Date: Thu, 30 Nov 2000 17:09:01 +1100 (EST) From: Bruce Evans <bde@zeta.org.au> To: Julian Elischer <julian@elischer.org> Cc: current@FreeBSD.ORG Subject: Re: slight improvement in locore.s? Message-ID: <Pine.BSF.4.21.0011301650450.4214-100000@besplex.bde.org> In-Reply-To: <3A2525DD.3D1E8D5A@elischer.org>
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On Wed, 29 Nov 2000, Julian Elischer wrote: > Bruce Evans wrote: > > > > On Thu, 23 Nov 2000, Julian Elischer wrote: > > > > > locore.s includes: > > > #define ALLOCPAGES(foo) \ > > > movl R(physfree), %esi ; \ > > > movl $((foo)*PAGE_SIZE), %eax ; \ > > > addl %esi, %eax ; \ > > > movl %eax, R(physfree) ; \ > > > movl %esi, %edi ; \ > > > movl $((foo)*PAGE_SIZE),%ecx ; \ > > > xorl %eax,%eax ; \ > > > cld ; \ > > > rep ; \ > > > stosb > > > > > > > > > might it be a very slight optimisation to change this to: > > > #define ALLOCPAGES(foo) \ > > > movl R(physfree), %esi ; \ > > > movl $((foo)*PAGE_SIZE), %eax ; \ > > > movl %eax, %ecx ; \ > > > addl %esi, %eax ; \ > > > movl %eax, R(physfree) ; \ > > > movl %esi, %edi ; \ > > > xorl %eax,%eax ; \ > > > cld ; \ > > > rep ; \ > > > stosb > > > > This can be improved more (3 instructions) by not loading physfree into > > the wrong register, and depending on stosb to increment the register, > > so your assembly code would look like? (just curious). This is completely untested: #define ALLOCPAGES(foo) \ movl R(physfree),%edi ; \ movl $(foo)*PAGE_SIZE,%ecx ; \ movl %edi,%esi; \ xorl %eax,%eax ; \ cld ; \ rep ; \ stosb ; \ movl %edi,R(physfree) I wouldn't write it like this if either space or time efficiency were important. > > but it should be written in C anyway. A relocation macro like R() > > should work just as well in C as in asm. In C, the above is: > > > > bzero(R(physfree), (foo) * PAGE_SIZE); > > R(physfree) += (foo) * PAGE_SIZE); > > return (R(physfree)); /* In unusual as well as wrong reg %esi. */ Oops, this should return the original value of physfree. %esi is used to hold this value, so it is not "wrong". Setting it takes one instruction, so my version is only 2 instructions shorter. Bruce To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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