[DebugInfo] No longer assign non-fixed-size types the size of a pointer by Teemperor · Pull Request #87569 · swiftlang/swift

Teemperor · 2026-02-27T16:59:40Z

CompletedDebugTypeInfo::getFromTypeInfo currently uses the storage type as
the preferred source of size information.
For address-only types (and any other type that has no fixed size at compile
time), the storage type is always changed to a pointer.

This currently causes that some non-fixed-size types (e.g., generics)
are assigned the size of a pointer as their actual size in the generated
debug information.

For example:

protocol P {
associatedtype S
}

func foo<B: P>(_ b: B) {
let x: (aaa: Int, bbb: B.S?) = (aaa: 0, bbb: nil)
_ = x
let y: (Fx: Int, Fy: Int) = (Fx: 0, Fy: 0)
_ = y
}

results in the following DWARF output where the first struct
is given size 8 (= the size of a pointer).

0x000000d4:   DW_TAG_structure_type
                DW_AT_name	("$sSi3aaa_1S4main1PPQzSg3bbbtD")
                DW_AT_linkage_name	("$sSi3aaa_1S4main1PPQzSg3bbbtD")
                DW_AT_byte_size	(0x08)
                DW_AT_APPLE_runtime_class	(DW_LANG_Swift)

0x000000f3:   DW_TAG_structure_type
                DW_AT_name	("$sSi2Fx_Si2FytD")
                DW_AT_linkage_name	("$sSi2Fx_Si2FytD")
                DW_AT_byte_size	(0x10)
                DW_AT_APPLE_runtime_class	(DW_LANG_Swift)

This change modifies CompletedDebugTypeInfo::getFromTypeInfo to ignore the
storage size for types that do not have fixed size. Generics now have no
specified size or 0 in DWARF. FixedArray<A, B> without substituted type/size
is now also emitted using the dedicated FixedArray code.

Teemperor · 2026-02-27T16:59:53Z

@swift-ci please test

Teemperor · 2026-02-27T17:14:39Z

test/DebugInfo/value-generics.swift


 extension Slab: Copyable where Element: Copyable {}

-// CHECK-DAG: !DICompositeType({{.*}}name: "$sxq_BVD"


I'm not sure if this change is an improvement or not, but the fact that we emit the new name is actually because we his the code path for the specific type here:

case TypeKind::BuiltinFixedArray: { if (Opts.DebugInfoLevel > IRGenDebugInfoLevel::ASTTypes) { // DISABLED IN THE TEST } unsigned FwdDeclLine = 0; return createOpaqueStruct(Scope, "Builtin.FixedArray", MainFile, // << HERE FwdDeclLine, SizeInBits, AlignInBits, Flags, MangledName); }

adrian-prantl · 2026-02-27T19:57:46Z

lib/IRGen/IRGenDebugInfo.cpp

+        // If we do not know the size of this type, pass a size of 0.
+        // FIXME: createEnumType should not require a sized debug info type.
+        if (!CompletedDbgTy)
+          CompletedDbgTy = CompletedDebugTypeInfo::get(DbgTy, 0);


I guess your FIXME makes that clear already, but this defeats the purpose of CompletedDebugTypeInfo. Why can't we call createOpaqueStructWithSizedContainer() here any more?

Tbh, I just want to keep this change as uninvasive as possible just so we can unblock main. Maybe we could call this, but then even more stuff breaks.

adrian-prantl · 2026-02-27T19:58:06Z

lib/IRGen/IRGenDebugInfo.cpp

+    const bool RequiresSize =
+      !isa<PrimaryArchetypeType>(DbgTy.getType()) &&
+      !DbgTy.getType()->hasArchetype() &&
+      !isa<TypeAliasType>(DbgTy.getType());


adrian-prantl · 2026-02-27T19:59:41Z

test/DebugInfo/value-generics.swift

 extension Slab: Copyable where Element: Copyable {}

-// CHECK-DAG: !DICompositeType({{.*}}name: "$sxq_BVD"
+// CHECK-DAG: !DICompositeType({{.*}}name: "Builtin.FixedArray"


I think that is fine, but can you CHECK for the mangled name instead (or in addition)? The mangled names are more important for LLDB's functionality than the human-readable ones.

