58 lines
1.7 KiB
C
58 lines
1.7 KiB
C
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/* ===-- ctzsi2.c - Implement __ctzsi2 -------------------------------------===
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*
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* The LLVM Compiler Infrastructure
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*
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* This file is dual licensed under the MIT and the University of Illinois Open
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* Source Licenses. See LICENSE.TXT for details.
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*
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* ===----------------------------------------------------------------------===
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*
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* This file implements __ctzsi2 for the compiler_rt library.
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*
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* ===----------------------------------------------------------------------===
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*/
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#include "int_lib.h"
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/* Returns: the number of trailing 0-bits */
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/* Precondition: a != 0 */
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COMPILER_RT_ABI si_int
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__ctzsi2(si_int a)
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{
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su_int x = (su_int)a;
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si_int t = ((x & 0x0000FFFF) == 0) << 4; /* if (x has no small bits) t = 16 else 0 */
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x >>= t; /* x = [0 - 0xFFFF] + higher garbage bits */
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su_int r = t; /* r = [0, 16] */
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/* return r + ctz(x) */
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t = ((x & 0x00FF) == 0) << 3;
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x >>= t; /* x = [0 - 0xFF] + higher garbage bits */
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r += t; /* r = [0, 8, 16, 24] */
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/* return r + ctz(x) */
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t = ((x & 0x0F) == 0) << 2;
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x >>= t; /* x = [0 - 0xF] + higher garbage bits */
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r += t; /* r = [0, 4, 8, 12, 16, 20, 24, 28] */
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/* return r + ctz(x) */
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t = ((x & 0x3) == 0) << 1;
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x >>= t;
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x &= 3; /* x = [0 - 3] */
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r += t; /* r = [0 - 30] and is even */
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/* return r + ctz(x) */
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/* The branch-less return statement below is equivalent
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* to the following switch statement:
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* switch (x)
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* {
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* case 0:
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* return r + 2;
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* case 2:
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* return r + 1;
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* case 1:
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* case 3:
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* return r;
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* }
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*/
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return r + ((2 - (x >> 1)) & -((x & 1) == 0));
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}
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