54 lines
1.5 KiB
C
54 lines
1.5 KiB
C
/* ===-- clzsi2.c - Implement __clzsi2 -------------------------------------===
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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 __clzsi2 for the compiler_rt library.
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*
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* ===----------------------------------------------------------------------===
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*/
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//
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// Ported by wsyeo for 8267 compatibility.
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//
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// Returns: the number of leading 0-bits
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// Precondition: a != 0
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unsigned int __clzsi2(unsigned int a)
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{
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unsigned int x = a;
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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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unsigned 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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