…entation Swift commit fcbebc5 added support for emitting Builtin.FixedArray as a DW_TAG_array_type. With swiftlang/swift#87569 this code path is now also taken for `Builtin.FixedArray<A, B>` which is the non-substituted `FixedArray` class. Before PR llvm#87569 the DWARF emitted for `FixedArray<A, B>` looked like this: DW_TAG_structure_type DW_AT_name ("$exq_BVD") DW_AT_linkage_name ("$exq_BVD") DW_AT_declaration (true) DW_AT_APPLE_runtime_class (DW_LANG_Swift) and the new DWARF that is emitted now takes the FixedArray code path which produces this DWARF: DW_TAG_array_type DW_AT_type (0x00006c70 "$eq_D") DW_TAG_structure_type DW_AT_name ("$eq_D") DW_AT_byte_size (0x00) DW_AT_APPLE_runtime_class (DW_LANG_Swift) The patch adds support for parsing this specific pattern. We curiously don't need to ever parse the substituted array types for embedded Swift, so this patch just adds enough support for the unsubstituted type.

augusto2112 · 2026-03-12T17:53:57Z

lib/IRGen/IRGenDebugInfo.cpp

+    // then only emit a forward declaration.
+    // TODO: This check can probably be simplified and generalized.
+    const bool RequiresSize = !isa<PrimaryArchetypeType>(DbgTy.getType()) &&
+                              !DbgTy.getType()->hasArchetype() &&


If the archetype is constrained to AnyObject (a class) the size is known.

struct W<T: AnyObject> { let t: T }

I'm not sure if there are other cases, I think not.

I guess constraining T to a class directly, a protocol that is itself constrained to any object. and maybe @objc protocols too.

I feel like this check (i.e., does this type have a fixed size) should already exist somewhere in the compiler and we should reuse it, but I don't know what is the right function here.

TypeInfo::IsFixedSize should do it, unless you don't have access to a TypeInfo here?

augusto2112

This is a nice change.

Like I mentioned on swiftlang/llvm-project#12571 though I think that you should figure out why FixedArrays are being emitted as $eq_D first.

augusto2112

Actually I think this change is correct. I commented on swiftlang/llvm-project#12571 what I think needs to be done to fix the FixedArray change.

…entation Swift commit fcbebc5 added support for emitting Builtin.FixedArray as a DW_TAG_array_type. With swiftlang/swift#87569 this code path is now also taken for `Builtin.FixedArray<A, B>` which is the non-substituted `FixedArray` class. Before PR llvm#87569 the DWARF emitted for `FixedArray<A, B>` looked like this: DW_TAG_structure_type DW_AT_name ("$exq_BVD") DW_AT_linkage_name ("$exq_BVD") DW_AT_declaration (true) DW_AT_APPLE_runtime_class (DW_LANG_Swift) and the new DWARF that is emitted now takes the FixedArray code path which produces this DWARF: DW_TAG_array_type DW_AT_type (0x00006c70 "$eq_D") DW_TAG_structure_type DW_AT_name ("$eq_D") DW_AT_byte_size (0x00) DW_AT_APPLE_runtime_class (DW_LANG_Swift) The patch adds support for parsing this specific pattern. We curiously don't need to ever parse the substituted array types for embedded Swift, so this patch just adds enough support for the unsubstituted type.

`CompletedDebugTypeInfo::getFromTypeInfo` currently uses the storage type as the preferred source of size information. For address-only types (and any other type that has no fixed size at compile time), the storage type is always changed to a pointer. This currently causes that some non-fixed-size types (e.g., generics) are assigned the size of a pointer as their actual size in the generated debug information. For example: ``` protocol P { associatedtype S } func foo<B: P>(_ b: B) { let x: (aaa: Int, bbb: B.S?) = (aaa: 0, bbb: nil) _ = x let y: (Fx: Int, Fy: Int) = (Fx: 0, Fy: 0) _ = y } ``` results in the following DWARF output where the first struct is given size 8 (= the size of a pointer). ``` 0x000000d4: DW_TAG_structure_type DW_AT_name ("$sSi3aaa_1S4main1PPQzSg3bbbtD") DW_AT_linkage_name ("$sSi3aaa_1S4main1PPQzSg3bbbtD") DW_AT_byte_size (0x08) DW_AT_APPLE_runtime_class (DW_LANG_Swift) 0x000000f3: DW_TAG_structure_type DW_AT_name ("$sSi2Fx_Si2FytD") DW_AT_linkage_name ("$sSi2Fx_Si2FytD") DW_AT_byte_size (0x10) DW_AT_APPLE_runtime_class (DW_LANG_Swift) ``` This change modifies `CompletedDebugTypeInfo::getFromTypeInfo` to ignore the storage size for types that do not have fixed size. Generics now have no specified size or 0 in DWARF. FixedArray<A, B> without substituted type/size is now also emitted using the dedicated FixedArray code.

