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S390: Derive float_t from FLT_EVAL_METHOD

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float_t supposedly represents the type that is used to evaluate float
expressions internally. While the isa supports single-precision float
operations, the port of glibc to s390 incorrectly deferred to the
generic definitions which, back then, tied float_t to double. gcc by
default evaluates float in single precision, so that scenario violates
the C standard (sections 5.2.4.2.2 and 7.12 in C11/C17). With
-fexcess-precision=standard, gcc evaluates float in double precision,
which aligns with the standard yet at the cost of added conversion
instructions.

With this patch, we drop the s390-specific definition of float_t and
defer to the default behavior, which aligns float_t with the
compiler-defined FLT_EVAL_METHOD in a standard-compliant way.

Checked on s390x-linux-gnu with 31-bit and 64-bit builds.
This commit is contained in:
Marius Hillenbrand 2020-11-30 15:53:59 +01:00 committed by Stefan Liebler
parent b5eeca8cfd
commit f88242af19
2 changed files with 7 additions and 24 deletions
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7
NEWS
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@ -46,6 +46,13 @@ Deprecated and removed features, and other changes affecting compatibility:
program is now installed in the /usr/bin subdirectory. Previously,
the /usr/sbin subdirectory was used.
* On s390(x), the type float_t is now derived from the macro
__FLT_EVAL_METHOD__ that is defined by the compiler, instead of being
hardcoded to double. This does not affect the ABI of any libraries
that are part of the GNU C Library, but may affect the ABI of other
libraries that use this type in their interfaces. The new definition
improves consistency with compiler behavior in many scenarios.
Changes to build and runtime requirements:
* On Linux, the system administrator needs to configure /dev/pts with

24
sysdeps/s390/bits/flt-eval-method.h
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@ -1,24 +0,0 @@
/* Define __GLIBC_FLT_EVAL_METHOD. S/390 version.
Copyright (C) 2016-2020 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#ifndef _MATH_H
# error "Never use <bits/flt-eval-method.h> directly; include <math.h> instead."
#endif
/* This value is used because of a historical mistake. */
#define __GLIBC_FLT_EVAL_METHOD 1