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|
; $Id: ASMBitNextClear.asm $
;; @file
; IPRT - ASMBitNextClear().
;
;
; Copyright (C) 2006-2023 Oracle and/or its affiliates.
;
; This file is part of VirtualBox base platform packages, as
; available from https://www.virtualbox.org.
;
; This program is free software; you can redistribute it and/or
; modify it under the terms of the GNU General Public License
; as published by the Free Software Foundation, in version 3 of the
; License.
;
; This program is distributed in the hope that it will be useful, but
; WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
; General Public License for more details.
;
; You should have received a copy of the GNU General Public License
; along with this program; if not, see <https://www.gnu.org/licenses>.
;
; The contents of this file may alternatively be used under the terms
; of the Common Development and Distribution License Version 1.0
; (CDDL), a copy of it is provided in the "COPYING.CDDL" file included
; in the VirtualBox 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.
;
; SPDX-License-Identifier: GPL-3.0-only OR CDDL-1.0
;
;*******************************************************************************
;* Header Files *
;*******************************************************************************
%include "iprt/asmdefs.mac"
BEGINCODE
;;
; Finds the next clear bit in a bitmap.
;
; @returns (32/64:eax, 16:ax+dx) Index of the first zero bit.
; @returns (32/64:eax, 16:ax+dx) -1 if no clear bit was found.
; @param msc:rcx gcc:rdi pvBitmap Pointer to the bitmap.
; @param msc:edx gcc:rsi cBits The number of bits in the bitmap. Multiple of 32.
; @param msc:r8d gcc:rcx iBitPrev The previous bit, start searching after it.
;
; @remarks Not quite sure how much sense it makes to do this in assembly, but
; it started out with the ASMBit* API, so that's why we still have it.
;
RT_BEGINPROC ASMBitNextClear
%if ARCH_BITS == 16
push bp
mov bp, sp
%endif
push xDI
;
; Align input registers: rdi=pvBitmap, ecx=iPrevBit
;
%if ARCH_BITS == 64
%ifdef ASM_CALL64_GCC
; ecx = iBitPrev param, rdi=pvBitmap param.
%else
mov rdi, rcx ; rdi=pvBits
mov ecx, r8d ; ecx=iPrevBit
mov r8d, edx ; r8d=cBits (saved for .scan_dwords)
%endif
mov r9, rdi ; Save rdi for bit calculation.
%elif ARCH_BITS == 32
mov edi, [esp + 8] ; edi=pvBits
mov ecx, [esp + 8 + 8] ; ecx=iPrevBit
%elif ARCH_BITS == 16
les di, [bp + 4] ; es:di=pvBits
mov ecx, [bp + 4 + 8] ; ecx=iPrevBit
%endif
;
; If iPrevBit and iPrevBit + 1 are in the same dword, inspect it for further bits.
;
inc ecx
mov eax, ecx
shr eax, 5
shl eax, 2 ; eax=byte offset into bitmap of current dword.
add xDI, xAX ; xDI=current dword address (of iPrevBit + 1).
and ecx, 31
jz .scan_dwords
%if ARCH_BITS == 16
mov edx, [es:di]
%else
mov edx, [xDI]
%endif
not edx ; edx = inverted current dword
shr edx, cl ; Shift out bits that we have searched.
jz .next_dword ; If zero, nothing to find. Go rep scasd.
shl edx, cl ; Shift it back so bsf will return the right index.
bsf edx, edx ; edx=index of first clear bit
shl eax, 3 ; Turn eax back into a bit offset of the current dword.
add eax, edx ; eax=bit offset
.return:
pop xDI
%if ARCH_BITS == 16
mov edx, eax
shr edx, 16
leave
%endif
ret
;
; Do dword scan.
;
; Skip empty dword.
.next_dword:
add xDI, 4 ; Skip the empty dword.
add eax, 4
.scan_dwords:
; First load and align bit count.
%if ARCH_BITS == 64
%ifdef ASM_CALL64_GCC
mov ecx, esi
%else
mov ecx, r8d
%endif
%elif ARCH_BITS == 32
mov ecx, [esp + 8 + 4]
%elif ARCH_BITS == 16
mov ecx, [bp + 4 + 4]
%endif
add ecx, 31
shr ecx, 5 ; ecx=bitmap size in dwords.
; Adjust ecx to how many dwords there are left to scan. (eax = current byte offset)
shr eax, 2 ; eax=current dword offset.
sub ecx, eax
jbe .return_failure
; Do the scanning.
cld
mov eax, 0ffffffffh
repe scasd
je .return_failure
; Find the bit in question.
sub xDI, 4 ; One step back.
%if ARCH_BITS == 16
movzx edi, di
xor eax, [es:xDI]
%else
xor eax, [xDI]
%endif
bsf eax, eax
jz .return_failure ; race paranoia
; Calc the bit offset.
%if ARCH_BITS == 16
sub di, [bp + 4]
movzx edi, di
%elif ARCH_BITS == 32
sub xDI, [esp + 8]
%elif ARCH_BITS == 64
sub xDI, r9
%endif
shl edi, 3 ; edi=bit offset of current dword.
add eax, edi
jmp .return
.return_failure:
mov eax, 0ffffffffh
jmp .return
ENDPROC ASMBitNextClear
|