Teemperor · 2026-03-18T13:58:40Z

@swift-ci please test and merge

Teemperor · 2026-03-18T14:35:43Z

@swift-ci please test

…entation Swift commit fcbebc5 added support for emitting Builtin.FixedArray as a DW_TAG_array_type. With swiftlang/swift#87569 this code path is now also taken for `Builtin.FixedArray<A, B>` which is the non-substituted `FixedArray` class. Before PR llvm#87569 the DWARF emitted for `FixedArray<A, B>` looked like this: DW_TAG_structure_type DW_AT_name ("$exq_BVD") DW_AT_linkage_name ("$exq_BVD") DW_AT_declaration (true) DW_AT_APPLE_runtime_class (DW_LANG_Swift) and the new DWARF that is emitted now takes the FixedArray code path which produces this DWARF: DW_TAG_array_type DW_AT_type (0x00006c70 "$eq_D") DW_TAG_structure_type DW_AT_name ("$eq_D") DW_AT_byte_size (0x00) DW_AT_APPLE_runtime_class (DW_LANG_Swift) The patch adds support for parsing this specific pattern. We curiously don't need to ever parse the substituted array types for embedded Swift, so this patch just adds enough support for the unsubstituted type. (cherry picked from commit 1e1af53)

Teemperor · 2026-03-19T07:10:35Z

@swift-ci please test

Teemperor · 2026-03-20T15:10:38Z

@swift-ci smoke test macOS

Teemperor requested a review from adrian-prantl as a code owner February 27, 2026 16:59

Teemperor commented Feb 27, 2026

View reviewed changes

Teemperor requested a review from augusto2112 February 27, 2026 17:15

Teemperor mentioned this pull request Feb 27, 2026

Assertion failed: SizeInBits < getSizeInBits(Var) && "piece covers entire var", file S:\SourceCache\swift\lib\IRGen\IRGenDebugInfo.cpp, line 3776 #87290

Open

adrian-prantl reviewed Feb 27, 2026

View reviewed changes

Teemperor force-pushed the StorageType branch from e489f28 to f287f11 Compare March 12, 2026 11:57

Teemperor mentioned this pull request Mar 12, 2026

[lldb][swift] Support parsing Builtin.FixedArray<A, B>'s DWARF representation swiftlang/llvm-project#12571

Merged

Teemperor force-pushed the StorageType branch from f287f11 to 8c4534b Compare March 12, 2026 16:05

augusto2112 reviewed Mar 12, 2026

View reviewed changes

augusto2112 approved these changes Mar 12, 2026

View reviewed changes

Teemperor changed the title ~~[WIP][DebugInfo] No longer assign non-fixed-size types the size of a …~~ [DebugInfo] No longer assign non-fixed-size types the size of a … Mar 13, 2026

Teemperor changed the title ~~[DebugInfo] No longer assign non-fixed-size types the size of a …~~ [DebugInfo] No longer assign non-fixed-size types the size of a pointer Mar 13, 2026

Teemperor force-pushed the StorageType branch from 8c4534b to b772384 Compare March 13, 2026 13:43

Teemperor enabled auto-merge March 18, 2026 15:08

Teemperor disabled auto-merge March 19, 2026 08:49

Teemperor enabled auto-merge March 19, 2026 08:49

Teemperor disabled auto-merge March 20, 2026 11:38

Teemperor enabled auto-merge March 20, 2026 15:10

Teemperor merged commit 345bda9 into swiftlang:main Mar 20, 2026
4 of 6 checks passed


		extension Slab: Copyable where Element: Copyable {}

		// CHECK-DAG: !DICompositeType({{.*}}name: "$sxq_BVD"

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Teemperor commented Feb 27, 2026 •

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