[SROA] Really remove associated dbg.declare when removing dead alloca

[uabelho]

Summary

There already was code that tried to remove the dbg.declare, but that code
was placed after we had called
I->replaceAllUsesWith(UndefValue::get(I->getType()));
on the alloca, so when we searched for the relevant dbg.declare, we
couldn't find it.

Now we do the search before we call RAUW so there is a chance to find it.

An existing testcase needed update due to this. Two dbg.declare with undef
were removed and then suddenly one of the two CHECKS failed.

Before this patch we got

call void @llvm.dbg.declare(metadata i24* undef, metadata !14, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 24)), !dbg !15
call void @llvm.dbg.declare(metadata %struct.prog_src_register* undef, metadata !14, metadata !DIExpression()), !dbg !15
call void @llvm.dbg.value(metadata i32 0, metadata !14, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32)), !dbg !15
call void @llvm.dbg.value(metadata i32 0, metadata !14, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 24)), !dbg !15

and with it we get

call void @llvm.dbg.value(metadata i32 0, metadata !14, metadata !DIExpression(DW_OP_LLVM_fragment, 0, 32)), !dbg !15
call void @llvm.dbg.value(metadata i32 0, metadata !14, metadata !DIExpression(DW_OP_LLVM_fragment, 32, 24)), !dbg !15

However, the CHECKs in the testcase checked things in a silly order, so
they only passed since they found things in the first dbg.declare. Now
we changed the order of the checks and the test passes.

[chandlerc]

I suspect Reid is in a much better position to look at debug info changes to SROA than I am, but let me know if I can help...

[rnk]

lgtm, I actually made this change as part of llvm.dbg.addr (D37768) and then reverted it to keep the diff minimal. Thanks!

[uabelho]

Rebased to top of tree.

lib/Transforms/Scalar/SROA.cpp

 /// We also record the alloca instructions deleted here so that they aren't
 /// subsequently handed to mem2reg to promote.
 void SROA::deleteDeadInstructions(
     SmallPtrSetImpl<AllocaInst *> &DeletedAllocas) {
   while (!DeadInsts.empty()) {
     Instruction *I = DeadInsts.pop_back_val();
     DEBUG(dbgs() << "Deleting dead instruction: " << *I << "\n");
 
+    // If the instruction is an alloca, find the possible dbg.declare connected
+    // to it, and remove it too. We must do this before calling RAUW or we will
+    // not be able to find it.
+    if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
+      DeletedAllocas.insert(AI);
+      if (DbgDeclareInst *DbgDecl = FindAllocaDbgDeclare(AI))
+        DbgDecl->eraseFromParent();
+    }
+
     I->replaceAllUsesWith(UndefValue::get(I->getType()));
 
     for (Use &Operand : I->operands())
       if (Instruction *U = dyn_cast<Instruction>(Operand)) {
         // Zero out the operand and see if it becomes trivially dead.
         Operand = nullptr;
         if (isInstructionTriviallyDead(U))
           DeadInsts.insert(U);
       }
 
-    if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
-      DeletedAllocas.insert(AI);
-      if (DbgDeclareInst *DbgDecl = FindAllocaDbgDeclare(AI))
-        DbgDecl->eraseFromParent();
-    }
-
     ++NumDeleted;
     I->eraseFromParent();
   }
 }
 
 /// \brief Promote the allocas, using the best available technique.
 ///
 /// This attempts to promote whatever allocas have been identified as viable in

test/DebugInfo/X86/sroasplit-5.ll

 ; ModuleID = 'pr22495.c'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; When SROA is creating new smaller allocas, it may add padding.
 ;
 ; There should be no debug info for the padding.
 ; CHECK-NOT: DW_OP_LLVM_fragment, 56
-; CHECK: DIExpression(DW_OP_LLVM_fragment, 32, 24)
-; CHECK-NOT: DW_OP_LLVM_fragment, 56
 ; CHECK: DIExpression(DW_OP_LLVM_fragment, 0, 32)
 ; CHECK-NOT: DW_OP_LLVM_fragment, 56
+; CHECK: DIExpression(DW_OP_LLVM_fragment, 32, 24)
+; CHECK-NOT: DW_OP_LLVM_fragment, 56
 %struct.prog_src_register = type { i32, i24 }
 
 ; Function Attrs: nounwind
 define i64 @src_reg_for_float() #0 !dbg !4 {
 entry:
   %retval = alloca %struct.prog_src_register, align 4
   %a = alloca %struct.prog_src_register, align 4
   %local = alloca i32, align 4

test/DebugInfo/X86/sroasplit-dbg-declare.ll

+; RUN: opt -S -sroa -o - %s | FileCheck %s
+
+; SROA should split the alloca in two new ones, each with its own dbg.declare.
+; The original alloca and dbg.declare should be removed.
+
+define void @f1() {
+entry:
+  %0 = alloca [9 x i32]
+  call void @llvm.dbg.declare(metadata [9 x i32]* %0, metadata !11, metadata !DIExpression()), !dbg !17
+  %1 = bitcast [9 x i32]* %0 to i8*
+  call void @llvm.memset.p0i8.i64(i8* %1, i8 0, i64 36, i32 16, i1 true)
+  %2 = getelementptr [9 x i32], [9 x i32]* %0, i32 0, i32 0
+  store volatile i32 1, i32* %2
+  ret void
+}
+
+; Function Attrs: nounwind readnone speculatable
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+; Function Attrs: argmemonly nounwind
+declare void @llvm.memset.p0i8.i64(i8* nocapture writeonly, i8, i64, i32, i1) #0
+
+attributes #0 = { argmemonly nounwind }
+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 6.0.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "foo.c", directory: "/bar")
+!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 6.0.0"}
+!7 = distinct !DISubprogram(name: "f1", scope: !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true, scopeLine: 2, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !10)
+!8 = !DISubroutineType(types: !9)
+!9 = !{null}
+!10 = !{!11}
+!11 = !DILocalVariable(name: "b", scope: !7, file: !1, line: 3, type: !12)
+!12 = !DICompositeType(tag: DW_TAG_array_type, baseType: !13, size: 288, elements: !15)
+!13 = !DIDerivedType(tag: DW_TAG_volatile_type, baseType: !14)
+!14 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!15 = !{!16}
+!16 = !DISubrange(count: 9)
+!17 = !DILocation(line: 3, column: 18, scope: !7)
+
+; CHECK-NOT:  = alloca [9 x i32]
+; CHECK-NOT:  call void @llvm.dbg.declare(metadata [9 x i32]*
+
+; CHECK:      %[[VAR1:.*]] = alloca i32
+; CHECK-NEXT: %[[VAR2:.*]] = alloca [8 x i32]
+; CHECK-NEXT: call void @llvm.dbg.declare(metadata i32* %[[VAR1]]
+; CHECK-NEXT: call void @llvm.dbg.declare(metadata [8 x i32]* %[[VAR2]]
+
+; CHECK-NOT:  = alloca [9 x i32]
+; CHECK-NOT:  call void @llvm.dbg.declare(metadata [9 x i32]*
+