llvm-project

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//===-- clzsi2.c - Implement __clzsi2 -------------------------------------===//
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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 __clzsi2 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 leading 0-bits
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// Precondition: a != 0
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COMPILER_RT_ABI int __clzsi2(si_int a) {
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  su_int x = (su_int)a;
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  si_int t = ((x & 0xFFFF0000) == 0) << 4; // if (x is small) t = 16 else 0
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  x >>= 16 - t;                            // x = [0 - 0xFFFF]
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  su_int r = t;                            // r = [0, 16]
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  // return r + clz(x)
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  t = ((x & 0xFF00) == 0) << 3;
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  x >>= 8 - t; // x = [0 - 0xFF]
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  r += t;      // r = [0, 8, 16, 24]
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  // return r + clz(x)
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  t = ((x & 0xF0) == 0) << 2;
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  x >>= 4 - t; // x = [0 - 0xF]
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  r += t;      // r = [0, 4, 8, 12, 16, 20, 24, 28]
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  // return r + clz(x)
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  t = ((x & 0xC) == 0) << 1;
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  x >>= 2 - t; // x = [0 - 3]
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  r += t;      // r = [0 - 30] and is even
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  // return r + clz(x)
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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 1:
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  //         return r + 1;
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  //     case 2:
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  //     case 3:
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  //         return r;
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  //     }
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  return r + ((2 - x) & -((x & 2) == 0));
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}
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