[PATCH] LoongArch: Optimize math barriers

[Andreas Schwab]

On Mai 07 2026, Deng Jianbo wrote:

> The constraints 'frm' used in math_opt_barrier and math_force_eval cause
> GCC choose fixed-point registers with lower costs in some cases, because
> in LoongArch ira_reg_class_subunion[FP_REGS][GR_REGS] is initialized to
> GR_REGS, extra move costs will be added when choosing FP_REGS. This
> results in unnecessary instructions to move values between FP_REGS and
> GR_REGS.

It looks like a bug in GCC (wrong cost calculations) if it generates
unnecessary moves when it has more choices.

> Most of cases in GLIBC math barriers related macros are invoked with
> floating-point type paramters, this patch removes "r" constraints,
> allowing GCC to keep values in floating-point registers and avoid the
> extra moves.

It does not *allow* that, it *forces* gcc to use an fp reg (or memory).
What if the value is in a general reg in the first place, wouldn't that
result in unnecessary moves?

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."