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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