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[DebugInfo] Prevent salvaged debug vars from being marked artificial #38942

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Merged
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Aug 20, 2021
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[DebugInfo] Prevent salvaged debug vars from being marked artificial #38942

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17 changes: 17 additions & 0 deletions lib/IRGen/IRGenDebugInfo.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2542,6 +2542,23 @@ void IRGenDebugInfoImpl::emitDbgIntrinsic(
!llvm::FindDbgDeclareUses(Storage).empty())
return;

// Fragment DIExpression cannot cover the whole variable
// or going out-of-bound.
if (auto Fragment = Expr->getFragmentInfo())
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Do we understand why we generate a fragment that covers the entire variable? Does this happen because we SROA a struct with a single member? Could we catch that case in IRGenDebugInfoImpl::emitVariableDeclaration() and not emit a DW_OP_LLVM_fragment?

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Yes this only happens when the struct only has one member. Originally I was going to solve this by preserving the bail out logic here and avoid emitting DW_OP_LLVM_fragment some place earlier as suggested. But then I found that it will be easier to just drop the fragment part here.

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That works!

if (auto VarSize = Var->getSizeInBits()) {
unsigned FragSize = Fragment->SizeInBits;
unsigned FragOffset = Fragment->OffsetInBits;
if (FragOffset + FragSize > *VarSize ||
FragSize == *VarSize) {
// Drop the fragment part
assert(Expr->isValid());
// Since this expression is valid, DW_OP_LLVM_fragment
// and its arguments must be the last 3 elements.
auto OrigElements = Expr->getElements();
Expr = DBuilder.createExpression(OrigElements.drop_back(3));
}
}

// A dbg.declare is only meaningful if there is a single alloca for
// the variable that is live throughout the function.
if (auto *Alloca = dyn_cast<llvm::AllocaInst>(Storage)) {
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3 changes: 1 addition & 2 deletions lib/SILOptimizer/Utils/InstOptUtils.cpp
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Expand Up @@ -2119,8 +2119,7 @@ void swift::salvageDebugInfo(SILInstruction *I) {
DestVal.getDefiningInstruction())) {
if (auto VarInfo = ASI->getVarInfo())
SILBuilder(SI, ASI->getDebugScope())
.createDebugValue(RegularLocation::getAutoGeneratedLocation(),
SI->getSrc(), *VarInfo);
.createDebugValue(SI->getLoc(), SI->getSrc(), *VarInfo);
}
}
}
17 changes: 17 additions & 0 deletions test/DebugInfo/debug_info_expression.sil
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Expand Up @@ -8,6 +8,10 @@ struct MyStruct {
var y: Builtin.Int64
}

struct SmallStruct {
var z : Builtin.Int64
}

sil_scope 1 { loc "file.swift":7:6 parent @test_fragment : $@convention(thin) () -> () }

// Testing op_fragment w/ debug_value_addr
Expand All @@ -16,6 +20,8 @@ bb0:
%2 = alloc_stack $MyStruct, var, name "my_struct", loc "file.swift":8:9, scope 1
// CHECK: %[[MY_STRUCT:.+]] = alloca %{{.*}}MyStruct
// CHECK: llvm.dbg.declare(metadata {{.*}}* %[[MY_STRUCT]], metadata ![[VAR_DECL_MD:[0-9]+]]
// CHECK: %[[SMALL_STRUCT:.+]] = alloca %{{.*}}SmallStruct
// CHECK: llvm.dbg.declare(metadata {{.*}}* %[[SMALL_STRUCT]], metadata ![[SMALL_VAR_DECL_MD:[0-9]+]]
%3 = struct_element_addr %2 : $*MyStruct, #MyStruct.x, loc "file.swift":9:17, scope 1
// CHECK: %[[FIELD_X:.*]] = getelementptr {{.*}} %[[MY_STRUCT]]
// CHECK-SIL: debug_value_addr %{{[0-9]+}} : $*Builtin.Int64
Expand All @@ -25,6 +31,17 @@ bb0:
// CHECK: llvm.dbg.value(metadata {{.*}}* %[[FIELD_X]], metadata ![[VAR_DECL_MD]]
// CHECK-SAME: !DIExpression(DW_OP_deref, DW_OP_LLVM_fragment, 0, 64)
// CHECK-NOT: ), !dbg ![[VAR_DECL_MD]]

%4 = alloc_stack $SmallStruct, var, name "small_struct", loc "file.swift":10:11, scope 1
%5 = struct_element_addr %4 : $*SmallStruct, #SmallStruct.z, loc "file.swift":11:13, scope 1
// CHECK: %[[FIELD_Z:.*]] = getelementptr {{.*}} %[[SMALL_STRUCT]]
// If the fragment covers the whole struct, we're not generating the
// DW_OP_LLVM_fragment part.
// CHECK: llvm.dbg.value(metadata {{.*}}* %[[FIELD_Z]], metadata ![[SMALL_VAR_DECL_MD]]
// CHECK-SAME: !DIExpression(DW_OP_deref)
debug_value_addr %5 : $*Builtin.Int64, var, (name "small_struct", loc "file.swift":10:11, scope 1), type $SmallStruct, expr op_fragment:#SmallStruct.z, loc "file.swift":11:13, scope 1
dealloc_stack %4 : $*SmallStruct

dealloc_stack %2 : $*MyStruct
%r = tuple()
return %r : $()
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9 changes: 9 additions & 0 deletions test/DebugInfo/sroa_mem2reg.sil
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Expand Up @@ -2,6 +2,8 @@
// RUN: %target-sil-opt -enable-sil-verify-all -sil-print-debuginfo -sroa -mem2reg %s -o %t.sil
// RUN: %FileCheck --check-prefix=CHECK-MEM2REG %s --input-file=%t.sil
// RUN: %target-swiftc_driver -Xfrontend -disable-debugger-shadow-copies -g -emit-ir %t.sil | %FileCheck --check-prefix=CHECK-IR %s
// RUN: %target-swiftc_driver -Xfrontend -disable-debugger-shadow-copies -g -c %t.sil -o %t.o
// RUN: %llvm-dwarfdump --debug-info %t.o | %FileCheck --check-prefix=CHECK-DWARF %s
sil_stage canonical

import Builtin
Expand Down Expand Up @@ -78,3 +80,10 @@ bb0(%0 : $Int64, %1 : $Int64):
// CHECK-IR-SAME: line: 7
// CHECK-IR: ![[ARG2_MD]] = !DILocalVariable(name: "in_y", arg: 2
// CHECK-IR-SAME: line: 7

// CHECK-DWARF-LABEL: DW_AT_name ("foo")
// CHECK-DWARF: DW_TAG_variable
// CHECK-DWARF: DW_AT_name ("my_struct")
// Shouldn't be marked artificial
// CHECK-DWARF-NOT: DW_AT_artificial (true)
// CHECK-DWARF: DW_TAG_{{.*}}