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Date: Thu, 3 Dec 2020 14:06:10 -0500
From: Rich Felker <dalias@...c.org>
To: Marius Hillenbrand <mhillen@...ux.ibm.com>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] s390x: derive float_t from compiler or default to
float
On Thu, Dec 03, 2020 at 05:53:52PM +0100, Marius Hillenbrand wrote:
> On 12/2/20 8:13 PM, Rich Felker wrote:
> > On Wed, Dec 02, 2020 at 06:09:44PM +0100, Marius Hillenbrand wrote:
> >> On 12/2/20 5:01 PM, Rich Felker wrote:
> >>> On Wed, Dec 02, 2020 at 09:25:04AM -0500, Rich Felker wrote:
> [...]
> >>> I'm looking at
> >>> https://gcc.gnu.org/pipermail/gcc-patches/2020-November/560225.html
> >>> which seems to be what you're talking about, and don't understand how
> >>> it's intended to work. It looks like it's running a test for target
> >>> behavior on the host compiler (there is no target compiler at the
> >>> point this test is run). Looking again, I guess that's why it's under
> >>> a condition for build==host==target.
> >>
> >> Right, that's the patch. The check only applies to a "native build",
> >> with the assumption that the build environment is the same as the
> >> intended target environment.
> >>
> >>> What happens when cross
> >>> compiling? Do you get the old behavior unless manually setting
> >>> --disable-s390-excess-float-precision?
> >>
> >> When cross compiling, we get the new behavior (the setting starts at
> >> "auto", which is never resolved to yes or no; so the AC_DEFINE is left out).
> >>
> >> In any case, manually setting
> >> --enable/disable-s390-excess-float-precision takes precedence.
> >
> > FWIW this means building GCC 11 for any older version of glibc or musl
> > will give a broken configuration unless you pass
> > --disable-s390-excess-float-precision to configure. I'm not sure if
> > anything should be done about that; at least I might want to handle it
> > in mcm...
>
> I will look into handling cross compiles in a more differentiating way...
I can't think of any valid way to detect (note: at configure time you
might not even have libc available) but it might be more reasonable to
take the conservative default and assume a libc that can't handle the
new definition. For example compiling old musl with the new definition
will be caused the math library to be miscompiled.
> > In any case this probably means I should include your patch in this
> > release cycle so at least current version builds right.
> >
> > BTW is there a -m option to override at runtime in order to test both
> > behaviors, so you don't have to build a new GCC from scratch to do it?
>
> Yes, in the current GCC, -fexcess-precision=standard or fast switches
> between the two behaviors (i.e., both __FLT_EVAL_METHOD__ and emitted
> code; "fast" corresponds to the "new" behavior).
I mean in a GCC 11 built for the new behavior, where
-fexcess-precision=standard will no longer give __FLT_EVAL_METHOD__==1
because float_t is expected to be defined as float and FLT_EVAL_METHOD
as 0. Is there a -m option to tell it "behave like old GCC, where
-fexcess-precision=standard implies evaluation in double?
Rich
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