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Date: Fri, 31 May 2024 17:37:33 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: James Y Knight <jyknight@...gle.com>
Cc: musl@...ts.openwall.com,
Nikolaos Chatzikonstantinou <nchatz314@...il.com>
Subject: Re: Re: Implementing csqrtl()
* James Y Knight <jyknight@...gle.com> [2024-05-29 11:13:12 -0400]:
> There is no impact on benchmarks by implementing fenv correctly, because,
> per the C standard, it is undefined behavior to access the floating-point
> status flags or to execute any code with non-default floating-point control
> modes in effect unless "fenv_access" is enabled -- either using "#pragma
> STDC FENV_ACCESS ON" in the code, or via an implementation-defined
> mechanism to set the default to on (in clang, the command-line option
> "-ffp-model=strict"). Because it's opt-in, only the code which actually
> requires it takes the performance hit.
note: the libc *must* opt-in if it tries to conform to c99 annex f.
(at least for library calls that are expected to work in non-default
rounding mode and math code that is specified to set exception flags.)
so fenv correctness has an impact on libc and thus on code that calls
into libc.
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