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/* ===-- clzsi2.c - Implement __clzsi2 -------------------------------------===
*
* The LLVM Compiler Infrastructure
*
* This file is dual licensed under the MIT and the University of Illinois Open
* Source Licenses. See LICENSE.TXT for details.
*
* ===----------------------------------------------------------------------===
*
* This file implements count leading zeros for 32bit arguments.
*
* ===----------------------------------------------------------------------===
*/
#include "../assembly.h"
.syntax unified
.text
#if __ARM_ARCH_ISA_THUMB == 2
.thumb
#endif
.p2align 2
#if __ARM_ARCH_ISA_THUMB == 2
DEFINE_COMPILERRT_THUMB_FUNCTION(__clzsi2)
#else
DEFINE_COMPILERRT_FUNCTION(__clzsi2)
#endif
#ifdef __ARM_FEATURE_CLZ
clz r0, r0
JMP(lr)
#else
/* Assumption: n != 0 */
/*
* r0: n
* r1: count of leading zeros in n + 1
* r2: scratch register for shifted r0
*/
mov r1, 1
/*
* Basic block:
* if ((r0 >> SHIFT) == 0)
* r1 += SHIFT;
* else
* r0 >>= SHIFT;
* for descending powers of two as SHIFT.
*/
#define BLOCK(shift) \
lsrs r2, r0, shift; \
movne r0, r2; \
addeq r1, shift \
BLOCK(16)
BLOCK(8)
BLOCK(4)
BLOCK(2)
/*
* The basic block invariants at this point are (r0 >> 2) == 0 and
* r0 != 0. This means 1 <= r0 <= 3 and 0 <= (r0 >> 1) <= 1.
*
* r0 | (r0 >> 1) == 0 | (r0 >> 1) == 1 | -(r0 >> 1) | 1 - (r0 >> 1)
* ---+----------------+----------------+------------+--------------
* 1 | 1 | 0 | 0 | 1
* 2 | 0 | 1 | -1 | 0
* 3 | 0 | 1 | -1 | 0
*
* The r1's initial value of 1 compensates for the 1 here.
*/
sub r0, r1, r0, lsr #1
JMP(lr)
#endif // __ARM_FEATURE_CLZ
END_COMPILERRT_FUNCTION(__clzsi2)
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