[Info-vax] Anyone still around ? :-)

[Louis Krupp]

On Thu, 10 Oct 2019 00:40:35 +0200, hb <end.of at inter.net> wrote:

>On 10/9/19 11:55 PM, Louis Krupp wrote:
>> On Tue, 8 Oct 2019 07:19:39 -0700 (PDT), osuvman50 at gmail.com wrote:
>> 
>>> I was considering making a post about the failure of the C compiler's optimizer
>>> to recognize and optimize the pattern:
>>>
>>>     choice = permutation % (CARDS_IN_DECK-i);
>>>     permutation = permutation / (CARDS_IN_DECK-i);
>>>
>>> but didn't think I'd get any constructive responses. (refactoring to eliminate a
>>> divide makes the whole program run 18% faster on both alpha and IA64).
>> 
>> For what it's worth, GNU C++ on Linux doesn't optimize it either, but
>> GNU Fortran does at optimization level 1. If you have a FORTRAN
>> compiler, you'll probably have to tweak this code a bit (getting rid
>> of the colons, for example) to try it:
>> 
>> subroutine s(permutation, choice, i)
>>     implicit none
>>     integer :: permutation, choice, i
>>     integer, parameter :: CARDS_IN_DECK = 52
>>     choice = mod(permutation, (CARDS_IN_DECK-i))
>>     permutation = permutation / (CARDS_IN_DECK-i)
>> end subroutine
>> 
>> Louis
>> 
>
>What kind of optimization do you expect?
>
>#define CARDS_IN_DECK 52
>extern int permutation,  choice;
>void foo (int i) {
>        choice = permutation % (CARDS_IN_DECK-i);
>        permutation = permutation / (CARDS_IN_DECK-i);
>}
>$
>$ gcc -m64 -S -O3 foo.c
>$ cat foo.s
>        .file   "foo.c"
>        .text
>        .p2align 4,,15
>        .globl  foo
>        .type   foo, @function
>foo:
>.LFB0:
>        .cfi_startproc
>        movl    permutation(%rip), %eax
>        movl    $52, %ecx
>        subl    %edi, %ecx
>        cltd
>        idivl   %ecx
>        movl    %edx, choice(%rip)
>        movl    %eax, permutation(%rip)
>        ret
>        .cfi_endproc
>.LFE0:
>        .size   foo, .-foo
>        .ident  "GCC: (Debian 8.3.0-6) 8.3.0"
>        .section        .note.GNU-stack,"", at progbits
>$ g++ -m64 -S -O3 foo.c
>$ cat foo.s
>        .file   "foo.c"
>        .text
>        .p2align 4,,15
>        .globl  _Z3fooi
>        .type   _Z3fooi, @function
>_Z3fooi:
>.LFB0:
>        .cfi_startproc
>        movl    permutation(%rip), %eax
>        movl    $52, %ecx
>        subl    %edi, %ecx
>        cltd
>        idivl   %ecx
>        movl    %edx, choice(%rip)
>        movl    %eax, permutation(%rip)
>        ret
>        .cfi_endproc
>.LFE0:
>        .size   _Z3fooi, .-_Z3fooi
>        .ident  "GCC: (Debian 8.3.0-6) 8.3.0"
>        .section        .note.GNU-stack,"", at progbits
>$

I expected what you got. My test case, which I should have posted, was
a bit different:

const int CARDS_IN_DECK = 52;

void s(int& permutation, int& choice, int i)
{
    choice = permutation % (CARDS_IN_DECK-i);
    permutation = permutation / (CARDS_IN_DECK-i);
}

and the results were:

    movl    (%rdi), %eax
    movl    $52, %ecx
    subl    %edx, %ecx
    cltd
    idivl   %ecx
    movl    %edx, (%rsi)
    movl    (%rdi), %eax
    cltd
    idivl   %ecx
    movl    %eax, (%rdi)
    ret

Changing permutation and choice to externals make all the difference,
for whatever reason...

Louis