S390: Derive float_t from FLT_EVAL_METHOD

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.

1 files changed, 7 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 725a9882c6..0820984547 100644
--- a/NEWS
+++ b/NEWS
@@ -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