[llvm] Omit member size from DWARF when desired (PR #161423)

[Tom Tromey via llvm-commits]

https://github.com/tromey updated https://github.com/llvm/llvm-project/pull/161423

>From a4b71a67c40f3feb792d2341f6c58318381ff554 Mon Sep 17 00:00:00 2001
From: Tom Tromey <tromey at adacore.com>
Date: Tue, 30 Sep 2025 12:30:09 -0600
Subject: [PATCH] Omit member size from DWARF when desired

It's reasonably normal for DWARF not to explicitly state the size of a
member of a composite type -- in this case the size is taken from the
type.  LLVM does allow a member to have a NULL size, but in this case
it emits a size of "0".  This patch cleans this up a bit, changing the
DWARF emission code to simply not emit a size in this situation.
---
 llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp   |  7 ++++---
 llvm/test/DebugInfo/X86/dynamic-bitfield.ll | 13 +++++++++++--
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index 62fb5eb011cf2..3cfe7cc12d5b6 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1889,11 +1889,12 @@ DIE &DwarfUnit::constructMemberDIE(DIE &Buffer, const DIDerivedType *DT) {
     bool IsBitfield = DT->isBitField();
 
     // Handle the size.
-    if (auto *Var = dyn_cast_or_null<DIVariable>(DT->getRawSizeInBits())) {
+    if (DT->getRawSizeInBits() == nullptr) {
+      // No size, just ignore.
+    } else if (auto *Var = dyn_cast<DIVariable>(DT->getRawSizeInBits())) {
       if (auto *VarDIE = getDIE(Var))
         addDIEEntry(MemberDie, dwarf::DW_AT_bit_size, *VarDIE);
-    } else if (auto *Exp =
-                   dyn_cast_or_null<DIExpression>(DT->getRawSizeInBits())) {
+    } else if (auto *Exp = dyn_cast<DIExpression>(DT->getRawSizeInBits())) {
       DIELoc *Loc = new (DIEValueAllocator) DIELoc;
       DIEDwarfExpression DwarfExpr(*Asm, getCU(), *Loc);
       DwarfExpr.setMemoryLocationKind();
diff --git a/llvm/test/DebugInfo/X86/dynamic-bitfield.ll b/llvm/test/DebugInfo/X86/dynamic-bitfield.ll
index c9148ca4582f6..f8935977c64e7 100644
--- a/llvm/test/DebugInfo/X86/dynamic-bitfield.ll
+++ b/llvm/test/DebugInfo/X86/dynamic-bitfield.ll
@@ -27,7 +27,7 @@ source_filename = "bitfield.c"
 !6 = !{}
 !7 = !{!0, !2}
 !8 = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "PackedBits", file: !5, line: 3, size: 40, elements: !9)
-!9 = !{!10, !12, !16}
+!9 = !{!10, !12, !16, !21}
 !10 = !DIDerivedType(tag: DW_TAG_member, name: "a", scope: !8, file: !5, line: 5, baseType: !11, size: 8)
 ; CHECK: DW_TAG_member
 ; CHECK-NEXT: DW_AT_name{{.*}}"a"
@@ -60,5 +60,14 @@ source_filename = "bitfield.c"
 ; CHECK:      DW_AT_bit_size             [DW_FORM_exprloc]	(DW_OP_lit27)
 ; CHECK-NEXT: DW_AT_data_bit_offset      [DW_FORM_exprloc]	(DW_OP_lit13)
 ; CHECK-NOT:  DW_AT_data_member_location
-; CHECK: DW_TAG
 !20 = !{!"clang version 3.9.0 (trunk 267633)"}
+!21 = !DIDerivedType(tag: DW_TAG_member, name: "d", scope: !8, file: !5, line: 7, baseType: !13, offset: !DIExpression(DW_OP_constu, 15), flags: DIFlagBitField)
+; CHECK: DW_TAG_member
+; CHECK-NEXT: DW_AT_name{{.*}}"d"
+; CHECK-NOT:  DW_TAG
+; CHECK-NOT:  DW_AT_bit_offset
+; CHECK-NOT:  DW_AT_byte_size
+; CHECK-NOT:  DW_AT_bit_size
+; CHECK:      DW_AT_data_bit_offset      [DW_FORM_exprloc]	(DW_OP_lit15)
+; CHECK-NOT:  DW_AT_data_member_location
+; CHECK: DW_TAG