[Lldb-commits] [lldb] [lldb] Prefer DW_AT_bit_size for DW_OP_convert type width (PR #208478)

[via lldb-commits]

https://github.com/firmiana402 created https://github.com/llvm/llvm-project/pull/208478

This PR fixes a DW_OP_convert issue where LLDB derives the conversion width from DW_AT_byte_size * 8 before considering DW_AT_bit_size, #208203

For base types that carry both DW_AT_byte_size and DW_AT_bit_size, such as C _BitInt(31), DW_AT_bit_size describes the actual value width. LLDB should therefore prefer DW_AT_bit_size when it is present, and only fall back to DW_AT_byte_size otherwise.


## Tests

Extended DWARFExpression.DW_OP_convert coverage with a base type that has both DW_AT_byte_size and DW_AT_bit_size, and verifies that the conversion uses the DW_AT_bit_size width.

>From b1c7afeed41aa3ff91eaadd752a9d78b5c13d277 Mon Sep 17 00:00:00 2001
From: firmiana402 <firmiana402 at gmail.com>
Date: Thu, 9 Jul 2026 22:36:44 +0800
Subject: [PATCH] [lldb] Prefer DW_AT_bit_size for DW_OP_convert type width

---
 .../Plugins/SymbolFile/DWARF/DWARFUnit.cpp    |  4 ++--
 .../Expression/DWARFExpressionTest.cpp        | 23 +++++++++++++++++++
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp b/lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp
index 4b02124e987e8..974e0f6f875f5 100644
--- a/lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp
+++ b/lldb/source/Plugins/SymbolFile/DWARF/DWARFUnit.cpp
@@ -726,9 +726,9 @@ DWARFUnit::GetDIEBitSizeAndSign(uint64_t relative_die_offset) const {
     return llvm::createStringError("cannot resolve DW_OP_convert type DIE");
   uint64_t encoding =
       die.GetAttributeValueAsUnsigned(DW_AT_encoding, DW_ATE_hi_user);
-  uint64_t bit_size = die.GetAttributeValueAsUnsigned(DW_AT_byte_size, 0) * 8;
+  uint64_t bit_size = die.GetAttributeValueAsUnsigned(DW_AT_bit_size, 0);
   if (!bit_size)
-    bit_size = die.GetAttributeValueAsUnsigned(DW_AT_bit_size, 0);
+    bit_size = die.GetAttributeValueAsUnsigned(DW_AT_byte_size, 0) * 8;
   if (!bit_size)
     return llvm::createStringError("unsupported type size");
   bool sign;
diff --git a/lldb/unittests/Expression/DWARFExpressionTest.cpp b/lldb/unittests/Expression/DWARFExpressionTest.cpp
index 75162ca0b5f3e..6609c9da51bb0 100644
--- a/lldb/unittests/Expression/DWARFExpressionTest.cpp
+++ b/lldb/unittests/Expression/DWARFExpressionTest.cpp
@@ -445,6 +445,16 @@ TEST(DWARFExpression, DW_OP_convert) {
               Form:            DW_FORM_data1
             - Attribute:       DW_AT_byte_size
               Form:            DW_FORM_data1
+        - Code:            0x00000003
+          Tag:             DW_TAG_base_type
+          Children:        DW_CHILDREN_no
+          Attributes:
+            - Attribute:       DW_AT_encoding
+              Form:            DW_FORM_data1
+            - Attribute:       DW_AT_byte_size
+              Form:            DW_FORM_data1
+            - Attribute:       DW_AT_bit_size
+              Form:            DW_FORM_data1
   debug_info:
     - Version:         4
       AddrSize:        8
@@ -493,6 +503,12 @@ TEST(DWARFExpression, DW_OP_convert) {
           Values:
             - Value:           0x000000000000000b # DW_ATE_numeric_string
             - Value:           0x0000000000000001
+        # 0x00000020:
+        - AbbrCode:        0x00000003
+          Values:
+            - Value:           0x0000000000000005 # DW_ATE_signed
+            - Value:           0x0000000000000004
+            - Value:           0x000000000000001f
         - AbbrCode:        0x00000000
 
 )";
@@ -502,6 +518,7 @@ TEST(DWARFExpression, DW_OP_convert) {
   uint8_t offs_sint64_t = 0x00000014;
   uint8_t offs_uchar = 0x00000017;
   uint8_t offs_schar = 0x0000001a;
+  uint8_t offs_sint31_t = 0x00000020;
 
   DWARFExpressionTester t(yamldata, /*cu_index=*/1);
   ASSERT_TRUE((bool)t.GetDwarfUnit());
@@ -555,6 +572,12 @@ TEST(DWARFExpression, DW_OP_convert) {
                                offs_schar, DW_OP_stack_value}),
                        ExpectScalar(8, 'A', is_signed));
 
+  // Prefer DW_AT_bit_size over DW_AT_byte_size when both are present.
+  EXPECT_THAT_EXPECTED(
+      t.Eval({DW_OP_const4u, 0x00, 0x00, 0x00, 0x40, DW_OP_convert,
+              offs_sint31_t, DW_OP_stack_value}),
+      ExpectScalar(31, 0x40000000, is_signed));
+
   //
   // Errors.
   //