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Fix Rotation Instruction Inference #87

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Fix Rotation Instruction Inference #87

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26 changes: 8 additions & 18 deletions include/pcg_extras.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -282,43 +282,33 @@ inline itype unxorshift(itype x, bitcount_t bits, bitcount_t shift)

/*
* Rotate left and right.
*
* In ideal world, compilers would spot idiomatic rotate code and convert it
* to a rotate instruction. Of course, opinions vary on what the correct
* idiom is and how to spot it. For clang, sometimes it generates better
* (but still crappy) code if you define PCG_USE_ZEROCHECK_ROTATE_IDIOM.
*/

template <typename itype>
inline itype rotl(itype value, bitcount_t rot)
{
constexpr bitcount_t bits = sizeof(itype) * 8;
constexpr bitcount_t mask = bits - 1;
#if PCG_USE_ZEROCHECK_ROTATE_IDIOM
return rot ? (value << rot) | (value >> (bits - rot)) : value;
#else
rot &= mask;
return (value << rot) | (value >> ((- rot) & mask));
#endif
}

template <typename itype>
inline itype rotr(itype value, bitcount_t rot)
{
constexpr bitcount_t bits = sizeof(itype) * 8;
constexpr bitcount_t mask = bits - 1;
#if PCG_USE_ZEROCHECK_ROTATE_IDIOM
return rot ? (value >> rot) | (value << (bits - rot)) : value;
#else
rot &= mask;
return (value >> rot) | (value << ((- rot) & mask));
#endif
}

/* Unfortunately, both Clang and GCC sometimes perform poorly when it comes
* to properly recognizing idiomatic rotate code, so for we also provide
* assembler directives (enabled with PCG_USE_INLINE_ASM). Boo, hiss.
* (I hope that these compilers get better so that this code can die.)
/* Older versions of this library with older versions of clang and gcc
* fail to infer rotation instructions. Inference is preferable to inline
* assembly when it works though. If you are on a gcc >= 4.9.0 or a clang
* >= 11.0 you should not use PCG_USE_INLINE_ASM.
*
* These overloads will be preferred over the general template code above.
* When PCG_USE_INLINE_ASM is set, these overloads will be preferred
* over the general template code above.
*/
#if PCG_USE_INLINE_ASM && __GNUC__ && (__x86_64__ || __i386__)

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