[COMMITTED 2/2] Remap __GLIBC_FLT_EVAL_METHOD to 0 if __FLT_EVAL_METHOD__ is -1

[Palmer Dabbelt <palmer@rivosinc.com>]

From: Kito Cheng <kito.cheng@sifive.com>

__GLIBC_FLT_EVAL_METHOD will effect the definition of float_t and
double_t, currently we'll set __GLIBC_FLT_EVAL_METHOD to 2 when
__FLT_EVAL_METHOD__ is -1, that means we'll define float_t and double_t
to long double.

However some target isn't natively (HW) support long double like AArch64 and
RISC-V, they defined long double as 128-bits IEEE 754 floating point type.

That means setting __GLIBC_FLT_EVAL_METHOD to 2 will cause very inefficient
code gen for those target who didn't provide native support for long
double, and that's violate the spirit float_t and double_t - most efficient
types at least as wide as float and double.

So this patch propose to remap __GLIBC_FLT_EVAL_METHOD to 0 rather than
2 when __FLT_EVAL_METHOD__ is -1, which means we'll use float/double
rather than long double for float_t and double_t.

Note: __FLT_EVAL_METHOD__ == -1 means the precision is indeterminable,
      which means compiler might using indeterminable precision during
      optimization/code gen, clang will set this value to -1 when fast
      math is enabled.

Note: Default definition float_t and double_t in current glibc:
      |  __GLIBC_FLT_EVAL_METHOD | float_t     | double_t
      |               0 or 16    | float       | double
      |               1          | double      | doulbe
      |               2          | long double | long double
      More complete list see math/math.h

Note: RISC-V has defined ISA extension to support 128-bits IEEE 754
      floating point operations, but only rare RISC-V core will implement that.

Related link:

[1] LLVM issue (__FLT_EVAL_METHOD__ is set to -1 with Ofast. #60781):
    https://github.com/llvm/llvm-project/issues/60781
[2] Last version of this patch: https://sourceware.org/pipermail/libc-alpha/2023-February/145622.html

Acked-by: Palmer Dabbelt <palmer@rivosinc.com> # RISC-V
Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>
Link: https://inbox.sourceware.org/libc-alpha/20230314151948.12892-1-kito.cheng@sifive.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
---
 bits/flt-eval-method.h | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/bits/flt-eval-method.h b/bits/flt-eval-method.h
index 75f57b9a0e..f9262d7d0b 100644
--- a/bits/flt-eval-method.h
+++ b/bits/flt-eval-method.h
@@ -26,14 +26,12 @@
    -1.  */
 
 /* In the default version of this header, follow __FLT_EVAL_METHOD__.
-   -1 is mapped to 2 (considering evaluation as long double to be a
-   conservatively safe assumption), and if __FLT_EVAL_METHOD__ is not
-   defined then assume there is no excess precision and use the value
-   0.  */
+   If __FLT_EVAL_METHOD__ is not defined or set to -1, assume there is no
+   excess precision and use the value 0 (this is correct for most targets). */
 
 #ifdef __FLT_EVAL_METHOD__
 # if __FLT_EVAL_METHOD__ == -1
-#  define __GLIBC_FLT_EVAL_METHOD	2
+#  define __GLIBC_FLT_EVAL_METHOD	0
 # else
 #  define __GLIBC_FLT_EVAL_METHOD	__FLT_EVAL_METHOD__
 # endif
-- 
2.40.0