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per Jens Arnold's suggestion:

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use platform clz instruction only on ARMv5+, where we know it returns 32 for a 0 input, and remove the special case for 0

use binary search only to find most significant four bits, then use lookup table to add number of leading zeroes within those bits

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@19888 a1c6a512-1295-4272-9138-f99709370657
This commit is contained in:
Andrew Mahone 2009-01-30 10:52:30 +00:00
parent b081a38131
commit ba4be51571
1 changed files with 8 additions and 18 deletions
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26
apps/plugins/pictureflow.c
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@ -337,11 +337,9 @@ static inline PFreal fmul(PFreal a, PFreal b)
return (a*b) >> PFREAL_SHIFT; return (a*b) >> PFREAL_SHIFT;
} }
/* ARMv5+ have a clz instruction. So do most modern desktop processors, and the /* ARMv5+ has a clz instruction equivalent to our function.
* simulator doesn't need to be as concerned about savings for inlining a calls
* to a clz function.
*/ */
#if defined(SIMULATOR) || (defined(CPU_ARM) && (ARM_ARCH > 4)) #if (defined(CPU_ARM) && (ARM_ARCH > 4))
static inline int clz(uint32_t v) static inline int clz(uint32_t v)
{ {
return __builtin_clz(v); return __builtin_clz(v);
@ -349,12 +347,14 @@ static inline int clz(uint32_t v)
/* Otherwise, use our clz, which can be inlined */ /* Otherwise, use our clz, which can be inlined */
#else #else
static const char clz_lut[16] = { 4, 3, 2, 2, 1, 1, 1, 1,
0, 0, 0, 0, 0, 0, 0, 0 };
/* This clz is based on the log2(n) implementation at /* This clz is based on the log2(n) implementation at
* http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog * http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogLookup
*/ */
static inline int clz(uint32_t v) static inline int clz(uint32_t v)
{ {
uint32_t r = 31; int r = 28;
if (v >= 0x10000) if (v >= 0x10000)
{ {
v >>= 16; v >>= 16;
@ -370,14 +370,7 @@ static inline int clz(uint32_t v)
v >>= 4; v >>= 4;
r -= 4; r -= 4;
} }
if (v & 0xc) return r + clz_lut[v];
{
v >>= 2;
r -= 2;
}
if (v & 2)
r -= 1;
return r;
} }
#endif #endif
@ -387,10 +380,7 @@ static inline int clz(uint32_t v)
static inline int allowed_shift(int32_t val) static inline int allowed_shift(int32_t val)
{ {
uint32_t uval = val ^ (val >> 31); uint32_t uval = val ^ (val >> 31);
if (!uval) return clz(uval) - 1;
return 31;
else
return clz(uval) - 1;
} }
/* Calculate num/den, with the result shifted left by PFREAL_SHIFT, by shifting /* Calculate num/den, with the result shifted left by PFREAL_SHIFT, by shifting