Re: Suggestion: Add -ffast-math to default BOOT_CFLAGS

[Nathanael Nerode <neroden at twcny dot rr dot com>]

Kelley Cook wrote:
> I just timed using the same compilier sources built with and without BOOT_CFLAGS="-O2 -g -ffast-math" to time building Gerald's Testcase.
>
> Repeated runs showed that the slowest version with -ffast-math was still faster than the quickest version without. Below is posted a sorted -ftime-report comparing the quickest results obtained from g++ built with and without -ffast-math. A 2 percent speedup is nothing to sneeze at.
>
> Note that I have not yet done a full all-languages and libraries regression test. But if those results do not change, it would seem prudent to build the compiler with -ffast-math. A one-liner patch to do that is attached.
>
> Also this patch does not propose to make '-ffast-math' the default compiler option -- that would be another discussion. It is only about whether or not to build gcc itself using -ffast-math.

I'm pretty sure we want to make sure of two things before trying this:

* no libraries should be compiled with -ffast-math (including libgcc), 
to avoid affecting user code

* no code which performs user code compilation at compile time should be
affected by -ffastmath, to avoid affecting user code. (I'm not sure 
whether that would actually mean not compiling them with -ffast-math -- 
a lot of this stuff is done by weird routines related to HOST_WIDE_INT, 
if I remember correctly, and I'm not sure how it would be affected.)

If we can separate those code segments out, compiling the 'ordinary' 
part of the compiler with -ffast-math seems reasonable -- but on the 
other hand, it would give different benefits, so needs to be benchmarked 
on its own.