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Re: [patch tree-optimization]: Improve handling of conditional-branches on targets with high branch costs
2011/10/26 Michael Matz <matz@suse.de>:
> Hi,
>
> On Wed, 26 Oct 2011, Kai Tietz wrote:
>
>> >> > int f(char *i, int j)
>> >> > {
>> >> > ? ? ? ?if (*i && j!=2)
>> >> > ? ? ? ? ? ? ? ?return *i;
>> >> > ? ? ? ?else
>> >> > ? ? ? ? ? ? ? ?return j;
>> >> > }
>> >>
>>
>> the case can be produced quite easily.
>>
>> extern int global = 0;
>>
>> ....
>> ? if (*a && global) ...
>
> See? ?You had to change the program to prove the transformation to be
> invalid. ?But my point was that the function we discuss about was exactly
> as above. ?It didn't have globals, or two loads, or a volatile, or
> anything else you can come up with where the transformation would be
> detectable (and hence invalid). ?I claim that you can't construct a
> program that can distinguish between this function:
>
> int f(char *i, int j)
> {
> ??if (*i && j!=2)
> ?? ?return *i;
> ??else
> ?? ?return j;
> }
>
> and this one:
>
> int f(char *i, int j)
> {
> ??if (*i & j!=2)
> ?? ?return *i;
> ??else
> ?? ?return j;
> }
>
> And if you can't construct such a program, then the initial transformation
> before the fold-const.c change _for this specific situation_ was correct.
>
>
> Ciao,
> Michael.
well, if such a function is used as inline and we know for it that j
has value != 2, then we have here a big difference. For your first
example, we still have to do the memory access to *i, even if we are
not interested in result. See here point 4 of 5.1.2.3 of C-spec.
For your second sample we don't need to do that, as the & itself is no
sequence-point and so we can eliminate the *i access without breaking
anything.
Regards,
Kai