[LLVM] Use DWARFv4 bitfields when tuning for GDB

GDB implemented data_bit_offset in https://sourceware.org/bugzilla/show_bug.cgi?id=12616
which has been present since GDB 8.0.

GCC started using it at GCC 11.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D135583

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index d73e58a..fa51546 100644 (file)
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -624,6 +624,13 @@ proposals are standardized and available in all major engines.
 DWARF Support in Clang
 ----------------------
 
+Previously when emitting DWARFv4 and tuning for GDB, Clang would use DWARF v2's
+``DW_AT_bit_offset`` and ``DW_AT_data_member_location``. Clang now uses DWARF v4's
+``DW_AT_data_bit_offset`` regardless of tuning.
+
+Support for ``DW_AT_data_bit_offset`` was added in GDB 8.0. For earlier versions,
+you can use the ``-gdwarf-3`` option to emit compatible DWARF.
+
 Arm and AArch64 Support in Clang
 --------------------------------
 

diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
index dfdae3c..3918899 100644 (file)
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -177,7 +177,12 @@ Changes to the Metadata Info
 Changes to the Debug Info
 ---------------------------------
 
-During this release ...
+Previously when emitting DWARF v4 and tuning for GDB, llc would use DWARF v2's
+``DW_AT_bit_offset`` and ``DW_AT_data_member_location``. llc now uses DWARF v4's
+``DW_AT_data_bit_offset`` regardless of tuning.
+
+Support for ``DW_AT_data_bit_offset`` was added in GDB 8.0. For earlier versions,
+you can use llc's ``-dwarf-version=3`` option to emit compatible DWARF.
 
 Changes to the LLVM tools
 ---------------------------------

diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
index 54af144..d3e2bef 100644 (file)
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -428,8 +428,7 @@ DwarfDebug::DwarfDebug(AsmPrinter *A)
   // https://sourceware.org/bugzilla/show_bug.cgi?id=11616
   UseGNUTLSOpcode = tuneForGDB() || DwarfVersion < 3;
 
-  // GDB does not fully support the DWARF 4 representation for bitfields.
-  UseDWARF2Bitfields = (DwarfVersion < 4) || tuneForGDB();
+  UseDWARF2Bitfields = DwarfVersion < 4;
 
   // The DWARF v5 string offsets table has - possibly shared - contributions
   // from each compile and type unit each preceded by a header. The string

diff --git a/llvm/test/DebugInfo/X86/bitfields-dwarf4.ll b/llvm/test/DebugInfo/X86/bitfields-dwarf4.ll
index 0186537..23ddee4 100644 (file)
--- a/llvm/test/DebugInfo/X86/bitfields-dwarf4.ll
+++ b/llvm/test/DebugInfo/X86/bitfields-dwarf4.ll
@@ -1,8 +1,9 @@
 ; RUN: llc -mtriple x86_64-apple-macosx -O0 -filetype=obj -o - %s \
 ; RUN: | llvm-dwarfdump -v -debug-info - | FileCheck %s
 ; RUN: llc -mtriple x86_64-gnu-linux -O0 -filetype=obj -o - %s \
-; RUN: | llvm-dwarfdump -v -debug-info - | FileCheck %s --check-prefix=LINUX
-; LINUX-NOT: DW_AT_data_bit_offset
+; RUN: | llvm-dwarfdump -v -debug-info - | FileCheck %s
+; RUN: llc -mtriple x86_64-gnu-linux -O0 -debugger-tune=gdb -filetype=obj -o - %s \
+; RUN: | llvm-dwarfdump -v -debug-info - | FileCheck %s
 ;
 ; Generated from:
 ;   #include <stdint.h>

diff --git a/llvm/test/DebugInfo/X86/packed_bitfields.ll b/llvm/test/DebugInfo/X86/packed_bitfields.ll
index c36df72..0e541f0 100644 (file)
--- a/llvm/test/DebugInfo/X86/packed_bitfields.ll
+++ b/llvm/test/DebugInfo/X86/packed_bitfields.ll
@@ -1,7 +1,5 @@
 ; RUN: llc -dwarf-version=2 -mtriple x86_64-apple-macosx -O0 -filetype=obj -o %t_2_le.o %s
 ; RUN: llvm-dwarfdump -v -debug-info %t_2_le.o | FileCheck %s
-; RUN: llc -dwarf-version=4 -debugger-tune=gdb -mtriple x86_64-apple-macosx -O0 -filetype=obj -o %t_4_le.o %s
-; RUN: llvm-dwarfdump -v -debug-info %t_4_le.o | FileCheck %s
 
 ; Produced at -O0 from:
 ; struct {