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
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