[LLVMdev] Best way to interface with MSVC _ftol2 runtime function for fptoui?

[Jakob Stoklund Olesen]

On Jan 24, 2012, at 2:30 PM, Joe Groff wrote:

> On Fri, Jan 20, 2012 at 2:10 PM, Jakob Stoklund Olesen <stoklund at 2pi.dk> wrote:
>> X86FloatingPoint.cpp with comments is all you get.
> 
> Thanks for your help, Jakob. Attached is a first-pass attempt at a
> patch. I don't want to post to -commits yet because I have no idea if
> this is fully correct, but it seems to work in simple test cases. Am I
> on the right track?

Yes, your definition of the new instruction looks sane.

However, you shouldn't expand the instruction right away in EmitInstrWithCustomInserter(), and leaving the pseudo and call instructions side by side is not going to work.

Just leave the pseudo-instruction alone until it hits X86FloatingPoint, where you can rewrite it.

> Could this patch ever break in cases where the
> operand's vreg doesn't happen to get mapped to ST0?

Yes, exactly. You need to make some more complicated test cases.

> I'm still a bit
> foggy on the internals of X86FloatingPoint.

Look at the code handling INLINE_ASM. You need to do the same, except you have fixed arguments STUses=1 and STClobbers=1, ST*=0. That should greatly simplify the code you need.

> One thing I noticed is that fptosi and fptoui both seem to always emit
> a redundant SSE load/store when SSE is enabled, because of the check
> at Target/X86/X86ISelLowering.cpp:7948. Can this check be easily
> modified so it doesn't store if the operand is already in memory and
> not actually in an SSE register? Should FP_TO_INTHelper switch over to
> using CVTTS?2SI insns when SSE is available?

When SSE is available, x87 registers are only ever used for f80.

/jakob