From: "Mattias Engdegård" <mattiase@acm.org>
To: Eli Zaretskii <eliz@gnu.org>
Cc: acohen@ust.hk, 54532@debbugs.gnu.org
Subject: bug#54532: [PATCH] sorting
Date: Thu, 24 Mar 2022 10:36:49 +0100 [thread overview]
Message-ID: <63887D60-7631-4583-A76C-B051FE5D9B78@acm.org> (raw)
In-Reply-To: <831qyretr4.fsf@gnu.org>
24 mars 2022 kl. 07.42 skrev Eli Zaretskii <eliz@gnu.org>:
> 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.
next prev parent reply other threads:[~2022-03-24 9:36 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-22 23:59 bug#54532: [PATCH] sorting Andrew Cohen
2022-03-23 12:02 ` Lars Ingebrigtsen
2022-03-23 13:30 ` Eli Zaretskii
2022-03-23 23:43 ` Andrew Cohen
2022-03-23 13:46 ` Eli Zaretskii
2022-03-23 23:31 ` Andrew Cohen
2022-03-23 20:24 ` Mattias Engdegård
2022-03-24 6:42 ` Eli Zaretskii
2022-03-24 7:22 ` Andrew Cohen
2022-03-24 8:55 ` Eli Zaretskii
2022-03-24 9:17 ` Andrew Cohen
2022-03-24 9:55 ` Mattias Engdegård
2022-03-24 9:36 ` Mattias Engdegård [this message]
2022-03-31 12:03 ` Lars Ingebrigtsen
2022-03-31 13:58 ` Eli Zaretskii
2022-03-31 23:47 ` Andrew Cohen
2022-04-01 6:26 ` Eli Zaretskii
2022-06-07 7:06 ` Stefan Kangas
[not found] ` <877d5tgd11.fsf@ust.hk>
2022-06-07 9:07 ` Stefan Kangas
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