[DebugInfo] Fix pr41180 : Loop Vectorization Debugify Failure

[Orlando]

Summary

Bug: https://bugs.llvm.org/show_bug.cgi?id=41180

In the bug test case the debug location was missing for the cmp instruction in the "middle block" BB. This patch fixes the bug by copying the debug location from the cmp of the scalar loop's terminator branch, if it exists.

The patch also fixes the debug location on the subsequent branch instruction. It was previously using the location of the of the original loop's pre-header block terminator. Both of these instructions will now map to the source line of the conditional branch in the original loop.

A regression test has been added that covers these issues.

[aprantl]

llvm/test/Transforms/LoopVectorize/middle-block-dbg.ll

6

Could you please include the source code this was generated from in the comment in case we need to regenerate it later?

9

please also check for the expected kind of instruction here.

[Orlando]

I've updated the test to include the source and commands to generate the IR it uses. Please note that I stripped out the call void @llvm.dbg.* lines from the test file by hand so any regenerated IR will look more cluttered.

[Orlando]

The test has been updated to check for the expected instructions.

[vsk]

Thanks, lgtm.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

2876

Consider adding a comment here explaining that the original loop must have a single latch block, because we already require it to have a single exit block?

[Orlando]

Updated a comment with some more info as vsk suggested.

Please could someone commit this change for me? I do not have commit access.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

for (auto &InductionEntry : *List) {
 
     // The old induction's phi node in the scalar body needs the truncated
     // value.
     for (BasicBlock *BB : LoopBypassBlocks)
       BCResumeVal->addIncoming(II.getStartValue(), BB);
     OrigPhi->setIncomingValue(BlockIdx, BCResumeVal);
   }
 
+  // We need the OrigLoop (scalar loop part) latch terminator to help
+  // produce correct debug info for the middle block BB instructions.
+  // The legality check stage guarantees that the loop will have a single
+  // latch.
+  assert(isa<BranchInst>(OrigLoop->getLoopLatch()->getTerminator()) &&
+         "Scalar loop latch terminator isn't a branch");
+  BranchInst *ScalarLatchBr =
+      cast<BranchInst>(OrigLoop->getLoopLatch()->getTerminator());
+
   // Add a check in the middle block to see if we have completed
   // all of the iterations in the first vector loop.
   // If (N - N%VF) == N, then we *don't* need to run the remainder.
   // If tail is to be folded, we know we don't need to run the remainder.
   Value *CmpN = Builder.getTrue();
-  if (!Cost->foldTailByMasking())
+  if (!Cost->foldTailByMasking()) {
     CmpN =
         CmpInst::Create(Instruction::ICmp, CmpInst::ICMP_EQ, Count,
                         CountRoundDown, "cmp.n", MiddleBlock->getTerminator());
-  ReplaceInstWithInst(MiddleBlock->getTerminator(),
-                      BranchInst::Create(ExitBlock, ScalarPH, CmpN));
+
+    // Provide correct stepping behaviour by using the same DebugLoc as the
+    // scalar loop latch branch cmp if it exists.
+    if (CmpInst *ScalarLatchCmp =
+            dyn_cast_or_null<CmpInst>(ScalarLatchBr->getCondition()))
+      cast<Instruction>(CmpN)->setDebugLoc(ScalarLatchCmp->getDebugLoc());
+  }
+
+  BranchInst *BrInst = BranchInst::Create(ExitBlock, ScalarPH, CmpN);
+  BrInst->setDebugLoc(ScalarLatchBr->getDebugLoc());
+  ReplaceInstWithInst(MiddleBlock->getTerminator(), BrInst);
 
   // Get ready to start creating new instructions into the vectorized body.
   Builder.SetInsertPoint(&*VecBody->getFirstInsertionPt());
 
   // Save the state.
   LoopVectorPreHeader = Lp->getLoopPreheader();
   LoopScalarPreHeader = ScalarPH;
   LoopMiddleBlock = MiddleBlock;

