54 lines
1.6 KiB
C
54 lines
1.6 KiB
C
//===-- ctzsi2.c - Implement __ctzsi2 -------------------------------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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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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#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 int __ctzsi2(si_int a) {
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su_int x = (su_int)a;
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si_int t = ((x & 0x0000FFFF) == 0)
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<< 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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return r + ((2 - (x >> 1)) & -((x & 1) == 0));
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}
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