Re: [PATCH] Various fixes for DWARF register size computation

[Jakub Jelinek <jakub@redhat.com>]

On Tue, Jan 03, 2023 at 12:15:23PM +0100, Florian Weimer wrote:
> --- a/gcc/debug.h
> +++ b/gcc/debug.h
> @@ -245,7 +245,18 @@ extern const struct gcc_debug_hooks vmsdbg_debug_hooks;
>  
>  /* Dwarf2 frame information.  */
>  
> -extern int dwarf_reg_sizes_constant ();
> +/* Query size information about DWARF registers.  */
> +struct dwarf_single_register_size
> +{
> +  dwarf_single_register_size();

Formatting, space before (

> @@ -334,27 +333,39 @@ generate_dwarf_reg_sizes (poly_uint16 *sizes)
>      targetm.init_dwarf_reg_sizes_extra (sizes);
>  }
>  
> -/* Return 0 if the DWARF register sizes are not constant, otherwise
> -   return the size constant.  */
> -
> -int
> -dwarf_reg_sizes_constant ()
> +dwarf_single_register_size::dwarf_single_register_size()

Likewise.

> +  for (int i = DWARF_FRAME_REGISTERS; i >= 0; --i)
> +    {
> +      unsigned short value;
> +      if (!sizes[i].is_constant (&value) || value != 0)

    if (!known_eq (sizes[i], 0))
?

Though, I still wonder, because all of this is a hack for a single target
- x86_64-linux -m64 - I think no other target has similar constant sizes, whether
it wouldn't be better to revert all this compiler side stuff and handle it
purely on the libgcc side - allow target headers to specify a simple
expression how to compute dwarf_reg_size + don't define dwarf_reg_size_table
array in that case and instead in a testcase verify that
__builtin_init_dwarf_reg_size_table initializes an array to the exact same
values as the libgcc/config/**/*.h overridden dwarf_reg_size version.
That way, for x86_64-linux we can use
((index) <= __LIBGCC_DWARF_FRAME_REGISTERS__ ? 8 : 0)
but could provide something reasonable even for other targets if it improves
the unwinder sufficiently.
Say s390x-linux -m64 is
((index) <= 32 ? 8 : (index) == 33 ? 4 : 0)
etc.

Or, if you want to do it on the compiler side, instead of predefining
__LIBGCC_DWARF_REG_SIZES_CONSTANT__ and __LIBGCC_DWARF_REG_MAXIMUM__
register conditionally a new builtin, __builtin_dwarf_reg_size,
which would be defined only if -fbuilding-libgcc and the compiler determines
dwarf_reg_size is desirable to be computed inline without a table and
would fold the builtin to say that
index <= 16U ? 8 : 0 on x86_64 -m64,
index <= 9U ? 4 : index - 11U <= 5U ? 12 : 0 on x86_64 -m32 etc.
and if the expression is too large/complex, wouldn't predefine the builtin.

Then you can
#if __has_builtin(__builtin_dwarf_reg_size)
use the builtin and don't provide the table + initialize it,
otherwise initialize + use the table.

Or, is it actually the use of table that is bad on the unwinder side,
or lack of a small upper bound for what you get from the table?
In that case you could predefine upper bound on the sizes instead (if
constant) and simply add if (size > __LIBGCC_DWARF_REG_SIZE_MAX__)
__builtin_unreachable ()).

	Jakub