Re: Is "optimize" attribute on fndecl handled differently?

[FX <fxcoudert at gmail dot com>]

tl;dr: there is a bug in optimization options saving/restoring. See after the break for simple C testcase.


> Unfortunately I can't find if/where there is some explanation for
> these frontend_set_flag_* options -- how they're supposed to work and
> who's responsible for setting them.

Itâs not them, but this got me looking into the right place.

In my case, opts.câs set_fast_math_flags() is called three times:

  1. first, from toplev_main -> decode_options -> default_options_optimization -> handle_generated_option -> handle_option -> common_handle_option
  2. then, from toplev_main -> decode_options -> read_cmdline_option -> handle_option -> common_handle_option
  3. then, from my own build_function_decl -> parse_optimize_options -> decode_options -> default_options_optimization -> handle_generated_option -> handle_option -> common_handle_option

At 1 and 3, itâs called with value = 0, and at 2, with value = 1. So it is indeed a bug: because we re-parse the defaults at 3, we reset some of the flags dependent on -ffast-math (the ones quoted before), overwriting their earlier value.

PS: There is also something similar with align_functions/align_jumps/align_loops flags, but I donât have time to chase it right now.

------------------------

This is easy to see with a simple C test case:

//__attribute__ ((optimize("strength-reduce")))
int foo (float x) { return __builtin_isnan(x); }

Compiled with -O3 -ffast-math, the isnan is simplified out (fast math means no NaNs). If you uncomment the attribute (chosen because itâs actually useless), and compile again with -O3 -ffast-math: the isnan test is not simplified any more. This is because the codepath through default_options_optimization() has overwritten the value of the flags handled in set_fast_math_flags(): flag_finite_math_only, flag_signed_zeros, flag_trapping_math and flag_unsafe_math_optimizations.

Iâm CCâing the maintainers who added the optimize attribute in the first place, as they might have an idea how to fix this. This is way beyond my league!


Thanks Steven for your help!
FX