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|
; $Id: ASMBitLastSetU64.asm $
;; @file
; IPRT - ASMBitLastSetU64().
;
;
; Copyright (C) 2006-2019 Oracle Corporation
;
; This file is part of VirtualBox Open Source Edition (OSE), as
; available from http://www.virtualbox.org. This file is free software;
; you can redistribute it and/or modify it under the terms of the GNU
; General Public License (GPL) as published by the Free Software
; Foundation, in version 2 as it comes in the "COPYING" file of the
; VirtualBox OSE distribution. VirtualBox OSE is distributed in the
; hope that it will be useful, but WITHOUT ANY WARRANTY of any kind.
;
; The contents of this file may alternatively be used under the terms
; of the Common Development and Distribution License Version 1.0
; (CDDL) only, as it comes in the "COPYING.CDDL" file of the
; VirtualBox OSE distribution, in which case the provisions of the
; CDDL are applicable instead of those of the GPL.
;
; You may elect to license modified versions of this file under the
; terms and conditions of either the GPL or the CDDL or both.
;
;*******************************************************************************
;* Header Files *
;*******************************************************************************
%include "iprt/asmdefs.mac"
BEGINCODE
;;
; Finds the last bit which is set in the given 64-bit integer.
;
; Bits are numbered from 1 (least significant) to 64.
;
; @returns (xAX) index [1..64] of the last set bit.
; @returns (xAX) 0 if all bits are cleared.
; @param msc:rcx gcc:rdi x86:stack u64 Integer to search for set bits.
;
; @cproto DECLASM(unsigned) ASMBitLastSetU64(uint64_t u64);
;
BEGINPROC_EXPORTED ASMBitLastSetU64
%if ARCH_BITS == 16
CPU 8086
push bp
mov bp, sp
; 63:48
mov ax, 64
mov cx, [bp + 2 + 2 + 6]
test cx, cx
jnz .next_bit
; 47:32
mov al, 48
or cx, [bp + 2 + 2 + 4]
jnz .next_bit
; 31:16
mov al, 32
or cx, [bp + 2 + 2 + 2]
jnz .next_bit
; 16:0
mov al, 16
or cx, [bp + 2 + 2 + 0]
jz .return_zero
; find the bit that was set.
.next_bit:
shl cx, 1
jc .return
dec ax
jmp .next_bit
.return_zero:
xor ax, ax
.return:
pop bp
ret
%else
%if ARCH_BITS == 64
%ifdef ASM_CALL64_GCC
bsr rax, rsi
%else
bsr rax, rcx
%endif
jz .return_zero
inc eax
.return:
ret
%elif ARCH_BITS == 32
; Check the 2nd dword then the first one.
bsr eax, dword [esp + 4 + 4]
jz .check_1st_dword
add eax, 32
ret
.check_1st_dword:
bsr eax, dword [esp + 4 + 0]
jz .return_zero
inc eax
ret
%else
%error "Missing or invalid ARCH_BITS."
%endif
.return_zero:
xor eax, eax
ret
%endif
ENDPROC ASMBitLastSetU64
|
