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Message-ID: <20111116004536.GV24939@port70.net>
Date: Wed, 16 Nov 2011 01:45:37 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: unneeded mremap calls in realloc
as discussed on irc, in realloc there is a mremap
where newlen is pagesize adjusted but oldlen is not
so oldlen==newlen almost always fails
run this simple test case with strace to see the issue:
#include <stdlib.h>
int main(){
char *p = 0;
int n;
for (n = 0; n < 500000; n++)
p = realloc(p, n);
free(p);
return 0;
}
the fix that significantly speeds up the above code:
(there might be better fix, eg why oldlen is not a
multiple of pagesize in the first place?)
diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
index abf3e8f..8e9b022 100644
--- a/src/malloc/malloc.c
+++ b/src/malloc/malloc.c
@@ -401,7 +401,7 @@ void *realloc(void *p, size_t n)
return new;
}
newlen = (newlen + PAGE_SIZE-1) & -PAGE_SIZE;
- if (oldlen == newlen) return p;
+ if (((oldlen + PAGE_SIZE-1) & -PAGE_SIZE) == newlen) return p;
base = __mremap(base, oldlen, newlen, MREMAP_MAYMOVE);
if (base == (void *)-1)
return newlen < oldlen ? p : 0;
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