84 lines
1.9 KiB
C++
84 lines
1.9 KiB
C++
/**
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* =========================================================================
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* File : bits.cpp
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* Project : 0 A.D.
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* Description : bit-twiddling.
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* =========================================================================
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*/
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// license: GPL; see lib/license.txt
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#include "precompiled.h"
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#include "bits.h"
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#if ARCH_IA32
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# include "lib/sysdep/ia32/ia32_asm.h" // ia32_asm_log2_of_pow2
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#endif
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int log2_of_pow2(uint n)
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{
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int bit_index;
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#if ARCH_IA32
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bit_index = ia32_asm_log2_of_pow2(n);
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#else
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if(!is_pow2(n))
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bit_index = -1;
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else
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{
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bit_index = 0;
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// note: compare against n directly because it is known to be a POT.
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for(uint bit_value = 1; bit_value != n; bit_value *= 2)
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bit_index++;
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}
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#endif
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debug_assert(-1 <= bit_index && bit_index < (int)sizeof(int)*CHAR_BIT);
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debug_assert(bit_index == -1 || n == (1u << bit_index));
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return bit_index;
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}
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int floor_log2(const float x)
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{
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const u32 i = *(u32*)&x;
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u32 biased_exp = (i >> 23) & 0xFF;
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return (int)biased_exp - 127;
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}
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uint round_up_to_pow2(uint x)
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{
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// fold upper bit into lower bits; leaves same MSB set but
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// everything below it 1. adding 1 yields next POT.
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x |= (x >> 1);
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x |= (x >> 2);
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x |= (x >> 4);
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x |= (x >> 8);
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x |= (x >> 16);
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// if ints are 64 bits, add "x |= (x >> 32);"
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cassert(sizeof(int)*CHAR_BIT == 32);
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return x+1;
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}
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// multiple must be a power of two.
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uintptr_t round_up(const uintptr_t n, const uintptr_t multiple)
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{
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debug_assert(is_pow2((long)multiple));
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const uintptr_t result = (n + multiple-1) & ~(multiple-1);
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debug_assert(n <= result && result < n+multiple);
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return result;
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}
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// multiple must be a power of two.
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uintptr_t round_down(const uintptr_t n, const uintptr_t multiple)
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{
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debug_assert(is_pow2((long)multiple));
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const uintptr_t result = n & ~(multiple-1);
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debug_assert(result <= n && n < result+multiple);
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return result;
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}
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