From: Paul Eggert <eggert@cs.ucla.edu>
To: Bruno Haible <bruno@clisp.org>
Cc: bug-gnulib@gnu.org
Subject: Re: Clang __built_assume
Date: Sat, 22 Aug 2020 10:32:18 -0700 [thread overview]
Message-ID: <4d87cd4d-f02c-761c-216e-65157754d039@cs.ucla.edu> (raw)
In-Reply-To: <4290937.bpigslQC6F@omega>
On 8/22/20 3:30 AM, Bruno Haible wrote:
> -#if _GL_HAS_BUILTIN_UNREACHABLE
> + features such as function calls not inlined by the compiler. */
> +
> +#if _GL_HAS_BUILTIN_ASSUME
> +/* Use a temporary variable, to avoid a clang warning
> + "the argument to '__builtin_assume' has side effects that will be discarded"
> + if R contains invocations of functions not marked as 'const'. */
> +# define assume(R) \
> + ((void) ({ __typeof__ (R) _gl_verify_temp = (R); \
> + __builtin_assume (_gl_verify_temp); }))
> +#elif _GL_HAS_BUILTIN_UNREACHABLE
> # define assume(R) ((R) ? (void) 0 : __builtin_unreachable ())
This is OK, but please also mention in the comment that __builtin_assume
generates better code for Clang 8 than __builtin_unreachable does, as that's the
only reason to put up with all this mess.
next prev parent reply other threads:[~2020-08-22 17:32 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-17 22:02 fixes for Clang builtins when compiling Emacs on Fedora Paul Eggert
2020-08-17 23:37 ` Bruno Haible
2020-08-18 1:10 ` Paul Eggert
2020-08-22 10:30 ` Clang __built_assume Bruno Haible
2020-08-22 17:32 ` Paul Eggert [this message]
2020-08-22 23:01 ` Clang __builtin_assume Bruno Haible
2020-08-23 13:46 ` clang's __diagnose_if__ and glibc fortify Bruno Haible
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