s390x: derive float_t from compiler or default to float

float_t should represent the type that is used to evaluate float
expressions internally. On s390x, float_t is currently set to double.
In contrast, the isa supports single-precision float operations and
compilers by default evaluate float in single precision, which
violates the C standard (sections 5.2.4.2.2 and 7.12 in C11/C17, to be
precise). With -fexcess-precision=standard, gcc evaluates float in
double precision, which aligns with the standard yet at the cost of
added conversion instructions.

gcc-11 will drop the special case to retrofit double precision
behavior for -fexcess-precision=standard so that __FLT_EVAL_METHOD__
will be 0 on s390x in any scenario.

To improve standards compliance and compatibility with future compiler
direction, this patch changes the definition of float_t to be derived
from the compiler's __FLT_EVAL_METHOD__.

arch/s390x/bits/alltypes.h.in

@@ -9,7 +9,11 @@
 TYPEDEF int wchar_t;
 #endif
 
+#if defined(__FLT_EVAL_METHOD__) && __FLT_EVAL_METHOD__ == 1
 TYPEDEF double float_t;
+#else
+TYPEDEF float float_t;
+#endif
 TYPEDEF double double_t;
 
 TYPEDEF struct { long long __ll; long double __ld; } max_align_t;

arch/s390x/bits/float.h

@@ -1,4 +1,8 @@
-#define FLT_EVAL_METHOD 1
+#ifdef __FLT_EVAL_METHOD__
+#define FLT_EVAL_METHOD __FLT_EVAL_METHOD__
+#else
+#define FLT_EVAL_METHOD 0
+#endif
 
 #define LDBL_TRUE_MIN 6.47517511943802511092443895822764655e-4966L
 #define LDBL_MIN 3.36210314311209350626267781732175260e-4932L