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Date:   Sun, 13 Mar 2022 04:34:33 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Max Kellermann <max.kellermann@gmail.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/4] pipe_fs_i.h: add pipe_buf_init()
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On Sun, Mar 13, 2022 at 02:48:10AM +0000, Matthew Wilcox wrote:

> That's not equivalent.  I think the better option here is to always
> initialise flags to 0 (and not have a parameter for it):
> 
> 			pipe_buf_init(buf, page, 0, 0, &anon_pipe_buf_ops);
> 			if (is_packetized(filp))
> 				buf->flags = PIPE_BUF_FLAG_PACKET;
> 			else
> 				buf->flags = PIPE_BUF_FLAG_CAN_MERGE;

Not equivalent in which sense?  IDGI...  Your variant is basically

	X = 0;
	if (Y == constant)
		X = 1;
	else
		X = 2;

If gcc can optimize that to 

	X = (Y == constant) ? 1 : 2;

it should be able to do the same to
	X = 1;
	if (Y != constant)
		X = 2;

	What obstacles are there, besides a (false) assumption that
X might alias Y?  Which would apply to both variants...  Granted, I'm
half-asleep right now, so I might be missing something obvious...