[PATCH] memcmp.3: note about optimization to bcmp in LLVM 9

[PATCH] memcmp.3: note about optimization to bcmp in LLVM 9

[Shawn Landden]

---
 man3/memcmp.3 | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/man3/memcmp.3 b/man3/memcmp.3
index 247eed223..6afca5ea4 100644
--- a/man3/memcmp.3
+++ b/man3/memcmp.3
@@ -84,10 +84,17 @@ because the required CPU time depends on the number of equal bytes.
 Instead, a function that performs comparisons in constant time is required.
 Some operating systems provide such a function (e.g., NetBSD's
 .BR consttime_memequal ()),
 but no such function is specified in POSIX.
 On Linux, it may be necessary to implement such a function oneself.
+.PP
+LLVM 9, released in 2019, and later will optimize a call
+.BR memcmp ()
+to a call to
+.BR bcmp (3)
+if the return value is only checked for equilivence to zero.
+.\" http://releases.llvm.org/9.0.0/docs/ReleaseNotes.html#noteworthy-optimizations
 .SH SEE ALSO
 .BR bcmp (3),
 .BR bstring (3),
 .BR strcasecmp (3),
 .BR strcmp (3),
-- 
2.20.1

Re: [PATCH] memcmp.3: note about optimization to bcmp in LLVM 9

[Shawn Landden]

11.01.2020, 10:02, "Shawn Landden" <shawn@git.icu>:
> ---
>  man3/memcmp.3 | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/man3/memcmp.3 b/man3/memcmp.3
> index 247eed223..6afca5ea4 100644
> --- a/man3/memcmp.3
> +++ b/man3/memcmp.3
> @@ -84,10 +84,17 @@ because the required CPU time depends on the number of equal bytes.
>  Instead, a function that performs comparisons in constant time is required.
>  Some operating systems provide such a function (e.g., NetBSD's
>  .BR consttime_memequal ()),
>  but no such function is specified in POSIX.
>  On Linux, it may be necessary to implement such a function oneself.
> +.PP
> +LLVM 9, released in 2019, and later will optimize a call
to
> +.BR memcmp ()
> +to a call to
> +.BR bcmp (3)
> +if the return value is only checked for equilivence to zero.
> +.\" http://releases.llvm.org/9.0.0/docs/ReleaseNotes.html#noteworthy-optimizations
>  .SH SEE ALSO
>  .BR bcmp (3),
>  .BR bstring (3),
>  .BR strcasecmp (3),
>  .BR strcmp (3),
> --
> 2.20.1

-- 
Shawn Landden

Re: [PATCH] memcmp.3: note about optimization to bcmp in LLVM 9

[Michael Kerrisk (man-pages)]

On 1/11/20 4:02 PM, Shawn Landden wrote:
> ---
>  man3/memcmp.3 | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/man3/memcmp.3 b/man3/memcmp.3
> index 247eed223..6afca5ea4 100644
> --- a/man3/memcmp.3
> +++ b/man3/memcmp.3
> @@ -84,10 +84,17 @@ because the required CPU time depends on the number of equal bytes.
>  Instead, a function that performs comparisons in constant time is required.
>  Some operating systems provide such a function (e.g., NetBSD's
>  .BR consttime_memequal ()),
>  but no such function is specified in POSIX.
>  On Linux, it may be necessary to implement such a function oneself.
> +.PP
> +LLVM 9, released in 2019, and later will optimize a call
> +.BR memcmp ()
> +to a call to
> +.BR bcmp (3)
> +if the return value is only checked for equilivence to zero.
> +.\" http://releases.llvm.org/9.0.0/docs/ReleaseNotes.html#noteworthy-optimizations

See my othermail. Wht is it relevant to document this 
compiler-specific detail?

Thanks,

Michael

>  .SH SEE ALSO
>  .BR bcmp (3),
>  .BR bstring (3),
>  .BR strcasecmp (3),
>  .BR strcmp (3),
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/