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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 DEFINE_CODE_STATE .p2align 2 DEFINE_COMPILERRT_FUNCTION(__clzsi2) #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) NO_EXEC_STACK_DIRECTIVE