[GVNHoist] drop debug location according to the debug info guide

[hiraditya]

Summary

According to the LLVM debug info update guide: https://llvm.org/docs/HowToUpdateDebugInfo.html, "Hoisting identical instructions which appear in several successor blocks into a predecessor block. In this case there is no single merged instruction. The rule for dropping locations applies".

Thanks to Yuanbo Li for reporting this.

[dblaikie]

Sounds about right to me.

[hiraditya]

issue addressed by this patch: https://github.com/llvm/llvm-project/issues/48443

llvm/lib/Transforms/Scalar/GVNHoist.cpp

if (Repl) {
       // Move the instruction at the end of HoistPt.
       Instruction *Last = DestBB->getTerminator();
       MD->removeInstruction(Repl);
       Repl->moveBefore(Last);
 
       DFSNumber[Repl] = DFSNumber[Last]++;
     }
 
+    // Drop debug location as per debug info update guide.
+    Repl->dropLocation();
     NR += removeAndReplace(InstructionsToHoist, Repl, DestBB, MoveAccess);
 
     if (isa<LoadInst>(Repl))
       ++NL;
     else if (isa<StoreInst>(Repl))
       ++NS;
     else if (isa<CallInst>(Repl))
       ++NC;

llvm/test/Transforms/GVNHoist/hoist-drop-debug-info.ll

+; RUN: opt -S -gvn-hoist < %s | FileCheck %s
+
+; Check that the debug info is dropped as per the debug info update guide
+
+@G = external global i32, align 4
+
+; CHECK-LABEL: @foo
+; CHECK-NOT:  store {{.*}}dbg
+define void @foo(i32 %c1) !dbg !6 {
+entry:
+  call void @llvm.dbg.value(metadata i32 0, metadata !9, metadata !DIExpression()), !dbg !11
+  switch i32 %c1, label %exit1 [
+    i32 0, label %sw0
+    i32 1, label %sw1
+  ], !dbg !11
+
+sw0:                                              ; preds = %entry
+  store i32 1, i32* @G, align 4, !dbg !12
+  br label %exit, !dbg !13
+
+sw1:                                              ; preds = %entry
+  store i32 1, i32* @G, align 4, !dbg !14
+  br label %exit, !dbg !15
+
+exit1:                                            ; preds = %entry
+  store i32 1, i32* @G, align 4, !dbg !16
+  ret void, !dbg !17
+
+exit:                                             ; preds = %sw1, %sw0
+  ret void, !dbg !18
+}
+
+; Function Attrs: nofree nosync nounwind readnone speculatable willreturn
+declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+
+attributes #0 = { nofree nosync nounwind readnone speculatable willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!3, !4}
+!llvm.module.flags = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "abc.ll", directory: "/")
+!2 = !{}
+!3 = !{i32 8}
+!4 = !{i32 1}
+!5 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "foo", linkageName: "foo", scope: null, file: !1, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !8)
+!7 = !DISubroutineType(types: !2)
+!8 = !{!9}
+!9 = !DILocalVariable(name: "1", scope: !6, file: !1, line: 1, type: !10)
+!10 = !DIBasicType(name: "ty32", size: 32, encoding: DW_ATE_unsigned)
+!11 = !DILocation(line: 1, column: 1, scope: !6)
+!12 = !DILocation(line: 2, column: 1, scope: !6)
+!13 = !DILocation(line: 3, column: 1, scope: !6)
+!14 = !DILocation(line: 4, column: 1, scope: !6)
+!15 = !DILocation(line: 5, column: 1, scope: !6)
+!16 = !DILocation(line: 6, column: 1, scope: !6)
+!17 = !DILocation(line: 7, column: 1, scope: !6)
+!18 = !DILocation(line: 8, column: 1, scope: !6)