On Thu, Oct 4, 2018 at 1:10 AM Jeff Law <l...@redhat.com> wrote:
>
> On 9/4/18 9:27 AM, Wilco Dijkstra wrote:
> >
> > ping
> >
> >
> >
> >
> > From: Wilco Dijkstra
> > Sent: 18 June 2018 15:01
> > To: GCC Patches
> > Cc: nd; Joseph Myers
> > Subject: [PATCH v3] Change default to -fno-math-errno
> >
> >
> > 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 <wdijk...@arm.com>
> >
> > * 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 I went back and reviewed all the discussion around this. I'm still
> having trouble getting comfortable with flipping the default -- unless
> we know ahead of time that the target runtime doesn't set errno on any
> of the math routines. That implies a target hook to describe the
> runtime's handling off errno in math functions. It also introduces
> target dependencies early in the GIMPLE pipeline which is generally
> counter to design goals around GIMPLE.
>
> Essentially if we flip the default, we run the risk that user code which
> does check this stuff will silently break. That's not a good position
> to take IMHO.
>
> One could also argue that if we're using -fno-math-errno and we see
> errno read after a math call without it being set/clobbered by some
> other non-math call then we should be warning the user. That'd be some
> kind of propagation engine in gimple. That would make flipping the
> default more reasonable -- we'd have a reasonable chance of warning the
> user that their code is potentially buggy and may need adjustment.
>
> That led me to wonder if we could prove that for the majority of FP
> intensive codes that even if the library set errno that nobody could
> possibly be reading it, then we could in large part treat those builtin
> calls as -fno-math-errno (we'd mark them somehow and check the mark at
> GIMPLE->RTL expansion time (and possibly other points) to allow us to
> generate machine instructions rather than library calls). That would
> get most of the benefit without the possibility to breaking user code.
One major issue here is that there's no way for GCC to know what
access constitutes an access to errno. We already have "hacks" in place
that even though things like pow() techincally clobber memory we only
consider (with TBAA) clobbers of 'int' typed memory.
glibc currently calls __errno_location to get at a pointer to errno, so
theoretically we could add a __attribute__((returns_errno_location))
and a __attribute__((errno)) for implementations that have a global
(TLS?) variable __errno. Then points-to analysis could track this,
but that also means this tracking would be weak.
The other issue is that we're treating -fno-math-errno as disabling
errno handling in general (not only for math functions). That would
have to be fixed when we switch the default (or even now... though
nobody uses errno set from malloc & friends which is where we abuse
the flag).
Richard.
> I'm willing to go with Joseph's recommendation here, but I'm not really
> comfortable acking the patch as-is on my own.
>
> jeff
