On 06/18/2018 08:01 AM, Wilco Dijkstra wrote:
> GCC currently defaults to -fmath-errno. This generates code assuming math
> functions set errno and the application checks errno. Few applications
> test errno and various systems and math libraries no longer set errno since it
> is optional. GCC generates much faster code for simple math functions with
> -fno-math-errno such as sqrt and lround (avoiding a call and PLT redirection).
> Therefore it is reasonable to change the default to -fno-math-errno. This is
> already the case for non-C languages. Only change the default for C99 and
> later.
>
> long f(float x) { return lroundf(x) + 1; }
>
> by default:
>
> f:
> str x30, [sp, -16]!
> bl lroundf
> add x0, x0, 1
> ldr x30, [sp], 16
> ret
>
> With -fno-math-errno:
>
> f:
> fcvtas x0, s0
> add x0, x0, 1
> ret
>
> Passes regress on AArch64. OK for commit?
>
> ChangeLog:
> 2018-06-18 Wilco Dijkstra <[email protected]>
>
> * common.opt (fmath-errno): Change default to 0.
> * opts.c (set_fast_math_flags): Force -fno-math-errno with -ffast-math.
> * c-family/c-opts.c (c_common_init_options_struct): Set flag_errno_math
> to special value.
> (c_common_post_options): Set flag_errno_math default based on language.
>
> doc/
> * invoke.texi (-fmath-errno) Update documentation.
>
> testsuite/
>
> * gcc.dg/errno-1.c: Add -fmath-errno.
> * gcc.dg/torture/pr68264.c: Likewise.
> * gcc.dg/tree-ssa/ssa-dse-15.c: Likewise.
> * gcc.target/aarch64/no-inline-lrint_1.c: Likewise.
So do we need to set or check math_errhandling & MATH_ERRNO at all? Or
are those really meant for the user to test? (which I think just
indicates we need to make sure they're set correctly).
WRT library behavior, I thought it was only complex arithmetic that had
the option of setting errno. I thought float/double math functions had
a requirement to set errno?
While I would love to see us drop the need to ensure errno is set as it
allows us to exploit processor capabilities, I want to make sure that we
stay conformant with the relevant standards.
jeff
>
