On Sat, Nov 21, 2020 at 01:57:32PM -0600, Segher Boessenkool wrote:
> On Fri, Nov 20, 2020 at 09:33:48PM -0700, Jeff Law wrote:
> > On 2/11/20 11:43 AM, Ulrich Weigand wrote:
> > > 1. If a component flag of -ffast-math (or -funsafe-math-optimizations)
> > > is explicitly set (or reset) on the command line, this should override
> > > any implicit change due to -f(no-)fast-math, no matter in which order
> > > the flags come on the command line. This change affects all flags.
> > >
> > > 2. Any component flag modified from its default by -ffast-math should
> > > be reset to the default by -fno-fast-math. This was previously
> > > not done for the following flags:
> > > -fcx-limited-range
> > > -fexcess-precision=
> > >
> > > 3. Once -ffinite-math-only is true, the -f(no-)signaling-nans flag has
> > > no meaning (if we have no NaNs at all, it does not matter whether
> > > there is a difference between quiet and signaling NaNs). Therefore,
> > > it does not make sense for -ffast-math to imply -fno-signaling-nans.
> > > This is also a documentation change.
> > >
> > > 4. -ffast-math is documented to imply -fno-rounding-math, however the
> > > latter setting is the default anyway; therefore it does not make
> > > sense to try to modify it from its default setting.
> > >
> > > 5. The __FAST_MATH__ preprocessor macro should be defined if and only
> > > if all the component flags of -ffast-math are set to the value that
> > > is documented as the effect of -ffast-math. The following flags
> > > were currently *not* so tested:
> > > -fcx-limited-range
> > > -fassociative-math
> > > -freciprocal-math
> > > -frounding-math
> > > (Note that we should still *test* for -fno-rounding-math here even
> > > though it is not set as per 4. -ffast-math -frounding-math should
> > > not set the __FAST_MATH__ macro.)
> > > This is also a documentation change.
>
> > It appears this was dropped on the floor.? It looks reasonable to me.?
> > Please retest and commit.? Thanks!
I've now retested on s390x-ibm-linux and committed to mainline.
Thanks for the review!
> It all makes sense, and is a nice improvement :-) But please mention it
> in the release notes? No doubt people did use non-sensical flag
> combinations, and they will be affected. Thanks!
Here's a proposed patch to update the gcc-11 changes.hmtl.
OK to commit?
Bye,
Ulrich
diff --git a/htdocs/gcc-11/changes.html b/htdocs/gcc-11/changes.html
index 46a6a37..c0f896a 100644
--- a/htdocs/gcc-11/changes.html
+++ b/htdocs/gcc-11/changes.html
@@ -58,6 +58,29 @@ a work-in-progress.</p>
is deprecated and will be removed in a future release. It should be
possible to use <code>--enable-cheaders=c_global</code> (the default)
with no change in behaviour. </li>
+
+ <li>Some inconsistencies in handling combinations of
<code>-ffast-math</code>,
+ <code>-fno-fast-math</code>, <code>-funsafe-math-optimizations</code>,
+ <code>-fno-unsafe-math-optimizations</code>, and their component flags
+ have been fixed. This might change the behavior of the compiler when
+ invoked with certain combinations of such command line options.
+ The behavior is now consistently:
+ <ul>
+ <li>If a component flag of <code>-ffast-math</code> or
+ <code>-funsafe-math-optimizations</code> is explicitly set or reset
+ on the command line, this will override any implicit change, no
matter
+ in which order the flags come on the command line.</li>
+ <li>Any component flag (which is not explicity set or reset on the
command
+ line) that was modified from its default by <code>-ffast-math</code>
or
+ <code>-funsafe-math-optimizations</code> is always reset to its
default
+ by a subsequent <code>-fno-fast-math</code> or
+ <code>-fno-unsafe-math-optimizations</code>.</li>
+ <li><code>-ffast-math</code> no longer implicitly changes
+ <code>-fsignaling-math</code>.
+ <li>The <code>__FAST_MATH__</code> preprocessor macro is set if and
+ only if all component flags of <code>-ffast-math</code> are set
+ to the value documented as the effect of
<code>-ffast-math</code>.</li>
+ </ul></li>
</ul>
--
Dr. Ulrich Weigand
GNU/Linux compilers and toolchain
[email protected]
