https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98776
Bug ID: 98776
Summary: DW_AT_low_pc is inconsistent with function entry
address, when enabling -fpatchable-function-entry
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: debug
Assignee: unassigned at gcc dot gnu.org
Reporter: lvlin at mail dot ustc.edu.cn
Target Milestone: ---
Target: x86,arm64
-fpatchable-function-entry will generate N NOPs at the beginning of each
function. Observe the binary compiled by gcc, the function entry address is
inconsistent with the value of DW_AT_low_pc in the corresponding DWARF data.
I used a toy example to describe the issue;
1.Compile the source file toy_exam.c
$ gcc -o toy_exam.gcc toy_exam.c -g -gdwarf-4
-fpatchable-function-entry=2 -save-temps
2.Check the symbolic address of the function fun_a
$ readelf -s toy_exam.gcc |grep -w fun_a
95: 00000000000007f0 80 FUNC GLOBAL DEFAULT 13 fun_a
3.Display assembler contents
objdump -d toy_exam.gcc |grep -A 8 -w \<fun_a\>:
00000000000007f0 <fun_a>:
7f0: d503201f nop
7f4: d503201f nop
7f8: a9be7bfd stp x29, x30, [sp, #-32]!
7fc: 910003fd mov x29, sp
800: 52800040 mov w0, #0x2 // #2
804: b90017e0 str w0, [sp, #20]
808: 528000a0 mov w0, #0x5 // #5
80c: b9001be0 str w0, [sp, #24]
4.dump dwarf info
$ llvm-dwarfdump toy_exam.gcc |grep -C 10 -w fun_a
0x00000315: DW_TAG_subprogram
DW_AT_external (true)
DW_AT_name ("fun_a")
DW_AT_decl_file ("/home/jianlin/code/test/toy_exam.c")
DW_AT_decl_line (14)
DW_AT_decl_column (0x06)
DW_AT_low_pc (0x00000000000007f8)
DW_AT_high_pc (0x0000000000000840)
DW_AT_frame_base (DW_OP_call_frame_cfa)
DW_AT_GNU_all_tail_call_sites (true)
DW_AT_sibling (0x0000035d)
5. Assembler code
fun_a:
.section __patchable_function_entries
.8byte .LPFE2
.text
.LPFE2:
nop
nop
.LFB7:
.loc 1 15 1
.cfi_startproc
stp x29, x30, [sp, -32]!
.cfi_def_cfa_offset 32
.cfi_offset 29, -32
.cfi_offset 30, -24
mov x29, sp
.loc 1 16 13
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/gcc-trunk/libexec/gcc/aarch64-unknown-linux-gnu/11.0.0/lto-wrapper
Target: aarch64-unknown-linux-gnu
Configured with: ../gcc/configure --prefix=/usr/gcc-trunk
--enable-languages=c,c++,fortran --disable-libquadmath
--disable-libquadmath-support --disable-werror --disable-bootstrap
--enable-gold
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.0 20210119 (experimental) (GCC)
the first instruction in the compile unit indicated by DW_AT_low_pc does not
include NOP.
GCC-9, GCC-10 and the latest master branch were respectively tested, and the
results were the same.
