Re: [PATCH] libstdc++: std::to_chars std::{,b}float16_t support

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 27 Oct 2022 at 09:00, Jakub Jelinek <jakub@redhat.com> wrote:
>
> Hi!
>
> The following patch on top of
> https://gcc.gnu.org/pipermail/libstdc++/2022-October/054849.html
> adds std::{,b}float16_t support for std::to_chars.
> When precision is specified (or for std::bfloat16_t for hex mode even if not),
> I believe we can just use the std::to_chars float (when float is mode
> compatible with std::float32_t) overloads, both formats are proper subsets
> of std::float32_t.
> Unfortunately when precision is not specified and we are supposed to emit
> shortest string, the std::{,b}float16_t strings are usually much shorter.
> E.g. 1.e7p-14f16 shortest fixed representation is
> 0.0001161 and shortest scientific representation is
> 1.161e-04 while 1.e7p-14f32 (same number promoted to std::float32_t)
> 0.00011610985 and
> 1.1610985e-04.
> Similarly for 1.38p-112bf16,
> 0.000000000000000000000000000000000235
> 2.35e-34 vs. 1.38p-112f32
> 0.00000000000000000000000000000000023472271
> 2.3472271e-34
> For std::float16_t there are differences even in the shortest hex, say:
> 0.01p-14 vs. 1p-22
> but only for denormal std::float16_t values (where all std::float16_t
> denormals converted to std::float32_t are normal), __FLT16_MIN__ and
> everything larger in absolute value than that is the same.  Unless
> that is a bug and we should try to discover shorter representations
> even for denormals...
> std::bfloat16_t has the same exponent range as std::float32_t, so all
> std::bfloat16_t denormals are also std::float32_t denormals and thus
> the shortest hex representations are the same.
>
> As documented, ryu can handle arbitrary IEEE like floating point formats
> (probably not wider than IEEE quad) using the generic_128 handling, but
> ryu is hidden in libstdc++.so.  As only few architectures support
> std::float16_t right now and some of them have special ISA requirements
> for those (e.g. on i?86 one needs -msse2) and std::bfloat16_t is right
> now supported only on x86 (again with -msse2), perhaps with aarch64/arm
> coming next if ARM is interested, but I think it is possible that more
> will be added later, instead of exporting APIs from the library to handle
> directly the std::{,b}float16_t overloads this patch instead exports
> functions which take a float which is a superset of those and expects
> the inline overloads to promote the 16-bit formats to 32-bit, then inside
> of the library it ensures they are printed right.
> With the added [[gnu::cold]] attribute because I think most users
> will primarily use these formats as storage formats and perform arithmetics
> in the excess precision for them and print also as std::float32_t the
> added support doesn't seem to be too large, on x86_64:
> readelf -Ws libstdc++.so.6.0.31 | grep float16_t
>    912: 00000000000ae824   950 FUNC    GLOBAL DEFAULT   13 _ZSt21__to_chars_bfloat16_tPcS_fSt12chars_format@@GLIBCXX_3.4.31
>   5767: 00000000000ae4a1   899 FUNC    GLOBAL DEFAULT   13 _ZSt20__to_chars_float16_tPcS_fSt12chars_format@@GLIBCXX_3.4.31
>    842: 000000000016d430   106 FUNC    LOCAL  DEFAULT   13 _ZN12_GLOBAL__N_113get_ieee_reprINS_23floating_type_float16_tEEENS_6ieee_tIT_EES3_
>    865: 0000000000170980  1613 FUNC    LOCAL  DEFAULT   13 _ZSt23__floating_to_chars_hexIN12_GLOBAL__N_123floating_type_float16_tEESt15to_chars_resultPcS3_T_St8optionalIiE.constprop.0.isra.0
>   7205: 00000000000ae824   950 FUNC    GLOBAL DEFAULT   13 _ZSt21__to_chars_bfloat16_tPcS_fSt12chars_format
>   7985: 00000000000ae4a1   899 FUNC    GLOBAL DEFAULT   13 _ZSt20__to_chars_float16_tPcS_fSt12chars_format
> so 3568 code bytes together or so.
>
> Tested with the attached test (which doesn't prove the shortest
> representation, just prints std::{,b}float16_t and std::float32_t
> shortest strings side by side, then tries to verify it can be
> emitted even into the exact sized range and can't be into range
> one smaller than that and tries to read what is printed
> back using from_chars float32_t overload (so there could be
> double rounding, but apparently there is none for the shortest strings).
> The only differences printed are for NaNs, where sNaNs are canonicalized
> to canonical qNaNs and as to_chars doesn't print NaN mantissa, even qNaNs
> other than the canonical one are read back just as the canonical NaN.
>
> Also attaching what Patrick wrote to generate the pow10_adjustment_tab,
> for std::float16_t only 1.0, 10.0, 100.0, 1000.0 and 10000.0 are powers
> of 10 in the range because __FLT16_MAX__ is 65504.0, and all of the above
> are exactly representable in std::float16_t, so we want to use 0 in
> pow10_adjustment_tab.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>

Unless I misunderstood something in Patrick's review, this is good and
can be incrementally improved.

OK for trunk, thanks.