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Re: [PATCH, i386]: Implement -mcmpxchg16b and -msahf options

On 3/14/07, Jan Hubicka <> wrote:
> Jan Hubicka wrote:
> >>+@item -mcmpxchg16b
> >>+@opindex -mcmpxchg16b
> >>+This option will enable GCC to use CMPXCHG16B instruction in generated
> >>code.

If the patch is not commited yet, perhaps
> +This option will enable GCC to use SAHF instruction in generated code.  Early
> +Intel CPUs with Intel 64 lacked LAHF and SAHF instructions supported by AMD64
> +until introduction of Pentium 4 G1 step in December 2005.  LAHF and SAHF are
> +load and store instructions, respectively, for certain status flags.  These
> +instructions are used for virtualization and floating-point condition handling.

I would leave out the virtualization - GCC won't use it in this context
so it is a bit misleading. It might be nice to make clear that sahf use
is very limited, since fcomi can be used in wast majority cases. But
this is all just nitpicking.

I hope that attached patch clarifies -msahf description.


Attachment: doc-sahf.diff
Description: Binary data

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