Teach SROA to handle allocas with more than one dbg.declare.

[aprantl]

Summary

It is technically legal for optimizations to create an alloca that is
used by more than one dbg.declare, if one or both of them are inlined
instances of aliasing variables.

[compnerd]

llvm/lib/Transforms/Scalar/SROA.cpp

4534

I wonder if its better to define a helper equality check function:

auto IdenticalInlinedFragment = [](const DbgVariableInstric *LHS, const DbgVariableIntrinsic *RHS) -> Bool {
  return LHS->getVariable() == RHS->getVariable() &&
    LHS->getDebugLoc()->getInlinedAt() == RHS->getDebugLoc()->getInlinedAt();
}

[davide]

LGTM with Saleem's comment addressed.

llvm/lib/Transforms/Scalar/SROA.cpp

bool SROA::splitAlloca(AllocaInst &AI, AllocaSlices &AS) {
   }
 
   NumAllocaPartitions += NumPartitions;
   MaxPartitionsPerAlloca.updateMax(NumPartitions);
 
   // Migrate debug information from the old alloca to the new alloca(s)
   // and the individual partitions.
   TinyPtrVector<DbgVariableIntrinsic *> DbgDeclares = FindDbgAddrUses(&AI);
-  if (!DbgDeclares.empty()) {
-    auto *Var = DbgDeclares.front()->getVariable();
-    auto *Expr = DbgDeclares.front()->getExpression();
-    auto VarSize = Var->getSizeInBits();
+  for (DbgVariableIntrinsic *DbgDeclare : DbgDeclares) {
+    auto *Expr = DbgDeclare->getExpression();
     DIBuilder DIB(*AI.getModule(), /*AllowUnresolved*/ false);
     uint64_t AllocaSize =
         DL.getTypeSizeInBits(AI.getAllocatedType()).getFixedSize();
     for (auto Fragment : Fragments) {
       // Create a fragment expression describing the new partition or reuse AI's
       // expression if there is only one partition.
       auto *FragmentExpr = Expr;
       if (Fragment.Size < AllocaSize || Expr->isFragment()) {
for (auto Fragment : Fragments) {
         // The new, smaller fragment is stenciled out from the old fragment.
         if (auto OrigFragment = FragmentExpr->getFragmentInfo()) {
           assert(Start >= OrigFragment->OffsetInBits &&
                  "new fragment is outside of original fragment");
           Start -= OrigFragment->OffsetInBits;
         }
 
         // The alloca may be larger than the variable.
+        auto VarSize = DbgDeclare->getVariable()->getSizeInBits();
         if (VarSize) {
           if (Size > *VarSize)
             Size = *VarSize;
           if (Size == 0 || Start + Size > *VarSize)
             continue;
         }
 
         // Avoid creating a fragment expression that covers the entire variable.
         if (!VarSize || *VarSize != Size) {
           if (auto E =
                   DIExpression::createFragmentExpression(Expr, Start, Size))
             FragmentExpr = *E;
           else
             continue;
         }
       }
 
-      // Remove any existing intrinsics describing the same alloca.
-      for (DbgVariableIntrinsic *OldDII : FindDbgAddrUses(Fragment.Alloca))
+      // Remove any existing intrinsics on the new alloca describing
+      // the variable fragment.
+      for (DbgVariableIntrinsic *OldDII : FindDbgAddrUses(Fragment.Alloca)) {
+        auto SameVariableFragment = [](const DbgVariableIntrinsic *LHS,
+                                       const DbgVariableIntrinsic *RHS) {
+          return LHS->getVariable() == RHS->getVariable() &&
+                 LHS->getDebugLoc()->getInlinedAt() ==
+                     RHS->getDebugLoc()->getInlinedAt();
+        };
+        if (SameVariableFragment(OldDII, DbgDeclare))
           OldDII->eraseFromParent();
+      }
 
-      DIB.insertDeclare(Fragment.Alloca, Var, FragmentExpr,
-                        DbgDeclares.front()->getDebugLoc(), &AI);
+      DIB.insertDeclare(Fragment.Alloca, DbgDeclare->getVariable(), FragmentExpr,
+                        DbgDeclare->getDebugLoc(), &AI);
     }
   }
   return Changed;
 }
 
 /// Clobber a use with undef, deleting the used value if it becomes dead.
 void SROA::clobberUse(Use &U) {
   Value *OldV = U;

