[PATCH] Remap __GLIBC_FLT_EVAL_METHOD to 0 if __FLT_EVAL_METHOD__ is -1
Xi Ruoyao
xry111@xry111.site
Thu Mar 9 14:33:51 GMT 2023
I'm wondering why the compiler will set __FLT_EVAL_METHOD__ to -1 in the
first place. On gcc91 (VisionFive v1 board) I get:
xry111@gcc91:~$ echo __FLT_EVAL_METHOD__ | cpp
# 0 "<stdin>"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "<stdin>"
0
On Thu, 2023-03-09 at 22:14 +0800, Kito Cheng via Libc-alpha wrote:
> ping
>
>
> On Sun, Feb 19, 2023 at 2:29 AM Palmer Dabbelt <palmer@rivosinc.com>
> wrote:
> >
> > On Thu, 16 Feb 2023 18:26:46 PST (-0800),
> > kito.cheng@sifive.com wrote:
> > > ---
> > > We were intending to update RISC-V's setting only, but after
> > > discussion
> > > with Wilco Dijkstra, we decide to change the generic one instead
> > > of RISC-V only
> > > since it also fix inefficient issue for float_t and double_t.
> > > ---
> > >
> > > __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
> > >
> > > Ref:
> > >
> > > [1] Definition of FLT_EVAL_METHOD from C99 spec:
> > > C99 Spec draft:
> > > (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf)
> > >
> > > Except for assignment and cast (which remove all extra range and
> > > precision), the values
> > > of operations with floating operands and values subject to the
> > > usual arithmetic
> > > conversions and of floating constants are evaluated to a format
> > > whose range and precision
> > > may be greater than required by the type. The use of evaluation
> > > formats is characterized
> > > by the implementation-defined value of FLT_EVAL_METHOD:
> > > 19)
> > >
> > > -1 indeterminable;
> > > 0 evaluate all operations and constants just to the range and
> > > precision of the
> > > type;
> > > 1 evaluate operations and constants of type float and double to
> > > the
> > > range and precision of the double type, evaluate long double
> > > operations and constants to the range and precision of the
> > > long double
> > > type;
> > > 2 evaluate all operations and constants to the range and
> > > precision of the
> > > long double type.
> > >
> > > All other negative values for FLT_EVAL_METHOD characterize
> > > implementation-defined
> > > behavior.
> > >
> > > [2] Definition of float_t and double_t in C99 spec:
> > >
> > > C99 Spec draft:
> > > (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf)
> > >
> > > 7.12
> > >
> > > ...
> > >
> > > The types
> > > float_t
> > > double_t
> > > are floating types at least as wide as float and double,
> > > respectively, and such that
> > > double_t is at least as wide as float_t. If FLT_EVAL_METHOD equals
> > > 0,
> > > float_t and double_t are float and double, respectively; if
> > > FLT_EVAL_METHOD equals 1, they are both double; if FLT_EVAL_METHOD
> > > equals
> > > 2, they are both long double; and for other values of
> > > FLT_EVAL_METHOD, they are
> > > otherwise implementation-defined.199)
> > >
> > > 199) The types float_t and double_t are intended to be the
> > > implementation’s most efficient types at
> > > least as wide as float and double, respectively. For
> > > FLT_EVAL_METHOD equal 0, 1, or 2, the
> > > type float_t is the narrowest type used by the implementation to
> > > evaluate floating expressions.
> > > ---
> > > bits/flt-eval-method.h | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/bits/flt-eval-method.h b/bits/flt-eval-method.h
> > > index 75f57b9a0e..b6be0a1e43 100644
> > > --- a/bits/flt-eval-method.h
> > > +++ b/bits/flt-eval-method.h
> > > @@ -26,14 +26,14 @@
> > > -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
> > > + -1 is mapped to 0 (considering evaluation as same precision 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. */
> > >
> > > #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
> >
> > Acked-by: Palmer Dabbelt <palmer@rivosinc.com> # RISC-V
> >
> > though we should have a global reviewed look at this now that it's
> > touching everything.
--
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University
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