Re: [RFC] x86: Disable traditional FPU instructions too

[H. Peter Anvin]

On 01/27/2014 08:37 AM, Borislav Petkov wrote:
> Hey guys,
> 
> should we do the below? It looks like we don't disable the generation of
> *all* FPU instructions on x86_64 (commit message below has the rationale
> why).
> 
> We do -msoft-float on 32-bit only and Micha says that -msoft-float and
> -mno-80387 are the same and the gcc manpage says:
> 
>            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.
> 
> and right after, it has also
> 
>        -mno-fp-ret-in-387
>            Do not use the FPU registers for return values of functions.
> 
>            The usual calling convention has functions return values of types "float"
>            and "double" in an FPU register, even if there is no FPU.  The idea is that
>            the operating system should emulate an FPU.
> 
>            The option -mno-fp-ret-in-387 causes such values to be returned in ordinary
>            CPU registers instead.
> 
> Btw, there's this -mno-fp-regs switch too which forces passing of FP
> results of functions in integer registers...
> 

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

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

	-hpa


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