[SLPVectorizer] Debug intrinsics shouldn't affect spill cost

[davide]

Summary

The SLPVectorizer doesn't skip DebugInfo instructions when computing the spill cost, although these instruction make no difference.

SLP: Spill Cost = 4.
SLP: Extract Cost = 0.
SLP: Total Cost = 2.
SLP: Found cost=2 for VF=2
SLP: Analyzing a store chain of length 2

vs

SLP: Spill Cost = 0.
SLP: Extract Cost = 0.
SLP: Total Cost = -2.
SLP: Found cost=-2 for VF=2
SLP: Decided to vectorize cost=-2

The proposed fix is to skip them.

Fixes PR37261 / rdar://problem/39794738

[davide]

llvm/test/Transforms/SLPVectorizer/AArch64/spillcost-di.ll

3–8

I'll fix the check lines before committing, they look like:

+; RUN: opt -S -slp-vectorizer %s -o - | FileCheck %s

[vsk]

Lgtm, thanks! You might consider stripping out the tbaa as well.

[hsaito]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

2559

If LLVM doesn't have a generic "skip for cost" utility already, I think it's time to start having one. It's very likely that this goes beyond just DbgInfoIntrinsic and functionality would be common to many optimizers. If there is one, we should use that instead.

[vsk]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

2559

I think you're right. Recently an instructionsWithoutDebug() helper method was introduced in BasicBlock, which is a step in the right direction. It would be nice to use that utility here, although that might be better as a follow-up, since the change looks invasive.

[fhahn]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

2559

Yep I think having such a utility would be great. When adding instructionsWithoutDebug(), I was also prototyping a more generic skipInstructions function, which takes a range iterator and instruction types to skip and returns an iterator that skips over the ignored instructions. I can put it somewhere on phabricator again.

[hsaito]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

2559

Sounds like a plan. I don't intend to slow down this patch waiting for that. Please follow up towards generic utility direction in the future.

[davide]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

2559

That would be great, Florian. I think it's going to be of great use now that we're trying to squash down these debuginfo bugs.

[davide]

thank you @vsk!

[fhahn]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

2559

Unfortunately the iterators here seem incompatible with range iterators.

But would a helper function like isFreeInstruction be useful, that returns true for instructions that won't result in any instructions in the generated code, e.g. dbg intrinsics, lifetime markers, assume. Here (and probably in plenty of more places), we should also ignore those.

[hsaito]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

2559

My thinking was that your iterator would use such a helper function and it's a matter of exposing it as a utility. I think we are now on the same page.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

                                  PrevInstIt =
                                      PrevInst->getIterator().getReverse();
     while (InstIt != PrevInstIt) {
       if (PrevInstIt == PrevInst->getParent()->rend()) {
         PrevInstIt = Inst->getParent()->rbegin();
         continue;
       }
 
-      if (isa<CallInst>(&*PrevInstIt) && &*PrevInstIt != PrevInst) {
+      // Debug informations don't impact spill cost.
+      if ((isa<CallInst>(&*PrevInstIt) &&
+           !isa<DbgInfoIntrinsic>(&*PrevInstIt)) &&
+          &*PrevInstIt != PrevInst) {
         SmallVector<Type*, 4> V;
         for (auto *II : LiveValues)
           V.push_back(VectorType::get(II->getType(), BundleWidth));
         Cost += TTI->getCostOfKeepingLiveOverCall(V);
       }
 
