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Message-ID: <20260613011842.GL3520958@port70.net>
Date: Sat, 13 Jun 2026 03:18:42 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: using builtins within musl
* Rich Felker <dalias@...c.org> [2026-06-12 19:11:12 -0400]:
> On Fri, Jun 12, 2026 at 11:43:41PM +0200, Szabolcs Nagy wrote:
> > builtins supposed to improve code generation and most useful when a
> > library call can be lowered to a few instructions. e.g. memcpy and
> > memset with a fixed small size can be a few load/store/move ops.
> >
> > unfortunately gcc creates a mess on x86_64 of code like
> >
> > if (n < 64) __builtin_memcpy(d,s,n);
> > if (n < 64) __builtin_memset(p,0,n);
> >
> > libc.so .text change:
> > arch diff size
> > x86_64: +4525 667424
> > riscv64: +724 613171
> > aarch64: -432 679747
> > arm: -152 658809
>
> Can you give a brief summary of what gcc does such a bad job of on
> x86_64? Does it inline something with a bunch of branching cases for
> different sizes or something? The results on the other archs don't
> look so bad.
x86_64 inlines more, i assume it is fast, but not the
best for size, e.g. try on godbolt:
void foo(char *d, char *s, long n)
{
if (n < 64) __builtin_memcpy(d,s,n);
}
on x86_64 the .text changes sorted by size diff:
-53 3836 res_msend.lo
-44 497 gethostbyaddr_r.lo
-33 548 sigaction.lo
-26 387 netlink.lo
-23 696 pthread_cancel.lo
-19 5678 malloc.lo
-13 356 getpw_r.lo
-12 116 __stack_chk_fail.lo
-12 193 mkdtemp.lo
...
+104 6621 crypt_blowfish.lo
+107 337 textdomain.lo
+112 1656 getifaddrs.lo
+116 650 execvp.lo
+128 2091 getnameinfo.lo
+149 1244 dn_comp.lo
+149 3908 vfscanf.lo
+150 19487 regcomp.lo
+182 439 calloc.lo
+182 439 libc_calloc.lo
+187 4414 __tz.lo
+204 2087 qsort.lo
+204 896 if_nameindex.lo
+240 2172 dcngettext.lo
+257 3433 crypt_sha256.lo
+357 1232 locale_map.lo
+372 3179 crypt_md5.lo
+616 4848 crypt_sha512.lo
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