Re: [patch] require sse3 for fisttp

[Eric Christopher <echristo at apple dot com>]

> We have discussed this a couple of months ago, where we decided that 
> -msse3 should enable fisttp.

I know, I was in this discussion.

> However, looking at your patch, I can't see why it should be better than 
> current implementation. Currently, fisttp is enabled only when -msse3 is 
> selected _or_ -march=nocona is specified. This is IMO the best 
> combination that has some logic in it:
>
> a) user specifies -msse3 and (even if it is counter-intuitive), fisttp 
> is generated. This is added to follow Intel arch manual.
> b) user specifies -march=nocona. As all nocona cores have fisttp, fisttp 
> is generated even without -msse3.
>
> The problem might arise for -march=nocona -mno-sse, where fisttp is 
> still generated. But since user knows that his code will run only on 
> nocona, but he wants to disable sse insn set (IMO this means usage of 
> xmm registers), everything should be OK.

No, this isn't OK if he has a core that is very like nocona, but doesn't 
have the sse3 instruction set. This will cause an invalid instruction error.

> Another problem is 64bit -march=k8 -mfpmath=387, where not all k8 
> processors have sse3 and fisttp. There is nothing we can do, but expect 
> user to provide -msse3 in this case.

Reasonable.

> I can't see the benefit of having fisttp disabled by default on nocona 
> (this is the only target_fisttp target).

Nocona already turns on sse3 by default which would enable fisttp.

-eric