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The return value of SDL_MostSignificantBitIndex32(0) is defined as -1.

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--HG--
extra : rebase_source : b9f0ed0a769617991e6eadf9d6a3efa17112d1a4
This commit is contained in:
Sam Lantinga 2013-08-07 16:32:51 -07:00
parent 5d1bee0f17
commit da3326eb95
1 changed files with 13 additions and 6 deletions
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19
include/SDL_bits.h
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@ -45,15 +45,18 @@ extern "C" {
* with 0. This operation can also be stated as "count leading zeroes" and * with 0. This operation can also be stated as "count leading zeroes" and
* "log base 2". * "log base 2".
* *
* \return Index of the most significant bit. * \return Index of the most significant bit, or -1 if the value is 0.
*/ */
SDL_FORCE_INLINE Sint8 SDL_FORCE_INLINE int
SDL_MostSignificantBitIndex32(Uint32 x) SDL_MostSignificantBitIndex32(Uint32 x)
{ {
#if defined(__GNUC__) && __GNUC__ >= 4 #if defined(__GNUC__) && __GNUC__ >= 4
/* Count Leading Zeroes builtin in GCC. /* Count Leading Zeroes builtin in GCC.
* http://gcc.gnu.org/onlinedocs/gcc-4.3.4/gcc/Other-Builtins.html * http://gcc.gnu.org/onlinedocs/gcc-4.3.4/gcc/Other-Builtins.html
*/ */
if (x == 0) {
return -1;
}
return 31 - __builtin_clz(x); return 31 - __builtin_clz(x);
#else #else
/* Based off of Bit Twiddling Hacks by Sean Eron Anderson /* Based off of Bit Twiddling Hacks by Sean Eron Anderson
@ -61,10 +64,14 @@ SDL_MostSignificantBitIndex32(Uint32 x)
* http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog * http://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
*/ */
const Uint32 b[] = {0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000}; const Uint32 b[] = {0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000};
const Uint8 S[] = {1, 2, 4, 8, 16}; const int S[] = {1, 2, 4, 8, 16};
Uint8 msbIndex = 0; int msbIndex = 0;
int i; int i;
if (x == 0) {
return -1;
}
for (i = 4; i >= 0; i--) for (i = 4; i >= 0; i--)
{ {