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clzsi2.S revision 1.1.1.2.4.2 
      1 /* ===-- clzsi2.c - Implement __clzsi2 -------------------------------------===
      2  *
      3  *               The LLVM Compiler Infrastructure
      4  *
      5  * This file is dual licensed under the MIT and the University of Illinois Open
      6  * Source Licenses. See LICENSE.TXT for details.
      7  *
      8  * ===----------------------------------------------------------------------===
      9  *
     10  * This file implements count leading zeros for 32bit arguments.
     11  *
     12  * ===----------------------------------------------------------------------===
     13  */
     14 #include "../assembly.h"
     15 
     16 	.syntax unified
     17 
     18 	.text
     19 	.align	2
     20 DEFINE_COMPILERRT_FUNCTION(__clzsi2)
     21 #ifdef __ARM_FEATURE_CLZ
     22 	clz	r0, r0
     23 	JMP(lr)
     24 #else
     25 	/* Assumption: n != 0 */
     26 
     27 	/*
     28 	 * r0: n
     29 	 * r1: count of leading zeros in n + 1
     30 	 * r2: scratch register for shifted r0
     31 	 */
     32 	mov	r1, 1
     33 
     34 	/*
     35 	 * Basic block:
     36 	 * if ((r0 >> SHIFT) == 0)
     37 	 *   r1 += SHIFT;
     38 	 * else
     39 	 *   r0 >>= SHIFT;
     40 	 * for descending powers of two as SHIFT.
     41 	 */
     42 
     43 #define BLOCK(shift) \
     44 	lsrs	r2, r0, shift; \
     45 	movne	r0, r2; \
     46 	addeq	r1, shift \
     47 
     48 	BLOCK(16)
     49 	BLOCK(8)
     50 	BLOCK(4)
     51 	BLOCK(2)
     52 
     53 	/*
     54 	 * The basic block invariants at this point are (r0 >> 2) == 0 and
     55 	 * r0 != 0. This means 1 <= r0 <= 3 and 0 <= (r0 >> 1) <= 1.
     56 	 *
     57 	 * r0 | (r0 >> 1) == 0 | (r0 >> 1) == 1 | -(r0 >> 1) | 1 - (r0 >> 1)
     58 	 * ---+----------------+----------------+------------+--------------
     59 	 * 1  | 1              | 0              | 0          | 1
     60 	 * 2  | 0              | 1              | -1         | 0
     61 	 * 3  | 0              | 1              | -1         | 0
     62 	 *
     63 	 * The r1's initial value of 1 compensates for the 1 here.
     64 	 */
     65 	sub	r0, r1, r0, lsr #1
     66 
     67 	JMP(lr)
     68 #endif // __ARM_FEATURE_CLZ
     69 END_COMPILERRT_FUNCTION(__clzsi2)
     70