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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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} |