       ++PrevInstIt;
     }

llvm/test/Transforms/SLPVectorizer/AArch64/spillcost-di.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+// Debug intrinsics shouldn't affect spill cost.
+
+// RUN: opt -S -slp-vectorizer %s | FileCheck %s
+
+// CHECK: %0 = bitcast i64* %x1 to <2 x i64>*, !dbg !26
+// CHECK: %1 = load <2 x i64>, <2 x i64>* %0, align 8, !dbg !26, !tbaa !29
+// CHECK: store <2 x i64> %1, <2 x i64>* %2, align 8, !dbg !36, !tbaa !29
+
+target triple = "aarch64"
+
+%struct.S = type { i64, i64 }
+
+define void @foo(i64 %n, i64 %i, %struct.S* %p) !dbg !7 {
+entry:
+  call void @llvm.dbg.value(metadata i64 %n, metadata !18, metadata !DIExpression()), !dbg !23
+  call void @llvm.dbg.value(metadata i64 %i, metadata !19, metadata !DIExpression()), !dbg !24
+  call void @llvm.dbg.value(metadata %struct.S* %p, metadata !20, metadata !DIExpression()), !dbg !25
+  %x1 = getelementptr inbounds %struct.S, %struct.S* %p, i64 %n, i32 0, !dbg !26
+  %0 = load i64, i64* %x1, align 8, !dbg !26, !tbaa !27
+  call void @llvm.dbg.value(metadata i64 %0, metadata !21, metadata !DIExpression()), !dbg !32
+  %y3 = getelementptr inbounds %struct.S, %struct.S* %p, i64 %n, i32 1, !dbg !33
+  %1 = load i64, i64* %y3, align 8, !dbg !33, !tbaa !34
+  call void @llvm.dbg.value(metadata i64 %1, metadata !22, metadata !DIExpression()), !dbg !35
+  %x5 = getelementptr inbounds %struct.S, %struct.S* %p, i64 %i, i32 0, !dbg !36
+  store i64 %0, i64* %x5, align 8, !dbg !37, !tbaa !27
+  %y7 = getelementptr inbounds %struct.S, %struct.S* %p, i64 %i, i32 1, !dbg !38
+  store i64 %1, i64* %y7, align 8, !dbg !39, !tbaa !34
+  ret void, !dbg !40
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.value(metadata, metadata, metadata) #1
+
+attributes #1 = { nounwind readnone speculatable }
+
+!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 7.0.0 (trunk 330946) (llvm/trunk 330976)", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "slp-reduced.c", directory: "/usr2/gberry/local/loop-align")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 7.0.0 (trunk 330946) (llvm/trunk 330976)"}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 6, type: !8, isLocal: false, isDefinition: true, scopeLine: 6, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !17)
+!8 = !DISubroutineType(types: !9)
+!9 = !{null, !10, !10, !11}
+!10 = !DIBasicType(name: "long int", size: 64, encoding: DW_ATE_signed)
+!11 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !12, size: 64)
+!12 = !DIDerivedType(tag: DW_TAG_typedef, name: "S", file: !1, line: 4, baseType: !13)
+!13 = distinct !DICompositeType(tag: DW_TAG_structure_type, file: !1, line: 1, size: 128, elements: !14)
+!14 = !{!15, !16}
+!15 = !DIDerivedType(tag: DW_TAG_member, name: "x", scope: !13, file: !1, line: 2, baseType: !10, size: 64)
+!16 = !DIDerivedType(tag: DW_TAG_member, name: "y", scope: !13, file: !1, line: 3, baseType: !10, size: 64, offset: 64)
+!17 = !{!18, !19, !20, !21, !22}
+!18 = !DILocalVariable(name: "n", arg: 1, scope: !7, file: !1, line: 6, type: !10)
+!19 = !DILocalVariable(name: "i", arg: 2, scope: !7, file: !1, line: 6, type: !10)
+!20 = !DILocalVariable(name: "p", arg: 3, scope: !7, file: !1, line: 6, type: !11)
+!21 = !DILocalVariable(name: "x", scope: !7, file: !1, line: 7, type: !10)
+!22 = !DILocalVariable(name: "y", scope: !7, file: !1, line: 8, type: !10)
+!23 = !DILocation(line: 6, column: 15, scope: !7)
+!24 = !DILocation(line: 6, column: 23, scope: !7)
+!25 = !DILocation(line: 6, column: 29, scope: !7)
+!26 = !DILocation(line: 7, column: 19, scope: !7)
+!27 = !{!28, !29, i64 0}
+!28 = !{!"", !29, i64 0, !29, i64 8}
+!29 = !{!"long", !30, i64 0}
+!30 = !{!"omnipotent char", !31, i64 0}
+!31 = !{!"Simple C/C++ TBAA"}
+!32 = !DILocation(line: 7, column: 10, scope: !7)
+!33 = !DILocation(line: 8, column: 19, scope: !7)
+!34 = !{!28, !29, i64 8}
+!35 = !DILocation(line: 8, column: 10, scope: !7)
+!36 = !DILocation(line: 9, column: 10, scope: !7)
+!37 = !DILocation(line: 9, column: 12, scope: !7)
+!38 = !DILocation(line: 10, column: 10, scope: !7)
+!39 = !DILocation(line: 10, column: 12, scope: !7)
+!40 = !DILocation(line: 11, column: 1, scope: !7)