[LoopRotate] Add test case to show dbg value problem

[bjope]

Summary

In commit d60f34c20a2f31335c8d5626e (llvm-svn 317128,
PR35113) MergeBlockIntoPredecessor was changed into
discarding some dbg.value intrinsics referring to
PHI values, post-splice due to loop rotation.

That elimination of dbg.value intrinsics does not
consider which dbg.value to keep based on the context.
Such as always keeping the one that comes first textually,
or the need to keep several of them in case the variable
is changing it's value several times inside the basic block.

In the past that hasn't been such a big problem since
CodeGenPrepare::placeDbgValues has moved the dbg.value
to be next to the PHI node anyway. But after commit
00e238896cd8ad3a7d7 CodeGenPrepare isn't doing that
any longer, so we need to be more careful when avoiding
duplicate dbg.value intrinsics in MergeBlockIntoPredecessor.

This patch is just a pre commit of the test case.

Change-Id: I2a428513312a7c3b62db453c03aaf00d436189de

llvm/test/Transforms/LoopRotate/dbg-value-duplicates-2.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -loop-rotate -S | FileCheck %s
+
+define dso_local i16 @main() local_unnamed_addr #0 !dbg !7 {
+; CHECK-LABEL: @main(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i16 9, metadata !12, metadata !DIExpression()), !dbg !13
+; CHECK-NEXT:    br label [[BB2:%.*]], !dbg !14
+; CHECK:       bb2:
+; CHECK-NEXT:    [[TMP1:%.*]] = phi i16 [ 9, [[ENTRY:%.*]] ], [ [[TMP5:%.*]], [[BB2]] ]
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i16 [[TMP1]], metadata !12, metadata !DIExpression()), !dbg !13
+; CHECK-NEXT:    [[TMP4:%.*]] = call i16 @wibble(i16 [[TMP1]]), !dbg !14
+; CHECK-NEXT:    [[TMP5]] = add nsw i16 [[TMP4]], [[TMP1]], !dbg !14
+; CHECK-NEXT:    [[TMP6:%.*]] = call i16 @wibble(i16 [[TMP4]]), !dbg !14
+; CHECK-NEXT:    [[TMP7:%.*]] = mul nsw i16 [[TMP6]], 3, !dbg !14
+; CHECK-NEXT:    [[TMP8:%.*]] = call i16 @wibble(i16 [[TMP7]]), !dbg !14
+; BUG: This dbg.value is expected to be placed directly after the add above.
+;      Otherwise variable "x" (!12) won't be described as having the correct value at the call to wibble just after the add.
+; CHECK-NEXT:    call void @llvm.dbg.value(metadata i16 [[TMP5]], metadata !12, metadata !DIExpression()), !dbg !13
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp slt i16 [[TMP5]], 17, !dbg !14
+; CHECK-NEXT:    br i1 [[TMP2]], label [[BB2]], label [[BB3:%.*]], !dbg !14
+; CHECK:       bb3:
+; CHECK-NEXT:    [[TMP10:%.*]] = call i16 @wibble(i16 8), !dbg !14
+; CHECK-NEXT:    ret i16 [[TMP10]], !dbg !14
+;
+entry:
+  call void @llvm.dbg.value(metadata i16 9, metadata !12, metadata !DIExpression()), !dbg !13
+  br label %bb1, !dbg !14
+
+bb1:
+  %tmp = phi i16 [ 9, %entry ], [ %tmp5, %bb2 ], !dbg !13
+  call void @llvm.dbg.value(metadata i16 %tmp, metadata !12, metadata !DIExpression()), !dbg !13
+  %tmp2 = icmp slt i16 %tmp, 17, !dbg !16
+  br i1 %tmp2, label %bb2, label %bb3, !dbg !16
+
+bb2:
+  %tmp4 = call i16 @wibble(i16 %tmp), !dbg !16
+  %tmp5 = add nsw i16 %tmp4, %tmp, !dbg !16
+  call void @llvm.dbg.value(metadata i16 %tmp5, metadata !12, metadata !DIExpression()), !dbg !13
+  %tmp6 = call i16 @wibble(i16 %tmp4), !dbg !16
+  %tmp7 = mul nsw i16 %tmp6, 3, !dbg !16
+  %tmp8 = call i16 @wibble(i16 %tmp7), !dbg !16
+  br label %bb1, !dbg !16
+
+bb3:
+  %tmp10 = call i16 @wibble(i16 8), !dbg !16
+  ret i16 %tmp10, !dbg !16
+}
+
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+declare i16 @wibble(i16) #2
+
+attributes #0 = { noinline nounwind }
+attributes #1 = { nounwind readnone speculatable willreturn }
+attributes #2 = { noinline nounwind readnone }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 10.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, globals: !2, nameTableKind: None)
+!1 = !DIFile(filename: "foo.c", directory: "")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 1}
+!6 = !{!"clang version 10.0.0"}
+!7 = distinct !DISubprogram(name: "main", scope: !1, file: !1, line: 8, type: !8, scopeLine: 8, flags: DIFlagAllCallsDescribed, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
+!8 = !DISubroutineType(types: !9)
+!9 = !{!10}
+!10 = !DIBasicType(name: "int", size: 16, encoding: DW_ATE_signed)
+!11 = !{!12}
+!12 = !DILocalVariable(name: "x", scope: !7, file: !1, line: 9, type: !10)
+!13 = !DILocation(line: 0, scope: !7)
+!14 = !DILocation(line: 11, column: 8, scope: !15)
+!15 = distinct !DILexicalBlock(scope: !7, file: !1, line: 11, column: 3)
+!16 = !DILocation(line: 11, column: 21, scope: !17)
+!17 = distinct !DILexicalBlock(scope: !15, file: !1, line: 11, column: 3)