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Re: [PATCH] Add malloc predictor (PR middle-end/83023).


On Wed, 1 Aug 2018, Martin Liška wrote:

On 08/01/2018 02:25 PM, Marc Glisse wrote:
On Wed, 1 Aug 2018, Martin Liška wrote:

On 07/27/2018 02:38 PM, Marc Glisse wrote:
On Fri, 27 Jul 2018, Martin Liška wrote:

So answer is yes, the builtin can be then removed.

Good, thanks. While looking at how widely it is going to apply, I noticed that the default, throwing operator new has attribute malloc and everything, but the non-throwing variant declared in <new> doesn't, so it won't benefit from the new predictor. I don't know if there is a good reason for this disparity...


Well in case somebody uses operator new:

    int* p1 = new int;
    if (p1)
     delete p1;

we optimize out that to if (true), even when one has used defined
operator new. Thus it's probably OK.

Throwing new is returns_nonnull (errors are reported with exceptions) so that's fine, but non-throwing new is not:

int* p1 = new(std::nothrow) int;

Here errors are reported by returning 0, so it is common to test if p1 is 0 and this is precisely the case that could benefit from a predictor but does not have the attribute to do so (there are also consequences on aliasing).

Then it can be handled with DECL_IS_OPERATOR_NEW, for those we can also set the newly introduced predictor.

Independently of whether you extend the predictor to DECL_IS_OPERATOR_NEW, it would be good for this nothrow operator new to get the aliasing benefits of attribute malloc. I'll open a PR.

(Jan's remark about functions with an inferred malloc attribute reminds me that at some point, the code was adding attribute malloc for functions that always return 0...)

By inferred do you mean function that are marked as malloc in IPA pure-const (propagate_malloc)?

Yes.

Example would be appreciated.

I used the past tense, I am not claiming this still happens.

--
Marc Glisse


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