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clzdi2.S revision 1.1.1.1.2.1 
      1 /* ===-- clzdi2.c - Implement __clzdi2 -------------------------------------===
      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 64bit arguments.
     11  *
     12  * ===----------------------------------------------------------------------===
     13  */
     14 #include "../assembly.h"
     15 
     16 	.syntax unified
     17 
     18 	.text
     19 	.p2align	2
     20 DEFINE_COMPILERRT_FUNCTION(__clzdi2)
     21 #ifdef __ARM_FEATURE_CLZ
     22 #ifdef __ARMEB__
     23 	cmp	r0, 0
     24 	itee ne
     25 	clzne	r0, r0
     26 	clzeq	r0, r1
     27 	addeq	r0, r0, 32
     28 #else
     29 	cmp	r1, 0
     30 	itee ne
     31 	clzne	r0, r1
     32 	clzeq	r0, r0
     33 	addeq	r0, r0, 32
     34 #endif
     35 	JMP(lr)
     36 #else
     37 	/* Assumption: n != 0 */
     38 
     39 	/*
     40 	 * r0: n
     41 	 * r1: upper half of n, overwritten after check
     42 	 * r1: count of leading zeros in n + 1
     43 	 * r2: scratch register for shifted r0
     44 	 */
     45 #ifdef __ARMEB__
     46 	cmp	r0, 0
     47 	moveq	r0, r1
     48 #else
     49 	cmp	r1, 0
     50 	movne	r0, r1
     51 #endif
     52 	movne	r1, 1
     53 	moveq	r1, 33
     54 
     55 	/*
     56 	 * Basic block:
     57 	 * if ((r0 >> SHIFT) == 0)
     58 	 *   r1 += SHIFT;
     59 	 * else
     60 	 *   r0 >>= SHIFT;
     61 	 * for descending powers of two as SHIFT.
     62 	 */
     63 #define BLOCK(shift) \
     64 	lsrs	r2, r0, shift; \
     65 	movne	r0, r2; \
     66 	addeq	r1, shift \
     67 
     68 	BLOCK(16)
     69 	BLOCK(8)
     70 	BLOCK(4)
     71 	BLOCK(2)
     72 
     73 	/*
     74 	 * The basic block invariants at this point are (r0 >> 2) == 0 and
     75 	 * r0 != 0. This means 1 <= r0 <= 3 and 0 <= (r0 >> 1) <= 1.
     76 	 *
     77 	 * r0 | (r0 >> 1) == 0 | (r0 >> 1) == 1 | -(r0 >> 1) | 1 - (r0 >> 1)
     78 	 * ---+----------------+----------------+------------+--------------
     79 	 * 1  | 1              | 0              | 0          | 1
     80 	 * 2  | 0              | 1              | -1         | 0
     81 	 * 3  | 0              | 1              | -1         | 0
     82 	 *
     83 	 * The r1's initial value of 1 compensates for the 1 here.
     84 	 */
     85 	sub	r0, r1, r0, lsr #1
     86 
     87 	JMP(lr)
     88 #endif // __ARM_FEATURE_CLZ
     89 END_COMPILERRT_FUNCTION(__clzdi2)
     90