Preserve the debug location when sinking compare instructions

[wolfgangp]

Summary

When the CodeGenPrepare pass sinks a compare instruction into the basic block of a user, it should preserve its debug location. Not doing so negatively affects source line attribution for debugging and AutoFDO.

Patch by Andrea Di Biagio

[wolfgangp]

ping...

[wolfgangp]

Ping...

[aprantl]

Seems reasonable to me. Thanks!

llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp

for (Value::user_iterator UI = CI->user_begin(), E = CI->user_end();
     CmpInst *&InsertedCmp = InsertedCmps[UserBB];
 
     if (!InsertedCmp) {
       BasicBlock::iterator InsertPt = UserBB->getFirstInsertionPt();
       assert(InsertPt != UserBB->end());
       InsertedCmp =
           CmpInst::Create(CI->getOpcode(), CI->getPredicate(),
                           CI->getOperand(0), CI->getOperand(1), "", &*InsertPt);
+      // Propagate the debug info.
+      InsertedCmp->setDebugLoc(CI->getDebugLoc());
     }
 
     // Replace a use of the cmp with a use of the new cmp.
     TheUse = InsertedCmp;
     MadeChange = true;
     ++NumCmpUses;
   }
 

llvm/trunk/test/DebugInfo/Generic/sunk-compare.ll

+; RUN: opt -S -codegenprepare < %s | FileCheck %s
+;
+; This test case has been generated by hand but is inspired by the
+; observation that compares that are sunk into the basic blocks where
+; their results are used did not retain their debug locs. This caused
+; sample profiling to attribute code to the wrong source lines. 
+;
+; We check that the compare instruction retains its debug loc after 
+; it is sunk into other.bb by the codegen prepare pass.
+; 
+; CHECK:       other.bb:
+; CHECK-NEXT:  icmp{{.*}}%x, 0, !dbg ![[MDHANDLE:[0-9]*]]
+; CHECK:       ![[MDHANDLE]] = !DILocation(line: 2
+;
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+
+define i32 @_Z3fooii(i32 %x, i32 %y) !dbg !5 {
+entry:
+  %cmp17 = icmp sgt i32 %x, 0, !dbg !6
+  br label %other.bb, !dbg !6
+
+other.bb:
+  br i1 %cmp17, label %exit1.bb, label %exit2.bb, !dbg !7
+
+exit1.bb:
+  %0 = add i32 %y, 42, !dbg !8
+  ret i32 %0, !dbg !8
+
+exit2.bb:
+  ret i32 44, !dbg !9
+
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "clang", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, globals: !2)
+!1 = !DIFile(filename: "test.cpp", directory: "/debuginfo/bug/cgp")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: !1, file: !1, line: 1, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: false, unit: !0, variables: !2)
+!6 = !DILocation(line: 2, column: 0, scope: !5)
+!7 = !DILocation(line: 3, column: 0, scope: !5)
+!8 = !DILocation(line: 4, column: 0, scope: !5)
+!9 = !DILocation(line: 5, column: 0, scope: !5)