80 lines
1.8 KiB
ArmAsm
80 lines
1.8 KiB
ArmAsm
/* ===-- clzsi2.c - Implement __clzsi2 -------------------------------------===
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*
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* The LLVM Compiler Infrastructure
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*
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* This file is dual licensed under the MIT and the University of Illinois Open
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* Source Licenses. See LICENSE.TXT for details.
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*
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* ===----------------------------------------------------------------------===
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*
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* This file implements count leading zeros for 32bit arguments.
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*
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* ===----------------------------------------------------------------------===
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*/
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#include "../assembly.h"
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.syntax unified
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.text
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#if __ARM_ARCH_ISA_THUMB == 2
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.thumb
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#endif
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.p2align 2
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#if __ARM_ARCH_ISA_THUMB == 2
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DEFINE_COMPILERRT_THUMB_FUNCTION(__clzsi2)
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#else
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DEFINE_COMPILERRT_FUNCTION(__clzsi2)
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#endif
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#ifdef __ARM_FEATURE_CLZ
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clz r0, r0
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JMP(lr)
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#else
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/* Assumption: n != 0 */
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/*
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* r0: n
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* r1: count of leading zeros in n + 1
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* r2: scratch register for shifted r0
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*/
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mov r1, 1
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/*
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* Basic block:
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* if ((r0 >> SHIFT) == 0)
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* r1 += SHIFT;
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* else
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* r0 >>= SHIFT;
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* for descending powers of two as SHIFT.
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*/
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#define BLOCK(shift) \
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lsrs r2, r0, shift; \
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movne r0, r2; \
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addeq r1, shift \
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BLOCK(16)
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BLOCK(8)
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BLOCK(4)
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BLOCK(2)
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/*
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* The basic block invariants at this point are (r0 >> 2) == 0 and
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* r0 != 0. This means 1 <= r0 <= 3 and 0 <= (r0 >> 1) <= 1.
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*
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* r0 | (r0 >> 1) == 0 | (r0 >> 1) == 1 | -(r0 >> 1) | 1 - (r0 >> 1)
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* ---+----------------+----------------+------------+--------------
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* 1 | 1 | 0 | 0 | 1
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* 2 | 0 | 1 | -1 | 0
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* 3 | 0 | 1 | -1 | 0
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*
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* The r1's initial value of 1 compensates for the 1 here.
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*/
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sub r0, r1, r0, lsr #1
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JMP(lr)
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#endif // __ARM_FEATURE_CLZ
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END_COMPILERRT_FUNCTION(__clzsi2)
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NO_EXEC_STACK_DIRECTIVE
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