linux-kernel - Re: [RFC] x86: Disable traditional FPU instructions too

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Message-ID: <20140127164544.GE6839@pd.tnic>
Date:	Mon, 27 Jan 2014 17:45:44 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jiri Kosina <jkosina@...e.cz>, Michael Matz <matz@...e.de>,
	Michal Hocko <mhocko@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] x86: Disable traditional FPU instructions too

On Mon, Jan 27, 2014 at 08:41:09AM -0800, H. Peter Anvin wrote:
> I don't think it'd hurt... although I think the above pretty much
> requires that the code contain actual floating-point types to ever be
> generated.

Yes, that is supposed to catch usage of the 'E' floating constant, for
example, in definitions. It is a build-time assertion, if you want. :)

> The issue with MMX/SSE is that an autovectorizing compiler could
> decide to use them for *integer* code.

Right, and we don't want the FPU state handling if it can be helped.

> -mno-fp-ret-in-387 in particular will only ever apply if a function
> return type is a floating-point type.

Yeah, this is for when using -msoft-float == -mno-80387 which still can
generate FPU insns according to the manpage:

           On machines where a function returns floating-point results in the 80387
           register stack, some floating-point opcodes may be emitted even if
           -msoft-float is used.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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