; OBJ:        ChangeCodeOffsetAndLineOffset: {CodeOffset: 0xD, LineOffset: 1}
-; OBJ:        ChangeCodeLength: 0xA
+; OBJ:        ChangeCodeLength: 0xF
 ; OBJ:      ]
 ; OBJ:    }
 ; OBJ:    Local {

llvm/test/DebugInfo/X86/tail-merge.ll

@@ -0,0 +1,76 @@
+; RUN: llc %s -mtriple=x86_64-unknown-unknown -use-unknown-locations=true -o - | FileCheck %s
+
+; Generated with "clang -gline-tables-only -c -emit-llvm -o - | opt -sroa -S"
+; from source:
+;
+; extern int foo(int);
+; extern int bar(int);
+;
+; int test(int a, int b) {
+;   if(b)
+;     a += foo(a);
+;   else
+;     a += bar(a);
+;   return a;
+; }
+
+; When tail-merging the debug location of the common tail should be removed.
+
+; CHECK-LABEL: test:
+; CHECK: movl	%edi, [[REG:%.*]]
+; CHECK: testl	%esi, %esi
+; CHECK: je	[[ELSE:.LBB[0-9]+_[0-9]+]]
+; CHECK: .loc	1 6 10
+; CHECK: callq	foo
+; CHECK: jmp	[[TAIL:.LBB[0-9]+_[0-9]+]]
+; CHECK: [[ELSE]]:
+; CHECK: .loc	1 8 10
+; CHECK: callq	bar
+; CHECK: [[TAIL]]:
+; CHECK: .loc	1 0 0
+; CHECK: addl	[[REG]], %eax
+; CHECK: .loc	1 9 3
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @test(i32 %a, i32 %b) !dbg !6 {
+entry:
+  %tobool = icmp ne i32 %b, 0, !dbg !8
+  br i1 %tobool, label %if.then, label %if.else, !dbg !8
+
+if.then:                                          ; preds = %entry
+  %call = call i32 @foo(i32 %a), !dbg !9
+  %add = add nsw i32 %a, %call, !dbg !10
+  br label %if.end, !dbg !11
+
+if.else:                                          ; preds = %entry
+  %call1 = call i32 @bar(i32 %a), !dbg !12
+  %add2 = add nsw i32 %a, %call1, !dbg !13
+  br label %if.end
+
+if.end:                                           ; preds = %if.else, %if.then
+  %a.addr.0 = phi i32 [ %add, %if.then ], [ %add2, %if.else ]
+  ret i32 %a.addr.0, !dbg !14
+}
+
+declare i32 @foo(i32)
+declare i32 @bar(i32)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "", isOptimized: false, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2)
+!1 = !DIFile(filename: "test.c", directory: "")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "test", scope: !1, file: !1, line: 4, type: !7, isLocal: false, isDefinition: true, scopeLine: 4, flags: DIFlagPrototyped, isOptimized: false, unit: !0, variables: !2)
+!7 = !DISubroutineType(types: !2)
+!8 = !DILocation(line: 5, column: 6, scope: !6)
+!9 = !DILocation(line: 6, column: 10, scope: !6)
+!10 = !DILocation(line: 6, column: 7, scope: !6)
+!11 = !DILocation(line: 6, column: 5, scope: !6)
+!12 = !DILocation(line: 8, column: 10, scope: !6)
+!13 = !DILocation(line: 8, column: 7, scope: !6)
+!14 = !DILocation(line: 9, column: 3, scope: !6)