[PATCH 0/6] Use correct function address in dwarf assembler
Doug Evans
dje@google.com
Tue Nov 4 23:01:00 GMT 2014
Yao Qi writes:
> When we use dwarf assembler, it is common to generate DW_AT_low_pc and
> DW_AT_high_pc attributes like this:
>
> subprogram {
> {name main}
> {low_pc main addr}
> {high_pc main+0x100 addr}
> }
>
> however, main is not resolved to function main's address on some
> targets such as arm thumb mode and powerpc64. One approach is to
> get function address by labels, like this:
>
> asm ("func_start: .globl func_start");
> static void func (void) {}
> asm ("func_end: .globl func_end");
>
> however, some compiler, such as clang, can't guarantee the order of
> these labels and function, so it isn't portable.
>
> In this patch series, we propose a new approach
>
> 1. get function start and end address correctly in a portable way,
> 2. don't affect too much on existing test cases.
>
> In patch 2/6, a new proc function_range is added. In this proc, the
> source file is compiled with debug info.
>
> int main (void)
> {
> asm ("main_label: .globl main_label");
> return 0;
> }
>
> we can get the offset of main_label in function main, and then compute
> the start address of main via main_label - main_label_offset. This is
> portable.
>
> In order to avoid much changes to existing test cases, we have to use
> function_range inside dwarf assembler. Then, I invent some attribute
> macros which look like DW attributes, but can be expanded to one or
> more standard attributes. The TAG_subprogram above will be simplified
> with macro attribute used, it becomes:
>
> subprogram {
> {MACRO_AT_func { func ${srcdir}/${subdir}/${srcfile} }}
> }
>
> With this macro attribute, attributes name, low_pc and high_pc are
> generated and set correctly. See more details in patch 2/6.
>
> Patch 4/6 is to use dwarf assembler to generate .S file, which is
> identical to .S current one we are using. Patch 3 and 5 is to use
> macro attributes to get function start and end address correctly.
> Patch 6 is remove labels and switch to use macro attribute to
> get function start address to make clang happy.
>
> I test the whole series by running tests under gdb.dwarf2/ on
> powerpc-linux (both 32-bit and 64-bit), arm-none-eabi (arm mode
> and thumb mode) and x86_64-linux.
>
> Comments are very welcome!
Hi.
In general I like this a lot.
Just a few outstanding questions mentioned in replies to
the relevant patches.
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