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clzdi2.S revision 1.1.1.3.6.1 
      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 	.text
     18 #if __ARM_ARCH_ISA_THUMB == 2
     19 	.thumb
     20 #endif
     21 
     22 
     23 	.p2align	2
     24 #if __ARM_ARCH_ISA_THUMB == 2
     25 DEFINE_COMPILERRT_THUMB_FUNCTION(__clzdi2)
     26 #else
     27 DEFINE_COMPILERRT_FUNCTION(__clzdi2)
     28 #endif
     29 #ifdef __ARM_FEATURE_CLZ
     30 #ifdef __ARMEB__
     31 	cmp	r0, 0
     32 	itee ne
     33 	clzne	r0, r0
     34 	clzeq	r0, r1
     35 	addeq	r0, r0, 32
     36 #else
     37 	cmp	r1, 0
     38 	itee ne
     39 	clzne	r0, r1
     40 	clzeq	r0, r0
     41 	addeq	r0, r0, 32
     42 #endif
     43 	JMP(lr)
     44 #else
     45 	/* Assumption: n != 0 */
     46 
     47 	/*
     48 	 * r0: n
     49 	 * r1: upper half of n, overwritten after check
     50 	 * r1: count of leading zeros in n + 1
     51 	 * r2: scratch register for shifted r0
     52 	 */
     53 #ifdef __ARMEB__
     54 	cmp	r0, 0
     55 	moveq	r0, r1
     56 #else
     57 	cmp	r1, 0
     58 	movne	r0, r1
     59 #endif
     60 	movne	r1, 1
     61 	moveq	r1, 33
     62 
     63 	/*
     64 	 * Basic block:
     65 	 * if ((r0 >> SHIFT) == 0)
     66 	 *   r1 += SHIFT;
     67 	 * else
     68 	 *   r0 >>= SHIFT;
     69 	 * for descending powers of two as SHIFT.
     70 	 */
     71 #define BLOCK(shift) \
     72 	lsrs	r2, r0, shift; \
     73 	movne	r0, r2; \
     74 	addeq	r1, shift \
     75 
     76 	BLOCK(16)
     77 	BLOCK(8)
     78 	BLOCK(4)
     79 	BLOCK(2)
     80 
     81 	/*
     82 	 * The basic block invariants at this point are (r0 >> 2) == 0 and
     83 	 * r0 != 0. This means 1 <= r0 <= 3 and 0 <= (r0 >> 1) <= 1.
     84 	 *
     85 	 * r0 | (r0 >> 1) == 0 | (r0 >> 1) == 1 | -(r0 >> 1) | 1 - (r0 >> 1)
     86 	 * ---+----------------+----------------+------------+--------------
     87 	 * 1  | 1              | 0              | 0          | 1
     88 	 * 2  | 0              | 1              | -1         | 0
     89 	 * 3  | 0              | 1              | -1         | 0
     90 	 *
     91 	 * The r1's initial value of 1 compensates for the 1 here.
     92 	 */
     93 	sub	r0, r1, r0, lsr #1
     94 
     95 	JMP(lr)
     96 #endif // __ARM_FEATURE_CLZ
     97 END_COMPILERRT_FUNCTION(__clzdi2)
     98