llvm/test/Transforms/SROA/dbg-inline.ll

+; Test that SROA can deal with allocas that have more than one
+; dbg.declare hanging off of it.
+
+; RUN: opt < %s -sroa -S | FileCheck %s
+source_filename = "/tmp/inlinesplit.cpp"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.15.0"
+
+%struct.pair = type { i64, i64 }
+
+; Function Attrs: noinline optnone ssp uwtable
+define i64 @_Z1g4pair(i64 %p.coerce0, i64 %p.coerce1) #0 !dbg !8 {
+entry:
+  %p = alloca %struct.pair, align 8
+  %0 = getelementptr inbounds %struct.pair, %struct.pair* %p, i32 0, i32 0
+  store i64 %p.coerce0, i64* %0, align 8
+  %1 = getelementptr inbounds %struct.pair, %struct.pair* %p, i32 0, i32 1
+  store i64 %p.coerce1, i64* %1, align 8
+  ; CHECK-DAG: call void @llvm.dbg.value(metadata i64 %p.coerce0, metadata ![[VAR:[0-9]+]], metadata !DIExpression(DW_OP_LLVM_fragment, 0, 64)), !dbg ![[LOC:[0-9]+]]
+  ; CHECK-DAG: call void @llvm.dbg.value(metadata i64 %p.coerce1, metadata ![[VAR]], metadata !DIExpression(DW_OP_LLVM_fragment, 64, 64)), !dbg ![[LOC]]
+  ; CHECK-DAG: call void @llvm.dbg.value(metadata i64 %p.coerce0, metadata ![[INLINED_VAR:[0-9]+]], metadata !DIExpression(DW_OP_LLVM_fragment, 0, 64)), !dbg ![[INLINED_LOC:[0-9]+]]
+  ; CHECK-DAG: call void @llvm.dbg.value(metadata i64 %p.coerce1, metadata ![[INLINED_VAR]], metadata !DIExpression(DW_OP_LLVM_fragment, 64, 64)), !dbg ![[INLINED_LOC]]
+  call void @llvm.dbg.declare(metadata %struct.pair* %p, metadata !17, metadata !DIExpression()), !dbg !18
+  call void @llvm.dbg.declare(metadata %struct.pair* %p, metadata !21, metadata !DIExpression()), !dbg !23
+  %a.i = getelementptr inbounds %struct.pair, %struct.pair* %p, i32 0, i32 0, !dbg !25
+  %x2 = load i64, i64* %0, align 8, !dbg !25
+  ret i64 %x2, !dbg !26
+}
+
+; Function Attrs: nounwind readnone speculatable willreturn
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+; Function Attrs: argmemonly nounwind willreturn
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nocapture writeonly, i8* noalias nocapture readonly, i64, i1 immarg) #2
+
+attributes #0 = { noinline ssp uwtable }
+attributes #1 = { nounwind readnone speculatable willreturn }
+attributes #2 = { argmemonly nounwind willreturn }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5, !6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 12.0.0 ([e-mail]:llvm/llvm-project 5110fd0343c2d06c8ae538741fbef13ece5e68de)", isOptimized: false, runtimeVersion: 0, emissionKind: FullDebug, enums: !2, nameTableKind: None, sysroot: "/")
+!1 = !DIFile(filename: "/tmp/inlinesplit.cpp", directory: "/Volumes/Data/llvm-project")
+!2 = !{}
+!3 = !{i32 7, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{i32 7, !"PIC Level", i32 2}
+!8 = distinct !DISubprogram(name: "g", linkageName: "_Z1g4pair", scope: !9, file: !9, line: 9, type: !10, scopeLine: 9, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !0, retainedNodes: !2)
+!9 = !DIFile(filename: "/tmp/inlinesplit.cpp", directory: "")
+!10 = !DISubroutineType(types: !11)
+!11 = !{!12, !13}
+!12 = !DIBasicType(name: "long long unsigned int", size: 64, encoding: DW_ATE_unsigned)
+!13 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "pair", file: !9, line: 1, size: 128, flags: DIFlagTypePassByValue, elements: !14, identifier: "_ZTS4pair")
+!14 = !{!15, !16}
+!15 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !13, file: !9, line: 1, baseType: !12, size: 64)
+!16 = !DIDerivedType(tag: DW_TAG_member, name: "b", scope: !13, file: !9, line: 1, baseType: !12, size: 64, offset: 64)
+!17 = !DILocalVariable(name: "p", arg: 1, scope: !8, file: !9, line: 9, type: !13)
+; CHECK: ![[LOC]] = !DILocation
+; CHECK-NOT: inlinedAt
+; CHECK: =
+!18 = !DILocation(line: 9, column: 27, scope: !8)
+!19 = !DILocation(line: 10, column: 12, scope: !8)
+!20 = !DILocation(line: 10, column: 10, scope: !8)
+!21 = !DILocalVariable(name: "p", arg: 1, scope: !22, file: !9, line: 5, type: !13)
+!22 = distinct !DISubprogram(name: "f", linkageName: "_ZL1f4pair", scope: !9, file: !9, line: 5, type: !10, scopeLine: 5, flags: DIFlagPrototyped, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition, unit: !0, retainedNodes: !2)
+; CHECK: ![[INLINED_LOC]] = !DILocation({{.*}}inlinedAt
+!23 = !DILocation(line: 5, column: 27, scope: !22, inlinedAt: !24)
+!24 = distinct !DILocation(line: 10, column: 10, scope: !8)
+!25 = !DILocation(line: 6, column: 12, scope: !22, inlinedAt: !24)
+!26 = !DILocation(line: 10, column: 3, scope: !8)