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remove 64-bit math from fdiv in pictureflow.c, replacing it with limited pre-shifting of input values

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git-svn-id: svn://svn.rockbox.org/rockbox/trunk@19886 a1c6a512-1295-4272-9138-f99709370657
This commit is contained in:
Andrew Mahone 2009-01-30 03:40:16 +00:00
parent 8751b94e48
commit 78409ff50c
1 changed files with 64 additions and 9 deletions
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73
apps/plugins/pictureflow.c
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@ -337,16 +337,72 @@ static inline PFreal fmul(PFreal a, PFreal b)
return (a*b) >> PFREAL_SHIFT; return (a*b) >> PFREAL_SHIFT;
} }
/* There are some precision issues when not using (long long) which in turn /* ARMv5+ have a clz instruction. So do most modern desktop processors, and the
takes very long to compute... I guess the best solution would be to optimize * simulator doesn't need to be as concerned about savings for inlining a calls
the computations so it only requires a single long */ * to a clz function.
*/
#if defined(SIMULATOR) || (defined(CPU_ARM) && (ARM_ARCH > 4))
static inline int clz(uint32_t v)
{
return __builtin_clz(v);
}
/* Otherwise, use our clz, which can be inlined */
#else
/* This clz is based on the log2(n) implementation at
* http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
*/
static inline int clz(uint32_t v)
{
uint32_t r = 31;
if (v > 0x8000)
{
v >>= 16;
r -= 16;
}
if (v & 0xff00)
{
v >>= 8;
r -= 8;
}
if (v & 0xf0)
{
v >>= 4;
r -= 4;
}
if (v & 0xc)
{
v >>= 2;
r -= 2;
}
if (v & 2)
r -= 1;
return r;
}
#endif
/* Return the maximum possible left shift for a signed int32, without
* overflow
*/
static inline int allowed_shift(int32_t val)
{
uint32_t uval = val ^ (val >> 31);
if (!uval)
return 31;
else
return clz(uval) - 1;
}
/* Calculate num/den, with the result shifted left by PFREAL_SHIFT, by shifting
* num and den before dividing.
*/
static inline PFreal fdiv(PFreal num, PFreal den) static inline PFreal fdiv(PFreal num, PFreal den)
{ {
long long p = (long long) (num) << (PFREAL_SHIFT * 2); int shift = allowed_shift(num);
long long q = p / (long long) den; shift = MIN(PFREAL_SHIFT, shift);
long long r = q >> PFREAL_SHIFT; num <<= shift;
den >>= PFREAL_SHIFT - shift;
return r; return num / den;
} }
#define fmin(a,b) (((a) < (b)) ? (a) : (b)) #define fmin(a,b) (((a) < (b)) ? (a) : (b))
@ -579,7 +635,6 @@ int create_track_index(const int slide_index)
return (track_count > 0) ? 0 : -1; return (track_count > 0) ? 0 : -1;
} }
/** /**
Determine filename of the album art for the given slide_index and Determine filename of the album art for the given slide_index and
store the result in buf. store the result in buf.