[PATCH v2] use lookup table for malloc bin index instead of float conversion

[Szabolcs Nagy <nsz@port70.net>]

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@aetey.se <u-uy74@aetey.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