On Wed, Jan 14, 2026 at 7:44 PM Y Song <[email protected]> wrote:
> Thanks. Now let us say I will not try to add those "changed function
> signatures" to dwarf
> and actually rely on dwarf locations to somehow recover parameters.
That isn't possible, so far as I can see - DWARF only describes how to
access the parameters from inside the function, not how they were passed on
the call. (eg: an unoptimized build will probably describe the parameters
as being in stack memory even though the values were passed in registers -
because the prologue moves the values from registers to memory, and the
DWARF locations only have to be valid after the prologue)
> I understand
> in some cases, it is not easy to recover if dwarf locations are too
> complicated and we
> might ignore those cases.
>
> I actually want to ask how to identify whether the return type is
> changed or not in dwarf.
> The following is an example:
>
> $ cat test.c
> #include <stdio.h>
> unsigned tar(int a);
> __attribute__((noinline)) static int foo(int a, int b)
> {
> return tar(a) + tar(a + 1);
> }
> __attribute__((noinline)) int bar(int a)
> {
> foo(a, 1);
> return 0;
> }
>
> In this particular case, the return value of foo() is actually not used
> and the compiler will optimize it away with returning void in llvm.
>
https://dwarfstd.org/issues/221105.1.html in DWARFv6 might help provide
some of this information & is closer to an ABI-encoding than we have for
parameters.
>
> $ clang -O2 -g -c test.c
> $ llvm-dwarfdump test.o
> ...
> 0x0000004e: DW_TAG_subprogram
> DW_AT_low_pc (0x0000000000000010)
> DW_AT_high_pc (0x0000000000000022)
> DW_AT_frame_base (DW_OP_reg7 RSP)
> DW_AT_call_all_calls (true)
> DW_AT_name ("foo")
> DW_AT_decl_file
> ("/home/yhs/tests/sig-change/deadret/test.c")
> DW_AT_decl_line (3)
> DW_AT_prototyped (true)
> DW_AT_calling_convention (DW_CC_nocall)
> DW_AT_type (0x00000096 "int")
>
> 0x0000005e: DW_TAG_formal_parameter
> DW_AT_location (indexed (0x1) loclist =
> 0x00000022:
> [0x0000000000000010, 0x0000000000000018): DW_OP_reg5
> RDI
> [0x0000000000000018, 0x000000000000001a): DW_OP_reg3
> RBX
> [0x000000000000001a, 0x0000000000000022):
> DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value)
> DW_AT_name ("a")
> DW_AT_decl_file
> ("/home/yhs/tests/sig-change/deadret/test.c")
> DW_AT_decl_line (3)
> DW_AT_type (0x00000096 "int")
>
> 0x00000067: DW_TAG_formal_parameter
> DW_AT_name ("b")
> DW_AT_decl_file
> ("/home/yhs/tests/sig-change/deadret/test.c")
> DW_AT_decl_line (3)
> DW_AT_type (0x00000096 "int")
> ...
> Assembly code:
> 0000000000000000 <bar>:
> 0: 50 pushq %rax
> 1: e8 0a 00 00 00 callq 0x10 <foo>
> 6: 31 c0 xorl %eax, %eax
> 8: 59 popq %rcx
> 9: c3 retq
> a: 66 0f 1f 44 00 00 nopw (%rax,%rax)
>
> 0000000000000010 <foo>:
> 10: 53 pushq %rbx
> 11: 89 fb movl %edi, %ebx
> 13: e8 00 00 00 00 callq 0x18 <foo+0x8>
> 18: ff c3 incl %ebx
> 1a: 89 df movl %ebx, %edi
> 1c: 5b popq %rbx
> 1d: e9 00 00 00 00 jmp 0x22 <foo+0x12>
>
> The compiler knows whether the return type has changed or not.
> Unfortunately the information is not available in dwarf.
>
> Any suggestions to add some additional information in dwarf so we can
> find whether the return type is changed or not?
>
--
Dwarf-discuss mailing list
[email protected]
https://lists.dwarfstd.org/mailman/listinfo/dwarf-discuss
