From mboxrd@z Thu Jan 1 00:00:00 1970 Path: news.gmane.io!.POSTED.blaine.gmane.org!not-for-mail From: Andrea Corallo Newsgroups: gmane.emacs.devel Subject: Re: master b0ba0d42b0f: * src/lisp.h (EQ): Improve generated code. Date: Thu, 28 Nov 2024 10:02:22 -0500 Message-ID: References: <173279015204.1017853.4020802222494409378@vcs3.savannah.gnu.org> <20241128103552.86CC34F4FDB@vcs3.savannah.gnu.org> Mime-Version: 1.0 Content-Type: text/plain Injection-Info: ciao.gmane.io; posting-host="blaine.gmane.org:116.202.254.214"; logging-data="14152"; mail-complaints-to="usenet@ciao.gmane.io" User-Agent: Gnus/5.13 (Gnus v5.13) Cc: Emacs Devel To: Pip Cet Original-X-From: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Thu Nov 28 16:02:56 2024 Return-path: Envelope-to: ged-emacs-devel@m.gmane-mx.org Original-Received: from lists.gnu.org ([209.51.188.17]) by ciao.gmane.io with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1tGg2i-0003WH-FR for ged-emacs-devel@m.gmane-mx.org; Thu, 28 Nov 2024 16:02:56 +0100 Original-Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1tGg2D-0007lL-Rv; Thu, 28 Nov 2024 10:02:25 -0500 Original-Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1tGg2C-0007kx-6R for emacs-devel@gnu.org; Thu, 28 Nov 2024 10:02:24 -0500 Original-Received: from fencepost.gnu.org ([2001:470:142:3::e]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1tGg2B-00056N-Sq; Thu, 28 Nov 2024 10:02:23 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=gnu.org; s=fencepost-gnu-org; h=MIME-Version:Date:References:In-Reply-To:Subject:To: From; bh=3vn0L4e7AxDzvqyqhz+oiAWfzDmDn7NX0SzsbaheOzs=; b=CBIqU2/FF0/EQf3080nO yb2s4W4l9b8KU1tIJnqIV+7Wx1hiHzS5lcBLhV42L3UIDkVhLOdoyW9tTEASxUGiCWhXPNvqqiQbW W02INZl8RKQnzTasgt01Y+D59yHKrk8vjwlr4K/+7zRwAjn1U7gPpYEEtvQQGO9Vw0VWMVVFoQw2p LRK58iiISQzunA71W5rcRZrnuvauxlvLpaJcSopwfLyhcjpOraLFFRIu0R0tsRYtjdEFkr12OifFF Z//8x+bIuuBtO+hgETystP8RrGA9UTuq+Q3d7O1mwZZwYgBMREP1sJtZS6e44yykknAjm+bHeeQjk kU7LBv7vskVtPw==; Original-Received: from acorallo by fencepost.gnu.org with local (Exim 4.90_1) (envelope-from ) id 1tGg2A-0003L5-S2; Thu, 28 Nov 2024 10:02:23 -0500 In-Reply-To: (Pip Cet's message of "Thu, 28 Nov 2024 13:53:40 +0000") X-BeenThere: emacs-devel@gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: "Emacs development discussions." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Original-Sender: emacs-devel-bounces+ged-emacs-devel=m.gmane-mx.org@gnu.org Xref: news.gmane.io gmane.emacs.devel:325821 Archived-At: Pip Cet writes: > On Thursday, November 28th, 2024 at 10:35, Andrea Corallo wrote: > >> branch: master >> commit b0ba0d42b0fdf70a20cd7a070128db8abe4a0826 >> Author: Andrea Corallo acorallo@gnu.org >> >> Commit: Andrea Corallo acorallo@gnu.org >> >> >> * src/lisp.h (EQ): Improve generated code. >> >> Outside compilation 'symbols_with_pos_enabled' is always false, so ask >> the compiler to organize the most likely execution path in a sequential >> fashion in order to favor run-time performance. > > Are we officially using __builtin_expect now? config.h AFAIU defines it to a nop if the compiler does not support it. > I think that's a major change to the way Emacs C code is written, and a decision which might benefit from further discussion. > > To quote the GCC manual: > In general, you should prefer to use actual profile feedback for this (-fprofile-arcs), as programmers are notoriously bad at predicting how their programs actually perform. > > Maybe we should use __builtin_expect_with_probability instead, in > those rare cases when we are certain we're making a correct > prediction? Or, my preference, avoid using __builtin_expect entirely, > so our scarce resources can be spent on more important issues? > > I also don't think the assumption you're telling GCC to make in this specific case (more than 90% of calls to EQ happen while syms_with_pos_enabled == false) is obviously correct. I think it is correct when we are not compiling, and as mentioned in the commit msg that's the case the patch is optimizing for. > IIUC, the current master branch won't even compile with a compiler that doesn't handle (or ignore) __builtin_expect. See my first point. Andrea