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Message-ID: <20161217140323.GM16379@port70.net>
Date: Sat, 17 Dec 2016 15:03:24 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: [PATCH v2] use lookup table for malloc bin index instead of float
conversion
float conversion is slow and big on soft-float targets.
The lookup table increases code size a bit on most hard float targets
(and adds 60byte rodata), performance can be a bit slower because of
position independent data access and cpu internal state dependence
(cache, extra branches), but the overall effect should be minimal
(common, small size allocations should be unaffected).
---
* u-uy74@...ey.se <u-uy74@...ey.se> [2016-12-17 08:36:00 +0100]:
> On Sat, Dec 17, 2016 at 12:50:58AM -0500, Rich Felker wrote:
> > Looks good mostly, but wouldn't it be better to drop the 4 unused
> > entries from the table and add -4's to the indices?
>
> Wouldn't this enlarge the code more than reduce the data?
most targets have a load instruction with small offset
and on some targets the compiler emits relocation against
(tab-4) instead of tab so the code size is not affected.
src/malloc/malloc.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
index b90636c..c38c46f 100644
--- a/src/malloc/malloc.c
+++ b/src/malloc/malloc.c
@@ -111,19 +111,29 @@ static int first_set(uint64_t x)
#endif
}
+static const unsigned char bin_tab[60] = {
+ 32,33,34,35,36,36,37,37,38,38,39,39,
+ 40,40,40,40,41,41,41,41,42,42,42,42,43,43,43,43,
+ 44,44,44,44,44,44,44,44,45,45,45,45,45,45,45,45,
+ 46,46,46,46,46,46,46,46,47,47,47,47,47,47,47,47,
+};
+
static int bin_index(size_t x)
{
x = x / SIZE_ALIGN - 1;
if (x <= 32) return x;
+ if (x < 512) return bin_tab[x/8-4];
if (x > 0x1c00) return 63;
- return ((union { float v; uint32_t r; }){(int)x}.r>>21) - 496;
+ return bin_tab[x/128-4] + 16;
}
static int bin_index_up(size_t x)
{
x = x / SIZE_ALIGN - 1;
if (x <= 32) return x;
- return ((union { float v; uint32_t r; }){(int)x}.r+0x1fffff>>21) - 496;
+ x--;
+ if (x < 512) return bin_tab[x/8-4] + 1;
+ return bin_tab[x/128-4] + 17;
}
#if 0
--
2.10.2
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