linux-kernel - RE: Unsigned widening casts of binary "not" operations..

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Message-ID: <AE90C24D6B3A694183C094C60CF0A2F6026B71F2@saturn3.aculab.com>
Date:	Tue, 23 Apr 2013 16:24:33 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Ingo Molnar" <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"David Miller" <davem@...emloft.net>,
	"Theodore Ts'o" <tytso@....edu>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	"Network Development" <netdev@...r.kernel.org>,
	<linux-ext4@...r.kernel.org>
Subject: RE: Unsigned widening casts of binary "not" operations..

> > If the narrower type is signed it is probably even more confusing!
> > The high bits will be preserved unless you are masking off bit 31.
> 
> Yes. However, that case doesn't trigger with the normal case of small
> values. So "~4" works fine with widening, in a way that "~4u" does
> not.

Thinks ... converting:
	foo &= ~bar;
to:
	foo = ~(~foo | bar);
would generally DTRT.
Whether the compiler has the relevant patterns to optimise
it is another question.

	David