clzsi2.S
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//===-- clzsi2.c - Implement __clzsi2 -------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// 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