Re: Release novops attribute for external use?

[Richard Guenther <richard dot guenther at gmail dot com>]

On Tue, Apr 13, 2010 at 12:15 PM, Bingfeng Mei <bmei@broadcom.com> wrote:
>>
>> Surely printf writes to global memory (it clobbers the stdout FILE*)
>>
> OK, the point is not about whether printf is pure or not. Instead, if
> programmer knows the callee function such as printf contains no
> memory access that affects operations inside caller function, and he
> would like to have a way to optimize the code. Our engineer gave following
> example:
>
> ? ?void myfunc(MyStruct *myStruct)
> ? ?{
> ? ? ?int a,b;
> ? ? ?a = myStruct->a;
> ? ? ?printf("a=%d\n",a);
> ? ? ?b = 2*mystruct->a; ? ? ?// I would like to have the compiler acting as if I had written b = 2*a;
> ? ? ...
> ? ?}
> Providing such attribute may be potentially dangerous. But it is just
> like "restrict" qualifier and some other attributes, putting responsibilty
> of correctness on the programmer. "novops" seems to achieve that effect,
> though its semantics doesn't match exactly what I described.

Indeed.  IPA pointer analysis will probably figure it out
automagically - that *myStruct didn't escape the unit.
Being able to annotate incoming pointers this way would
maybe be useful.

>> As for the original question - novops is internal only because its
>> semantics is purely internal and changes with internal aliasing
>> changes.
>>
>> Now, we still lack a compelling example to see what exact semantics
>> you are requesting? ?I suppose it might be close to a pure but
>> volatile function? ?Which you could simulate by
>>
>> dummy = pure_fn ();
>> asm ("" : "g" (dummy));
>>
>> or even
>>
>> volatile int dummy = pure_fn ();
>
> These two methods still generate extra code to reload variables

The latter works for me (ok, the store to dummy is retained):

extern int myprintf(int) __attribute__((pure));
int myfunc (int *p)
{
  int a;
  a = *p;
  volatile int dummy = myprintf(a);
  return a + *p;
}

myfunc:
.LFB0:
        pushq   %rbx
.LCFI0:
        subq    $16, %rsp
.LCFI1:
        movl    (%rdi), %ebx
        movl    %ebx, %edi
        call    myprintf
        movl    %eax, 12(%rsp)
        leal    (%rbx,%rbx), %eax
        addq    $16, %rsp
.LCFI2:
        popq    %rbx
.LCFI3:
        ret

so we load from %rdi only once.

Richard.

> Bingfeng
>
>
>>
>> Richard.
>>
>> > Bingfeng
>> >
>> >> -----Original Message-----
>> >> From: gcc-owner@gcc.gnu.org [mailto:gcc-owner@gcc.gnu.org] On
>> >> Behalf Of Andrew Haley
>> >> Sent: 12 April 2010 17:34
>> >> To: gcc@gcc.gnu.org
>> >> Subject: Re: Release novops attribute for external use?
>> >>
>> >> On 04/12/2010 05:27 PM, Bingfeng Mei wrote:
>> >> > Hello,
>> >> > One of our engineers requested a feature so that
>> >> > compiler can avoid to re-load variables after a function
>> >> > call if it is known not to write to memory. It should
>> >> > slash considerable code size in our applications. I found
>> >> > the existing "pure" and "const" cannot meet his requirements
>> >> > because the function is optimized out if it doesn't return
>> >> > a value.
>> >>
>> >> If a function doesn't write to memory and it doesn't return a
>> >> value, what is the point of calling it?
>> >>
>> >> Andrew.
>> >>
>> >>
>> >
>>
>>
>