Re: optimization/10189: pentium4 breaks suns libm code for __ieee754_pow(double x, double y)

From: Loren James Rittle <>
Date: Wed, 26 Mar 2003 14:58:27 -0600 (CST)
> Beautiful email!!

Thanks David but, although the exact words are mine, I felt deja vu
while writing them.  I have been asked to contribute it to the gcc bug
reporting docs as a warning against using the compiler in novel modes
unless you actually first test it as I will describe.  Then we will
ask all reporters of CPU switch PRs whether they did or can do that
test before attempting to stare at a full package/kernel failure mode.

I will attempt to be a little more pro-active in watching the GNATS at for FreeBSD.  There appears to be an near endless supply
of people that wish to add these CPU flags to kernel builds. ;-)

>>     Special secret #2:  Although the FSF-side does want to improve all
>>     code generation (and I think proper PRs RE CPU switches will be
>>     looked at by someone given enough time) be aware that -O2 without
>>     special arch flags is probably the most stable for any given CPU
>>     for any given gcc release.  Do you really want to trust a kernel
>>     built with optimization flags and arch flags that near zero or zero
>>     people have fully tested?  Doubtful.  However, inline with secret
>>     #1 and by virtual of being digital, if even one person tests it
>>     (i.e. yourself) and it appears OK, then it is probably safe to at
>>     least attempt to build a kernel and run it.
> FreeBSD has for years recommended -O[1] vs. -O2.  Do you think there is
> value in having the GCC test suite runs you do at do runs
> with both settings?

Actually (slight backpettle), all of the modern DG test suite in gcc
are run at the broad range of -O0,1,2,3.  OTOH, by default, everyone
is bootstrapping the compiler at -O2 everyday.

> To also do runs with the newer CPU types?

This would be quite revealing.  I would like to extend the automatic
regression checkers to cover that but, yow, I'm already eating a lot
of cycles on those machines.  Added to list of things to check.

Received on Wed Mar 26 2003 - 11:58:32 UTC

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