Had a go at implementing checked-multiply differently, but the results worked out slower

This commit is contained in:
Neil Brown 2009-01-23 12:44:37 +00:00
parent b7e296943a
commit ecf840fe9f

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@ -114,7 +114,7 @@ static inline int occam_check_retype (int src, int dest, const char *pos) {
}
// Some things taken from http://www.fefe.de/intof.html
#define __HALF_MAX_SIGNED(type) ((type)1 << (sizeof(type)*8-2))
#define __HALF_MAX_SIGNED(type) ((type)1 << (sizeof(type)*CHAR_BIT-2))
#define __MAX_SIGNED(type) (__HALF_MAX_SIGNED(type) - 1 + __HALF_MAX_SIGNED(type))
#define __MIN_SIGNED(type) (-1 - __MAX_SIGNED(type))
@ -140,9 +140,51 @@ static inline int occam_check_retype (int src, int dest, const char *pos) {
#define MAKE_SUBTRF(type) \
static inline type occam_subtr_##type (type, type, const char *) occam_unused; \
static inline type occam_subtr_##type (type a, type b, const char *pos) { return a - b;}
#define MAKE_MUL(type) \
//Here is the plan:
// - For any values that are smaller than sizeof(long), cast them to long and check that way (TODO).
// - For all other values, split the number. Take N to be the length of the word
// and n to be N/2. Split the bits into top(A|B) and bottom(A|B) So we have:
//A = topA*2^n + bottomA
//B = topB*2^n + bottomB
//A*B = (topA*2^n + bottomA) * (topB*2^n + bottomB)
// = topA*topB*2^N + 2^n*(topA*bottomB + topB*bottomA) + (bottomA*bottomB)
//
//If topA*topB is non-zero, the multiplication will definitely overflow. So
//the multiplication can only be valid if either topA or topB is zero.
//Then we can see if the middle bracketed multiplication is >= 2^n, in which case
//the calculation would also overflow. Also, given that either topA or topB is zero,
//only one of the components of the bracketed sum is non-zero. Finally we do an occam add-style addition
//of the middle and end, again checking for overflow.
#define abs(x) ((x) < 0 ? -x : x)
/* This is my other attempt at the function, but it turned out slower:
const utype topA = (utype)abs(a) >> (sizeof(type)*CHAR_BIT/2); \
const utype topB = (utype)abs(b) >> (sizeof(type)*CHAR_BIT/2); \
const utype bottomA = (utype)abs(a) & (((utype)1<<(sizeof(type)*CHAR_BIT/2))-1); \
const utype bottomB = (utype)abs(b) & (((utype)1<<(sizeof(type)*CHAR_BIT/2))-1); \
const utype pos_res = ((a > 0 && b > 0) || (a < 0 && b < 0)) ? 1 : 0; \
if (topA != 0 && topB != 0) { \
occam_stop(pos,3,"integer overflow when doing %d * %d", a, b); \
} else { \
const utype mid = topA != 0 ? topA*bottomB : topB*bottomA; \
const utype mid_shift = mid << (sizeof(type)*CHAR_BIT/2); \
const utype low = bottomA*bottomB; \
if ((mid>>((utype)1<<(sizeof(type)*CHAR_BIT/2))==0)&&(((utype)__MAX(type))+1-pos_res-mid_shift>=low)) {\
return (pos_res == 1 ? (type)(mid_shift+low) : -(type)mid_shift-(type)low); \
} else { \
occam_stop(pos,3,"integer overflow when doing %d * %d", a, b); \
} \
} \
}
*/
#define MAKE_MUL(type,utype) \
static inline type occam_mul_##type (type, type, const char *) occam_unused; \
static inline type occam_mul_##type (type a, type b, const char *pos) { \
static inline type occam_mul_##type (const type a, const type b, const char *pos) { \
if (( (a < 0 ? -a : a) >> ((sizeof(type)*CHAR_BIT/2)-1) \
| (b < 0 ? -b : b) >> ((sizeof(type)*CHAR_BIT/2)-1)) == 0) { \
/*overflow not possible on such small numbers*/ \
@ -260,7 +302,7 @@ static inline int occam_check_retype (int src, int dest, const char *pos) {
MAKE_RANGE_CHECK(type,flag) \
MAKE_ADD(type) \
MAKE_SUBTR(type) \
MAKE_MUL(type) \
MAKE_MUL(type,utype) \
MAKE_DIV(type) \
MAKE_REM(type) \
MAKE_NEGATE(type) \
@ -273,7 +315,7 @@ static inline int occam_check_retype (int src, int dest, const char *pos) {
MAKE_RANGE_CHECK(uint8_t, "%d")
MAKE_ADD(uint8_t)
MAKE_SUBTR(uint8_t)
MAKE_MUL(uint8_t)
MAKE_MUL(uint8_t,uint8_t)
MAKE_DIV(uint8_t)
MAKE_SHIFT(uint8_t,uint8_t)
MAKE_PLUS(uint8_t)