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Date: Wed, 16 Nov 2011 01:45:37 +0100
From: Szabolcs Nagy <>
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);
	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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