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| author | Kito Cheng <kito.cheng@sifive.com> | 2023-03-14 23:19:48 +0800 |
|---|---|---|
| committer | Palmer Dabbelt <palmer@rivosinc.com> | 2023-04-28 07:02:48 -0700 |
| commit | a225cb3ee9a22021312ae25c37595cd9d1995a1f (patch) | |
| tree | 8268a859965d0bea7009a6434abf75f5998c3f50 /bits | |
| parent | 117e8b341c5c0ace8d65feeef136fececb3fdc9c (diff) | |
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Remap __GLIBC_FLT_EVAL_METHOD to 0 if __FLT_EVAL_METHOD__ is -1
__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>
Diffstat (limited to 'bits')
| -rw-r--r-- | bits/flt-eval-method.h | 8 |
1 files 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 |