llvm/test/Transforms/LoopVectorize/middle-block-dbg.ll

+; RUN: opt -loop-vectorize -force-vector-width=2 -S < %s | FileCheck %s
+;
+; Confirm that the DebugLoc info for the instructions in the middle block of a
+; vectorized loop are correct. The Cmp and Br instructions should map to the
+; same source lines as the Cmp and Br of the scalar loop.
+
+; CHECK-LABEL: middle.block:
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq {{.*}}!dbg ![[DL:[0-9]+]]
+; CHECK-NEXT: br i1 [[CMP]]{{.*}} !dbg ![[DL]]
+; CHECK: ![[DL]] = !DILocation(line: 6,
+
+; This IR can be generated by running:
+; clang -g -O2 -emit-llvm -S -mllvm -opt-bisect-limit=68 vec.cpp -o - | opt -loop-vectorize -force-vector-width=2 -S -o vec.ll
+;
+; Where vec.cpp contains:
+;
+; extern int x;
+; extern int y;
+; void a() {
+;     const int len = x;
+;     int b[len];
+;     for(int i = 0; i< len; ++i)
+;         b[i] = x;
+;
+;     y = b[x] + b[x-5];
+; }
+
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+
+@"?x@@3HA" = external dso_local local_unnamed_addr global i32, align 4
+@"?y@@3HA" = external dso_local local_unnamed_addr global i32, align 4
+
+define dso_local void @"?a@@YAXXZ"() local_unnamed_addr #0 !dbg !8 {
+entry:
+  %0 = load i32, i32* @"?x@@3HA", align 4, !dbg !23, !tbaa !24
+  %1 = zext i32 %0 to i64, !dbg !28
+  %vla = alloca i32, i64 %1, align 16, !dbg !28
+  %cmp10 = icmp sgt i32 %0, 0, !dbg !30
+  br i1 %cmp10, label %for.body.preheader, label %for.cond.cleanup, !dbg !30
+
+for.body.preheader:
+  br label %for.body, !dbg !31
+
+for.cond.cleanup.loopexit:
+  %idxprom1.phi.trans.insert = sext i32 %0 to i64
+  %arrayidx2.phi.trans.insert = getelementptr inbounds i32, i32* %vla, i64 %idxprom1.phi.trans.insert
+  %.pre = load i32, i32* %arrayidx2.phi.trans.insert, align 4, !dbg !33, !tbaa !24
+  br label %for.cond.cleanup, !dbg !33
+
+for.cond.cleanup:
+  %2 = phi i32 [ %.pre, %for.cond.cleanup.loopexit ], [ undef, %entry ], !dbg !33
+  %sub = add nsw i32 %0, -5, !dbg !33
+  %idxprom3 = sext i32 %sub to i64, !dbg !33
+  %arrayidx4 = getelementptr inbounds i32, i32* %vla, i64 %idxprom3, !dbg !33
+  %3 = load i32, i32* %arrayidx4, align 4, !dbg !33, !tbaa !24
+  %add = add nsw i32 %3, %2, !dbg !33
+  store i32 %add, i32* @"?y@@3HA", align 4, !dbg !33, !tbaa !24
+  ret void, !dbg !34
+
+for.body:
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %for.body.preheader ]
+  %arrayidx = getelementptr inbounds i32, i32* %vla, i64 %indvars.iv, !dbg !31
+  store i32 %0, i32* %arrayidx, align 4, !dbg !31, !tbaa !24
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1, !dbg !35
+  %exitcond = icmp eq i64 %indvars.iv.next, %1, !dbg !30
+  br i1 %exitcond, label %for.cond.cleanup.loopexit, label %for.body, !dbg !30, !llvm.loop !36
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+!llvm.ident = !{!7}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, file: !1, producer: "clang version 9.0.0 (https://github.com/llvm/llvm-project.git 045b8544fd2c4e14f7e72e0df2bc681d823b0838)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "vec.cpp", directory: "C:\5CUsers\5Cgbhyamso\5Cdev\5Cllvm\5Csamples", checksumkind: CSK_MD5, checksum: "fed997f50117f5514a69caf1c2fb2c49")
+!2 = !{}
+!3 = !{i32 2, !"CodeView", i32 1}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 2}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!7 = !{!"clang version 9.0.0 (https://github.com/llvm/llvm-project.git 045b8544fd2c4e14f7e72e0df2bc681d823b0838)"}
+!8 = distinct !DISubprogram(name: "a", linkageName: "?a@@YAXXZ", scope: !1, file: !1, line: 3, type: !9, scopeLine: 3, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !11)
+!9 = !DISubroutineType(types: !10)
+!10 = !{null}
+!11 = !{!12, !15, !17, !21}
+!12 = !DILocalVariable(name: "len", scope: !8, file: !1, line: 4, type: !13)
+!13 = !DIDerivedType(tag: DW_TAG_const_type, baseType: !14)
+!14 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!15 = !DILocalVariable(name: "__vla_expr0", scope: !8, type: !16, flags: DIFlagArtificial)
+!16 = !DIBasicType(name: "long long unsigned int", size: 64, encoding: DW_ATE_unsigned)
+!17 = !DILocalVariable(name: "b", scope: !8, file: !1, line: 5, type: !18)
+!18 = !DICompositeType(tag: DW_TAG_array_type, baseType: !14, elements: !19)
+!19 = !{!20}
+!20 = !DISubrange(count: !15)
+!21 = !DILocalVariable(name: "i", scope: !22, file: !1, line: 6, type: !14)
+!22 = distinct !DILexicalBlock(scope: !8, file: !1, line: 6)
+!23 = !DILocation(line: 4, scope: !8)
+!24 = !{!25, !25, i64 0}
+!25 = !{!"int", !26, i64 0}
+!26 = !{!"omnipotent char", !27, i64 0}
+!27 = !{!"Simple C++ TBAA"}
+!28 = !DILocation(line: 5, scope: !8)
+!29 = !DILocation(line: 0, scope: !8)
+!30 = !DILocation(line: 6, scope: !22)
+!31 = !DILocation(line: 7, scope: !32)
+!32 = distinct !DILexicalBlock(scope: !22, file: !1, line: 6)
+!33 = !DILocation(line: 9, scope: !8)
+!34 = !DILocation(line: 10, scope: !8)
+!35 = !DILocation(line: 6, scope: !32)
+!36 = distinct !{!36, !30, !37}
+!37 = !DILocation(line: 7, scope: !22)