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Re: [patch, fortran] Load scalar intent-in variables at the beginning of procedures
On Wed, Nov 20, 2019 at 11:35 PM Thomas König <tk@tkoenig.net> wrote:
>
> Am 20.11.19 um 21:45 schrieb Janne Blomqvist:
> > BTW, since this is done for the purpose of optimization, have you done
> > testing on some suitable benchmark suite such as polyhedron, whether
> > it a) generates any different code b) does it make it go faster?
>
> I haven't run any actual benchmarks.
>
> However, there is a simple example which shows its advantages.
> Consider
>
> subroutine foo(n,m)
> m = 0
> do 100 i=1,100
> call bar
> m = m + n
> 100 continue
> end
>
> (I used old-style DO loops just because :-)
>
> Without the optimization, the inner loop is translated to
>
> .L2:
> xorl %eax, %eax
> call bar_
> movl (%r12), %eax
> addl %eax, 0(%rbp)
> subl $1, %ebx
> jne .L2
>
> and with the optimization to
>
> .L2:
> xorl %eax, %eax
> call bar_
> addl %r12d, 0(%rbp)
> subl $1, %ebx
> jne .L2
>
> so the load of the address is missing. (Why do we zero %eax
> before each call? It should not be a variadic call right?)
Not sure. Maybe some belt and suspenders thing? I guess someone better
versed in ABI minutiae knows better. It's not Fortran-specific though,
the C frontend does the same when calling a void function.
AFAIK on reasonably current OoO CPU's xor'ing a register with itself
is handled by the renamer and doesn't consume an execute slot, so it's
in effect a zero-cycle instruction. Still bloats the code slightly,
though.
> Of course, Fortran language rules specify that the call to bar
> cannot do anything to n
Hmm, does it? What about the following modification to your testcase:
module nmod
integer :: n
end module nmod
subroutine foo(n,m)
m = 0
do 100 i=1,100
call bar
m = m + n
100 continue
end subroutine foo
subroutine bar()
use nmod
n = 0
end subroutine bar
program main
use nmod
implicit none
integer :: m
n = 1
m = 0
call foo(n, m)
print *, m
end program main
> So, a copy in / copy out for variables where we can not be sure that
> no value is assigned? Does anybody see a downside for that?)
In principle sounds good, unless my concerns above are real and affect
this case too.
> > Is there a risk of performance regressions due to higher register pressure?
>
> I don't think so. Either the compiler realizes that it can
> keep the variable in a register (then it makes no difference),
> or it has to load it fresh from its address (then there is
> one additional register needed).
Yes, true. Good point.
--
Janne Blomqvist