ref: fc08d0a6d63a5453a8928bd5f77e30cf73f13073
dir: /libentcode/ecintrin.h/
/*Some common macros for potential platform-specific optimization.*/ #include <math.h> #include <limits.h> #if !defined(_ecintrin_H) # define _ecintrin_H (1) /*Some specific platforms may have optimized intrinsic or inline assembly versions of these functions which can substantially improve performance. We define macros for them to allow easy incorporation of these non-ANSI features.*/ /*Note that we do not provide a macro for abs(), because it is provided as a library function, which we assume is translated into an intrinsic to avoid the function call overhead and then implemented in the smartest way for the target platform. With modern gcc (4.x), this is true: it uses cmov instructions if the architecture supports it and branchless bit-twiddling if it does not (the speed difference between the two approaches is not measurable). Interestingly, the bit-twiddling method was patented in 2000 (US 6,073,150) by Sun Microsystems, despite prior art dating back to at least 1996: http://web.archive.org/web/19961201174141/www.x86.org/ftp/articles/pentopt/PENTOPT.TXT On gcc 3.x, however, our assumption is not true, as abs() is translated to a conditional jump, which is horrible on deeply piplined architectures (e.g., all consumer architectures for the past decade or more) when the sign cannot be reliably predicted.*/ /*Modern gcc (4.x) can compile the naive versions of min and max with cmov if given an appropriate architecture, but the branchless bit-twiddling versions are just as fast, and do not require any special target architecture. Earlier gcc versions (3.x) compiled both code to the same assembly instructions, because of the way they represented ((_b)>(_a)) internally.*/ #define EC_MAXI(_a,_b) ((_a)-((_a)-(_b)&-((_b)>(_a)))) #define EC_MINI(_a,_b) ((_a)+((_b)-(_a)&-((_b)<(_a)))) /*This has a chance of compiling branchless, and is just as fast as the bit-twiddling method, which is slightly less portable, since it relies on a sign-extended rightshift, which is not guaranteed by ANSI (but present on every relevant platform).*/ #define EC_SIGNI(_a) (((_a)>0)-((_a)<0)) /*Slightly more portable than relying on a sign-extended right-shift (which is not guaranteed by ANSI), and just as fast, since gcc (3.x and 4.x both) compile it into the right-shift anyway.*/ #define EC_SIGNMASK(_a) (-((_a)<0)) /*Clamps an integer into the given range. If _a>_c, then the lower bound _a is respected over the upper bound _c (this behavior is required to meet our documented API behavior). _a: The lower bound. _b: The value to clamp. _c: The upper boud.*/ #define EC_CLAMPI(_a,_b,_c) (EC_MAXI(_a,EC_MINI(_b,_c))) /*Count leading zeros. This macro should only be used for implementing ec_ilog(), if it is defined. All other code should use EC_ILOG() instead.*/ #if __GNUC_PREREQ(3,4) # if INT_MAX>=2147483647 # define EC_CLZ0 sizeof(unsigned)*CHAR_BIT # define EC_CLZ(_x) (__builtin_clz(_x)) # elif LONG_MAX>=2147483647L # define EC_CLZ0 sizeof(unsigned long)*CHAR_BIT # define EC_CLZ(_x) (__builtin_clzl(_x)) # endif #endif #if defined(EC_CLZ) /*Note that __builtin_clz is not defined when _x==0, according to the gcc documentation (and that of the BSR instruction that implements it on x86), so we have to special-case it.*/ # define EC_ILOG(_x) (EC_CLZ0-EC_CLZ(_x)&-!!(_x)) #else # define EC_ILOG(_x) (ec_ilog(_x)) #endif #endif