Re: Add option for whether ceil etc. can raise "inexact", adjust x86 conditions

[Joseph Myers <joseph at codesourcery dot com>]

On Tue, 15 Aug 2017, Martin Jambor wrote:

> I am not sure what to do about this, to me it seems that the
> -ffp-int-builtin-inexact simply has a wrong default value, at least
> for x86_64, as it was added in order not to slow code down but does
> exactly that (all of the slowdown of course disappears when
> -fno-fp-int-builtin-inexact is used).
> 
> Or is the situation somehow more complex?

It's supposed to be that -ffp-int-builtin-inexact allows inexact to be 
raised, and is on by default, and -fno-fp-int-builtin-inexact is the 
nondefault option that disallows it from being raised and may result in 
slower code generation.

As I understand it, your issue is actually with inline SSE expansions of 
certain functions.  Before my patch, those had !flag_trapping_math 
conditionals.  My patch changed that to the logically correct 
(TARGET_ROUND || !flag_trapping_math || flag_fp_int_builtin_inexact), that 
being the conditions under which the expansion in question is correct.  
Your problem is that the expansion, though correct under those conditions, 
is slow compared to an IFUNC implementation of the library function.

Maybe that means that expansion should be disabled under some conditions 
where it is correct but suboptimal.  It should be kept for TARGET_ROUND, 
because then it's expanding to a single instruction.  But for 
!TARGET_ROUND, it's a tuning question (e.g. if tuning for a processor that 
would satisfy TARGET_ROUND, or for -mtune=generic, and building with 
recent-enough glibc, the expansion should be avoided as suboptimal, on the 
expectation that at runtime an IFUNC is likely to be available - or given 
the size of the generic SSE expansion, maybe it should be avoided more 
generally than that).

-- 
Joseph S. Myers
joseph@codesourcery.com