Subject: Re: bug#54532: [PATCH] sorting

From: Mattias Engdegård <[email-masked]>
Date: Thu, 24 Mar 2022 10:36:49 +0100

24 mars 2022 kl. 07.42 skrev Eli Zaretskii <[email-masked]>:

> That instance was
> at the end of the function, right before it returns, and I wonder what
> kind of optimization opportunities that could present.

I don't think we need to justify every single `eassume` on the concrete utility for a compiler; in general, the more information we give it, the better code it can produce. It just doesn't hurt to do so.

In fact, the only reason we have `eassert` at all is for assertions that may be time-consuming or otherwise affect the execution (that is, expressions that the compiler just can't optimise away). For anything else, `eassume` is strictly better since it does all that `eassert` does, but with the extra optimisation hints.

Now in this concrete case, we state that `lastofs` and `ofs` are equal at the point when we are about to return `ofs`, and that gives the compiler the option to return `lastofs` instead, should that be more convenient in some way.

The compiler also knows that lastofs >= ofs because of the loop condition, which means that it can deduce that lastofs > ofs can never occur which can have various uses -- for example, in the statement

      ptrdiff_t m = lastofs + ((ofs - lastofs) >> 1);

it would know that the argument being shifted is nonnegative, which might be useful in instruction selection. And so on.