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Rationalise FIND_FIRST_BIT/FindFirstBit2x64

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Use builtins for FindFirstBit2x64.
Apply and masks to prevent undefined behaviour if inputs out of range
This commit is contained in:
Jonathan G Rennison
2021-11-01 23:14:29 +00:00
parent 1e0bea5b8d
commit de5de583e3
1 changed files with 8 additions and 6 deletions
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14
src/core/bitmath_func.hpp
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@@ -204,7 +204,7 @@ extern const uint8 _ffb_64[64];
* @param x The 6-bit value to check the first zero-bit
* @return The first position of a bit started from the LSB or 0 if x is 0.
*/
#define FIND_FIRST_BIT(x) _ffb_64[(x)]
#define FIND_FIRST_BIT(x) _ffb_64[(x) & 0x3F]
#endif
@@ -246,10 +246,7 @@ uint8 FindLastBit(uint64 x);
*
* This function returns the position of the first bit set in the
* integer. It does only check the bits of the bitmask
* 0x3F3F (0011111100111111) and checks only the
* bits of the bitmask 0x3F00 if and only if the
* lower part 0x00FF is 0. This results the bits at 0x00C0 must
* be also zero to check the bits at 0x3F00.
* 0x3F3F (0011111100111111).
*
* @param value The value to check the first bits
* @return The position of the first bit which is set
@@ -257,11 +254,16 @@ uint8 FindLastBit(uint64 x);
*/
static inline uint8 FindFirstBit2x64(const int value)
{
if ((value & 0xFF) == 0) {
#ifdef WITH_BITMATH_BUILTINS
return FindFirstBit(value & 0x3F3F);
#else
if (value == 0) return 0;
if ((value & 0x3F) == 0) {
return FIND_FIRST_BIT((value >> 8) & 0x3F) + 8;
} else {
return FIND_FIRST_BIT(value & 0x3F);
}
#endif
}
/**