linux-kernel - Re: "m" constraints, jumps, and alternatives

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Message-ID: <5306CB60.3070302@zytor.com>
Date:	Thu, 20 Feb 2014 19:43:28 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	"H.J. Lu" <hjl.tools@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: "m" constraints, jumps, and alternatives

On 02/20/2014 07:30 PM, Brian Gerst wrote:
> 
> The percpu code uses %P to force absolute addressing mode, instead of
> rip-relative.
> 

   P -- if PIC, print an @PLT suffix.

Clearly it has more effects than that... as the code does reveal:

      /* Avoid (%rip) for call operands.  */
      if (CONSTANT_ADDRESS_P (x) && code == 'P'
          && !CONST_INT_P (x))

That should work... in the probably extremely rare case that any of
these references go to global addresses then they spend an extra byte
(and a little more work during kASLR relocation or module insertion).

	-hpa

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