llvm dev - May 2011 - [LLVMdev] Floating Point Register Allocation in X86 backend

Hi Guys,

I was working on some floating point intensive benchmarks and realize that
the floating point register allocation in llvm assumes that there are only 7
floating point registers in X86, whereas the hardware has 8.

Line number

00266   assert(Reg >= X86::FP0 && Reg <= X86::FP6 &&
"Expected FP register!");


of X86FloatingPoint.cpp.

Is there any reason for only counting from 0 to 6, when there are actually
8  in hardware ?

Is there an assumption somewhere else, that I am missing.



Thanks and Regards

Aparna Kotha

Graduate Student
University of Maryland, College Park
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On May 25, 2011, at 11:09 AM, aparna kotha wrote:
> Hi Guys, 
> 
> I was working on some floating point intensive benchmarks and realize that
the floating point register allocation in llvm assumes that there are only 7
floating point registers in X86, whereas the hardware has 8.
> 
> Line number 
> 00266   assert(Reg >= X86::FP0 && Reg <= X86::FP6 &&
"Expected FP register!");
> 
> of X86FloatingPoint.cpp. 
> 
> Is there any reason for only counting from 0 to 6, when there are actually
8  in hardware ?
It has to do with the weird tricks that are needed to generate code for a stack
machine.
> Is there an assumption somewhere else, that I am missing. 
Yes, the default cpu on Linux is i386 which doesn't have SSE support.

Use SSE if you care about floating point performance. I think -mcpu=... is all
you need.

/jakob

Right. But there are 8 registers on the floating point stack from ST0 to ST7
and I think llvm is only using ST0 to ST6 in some code fragments. Could this
be because of the assumption that X86::FP registers run from X86::FP0 to
X86:FP6 ?




--Aparna

On Wed, May 25, 2011 at 2:28 PM, Jakob Stoklund Olesen <stoklund at
2pi.dk>wrote:
>
> On May 25, 2011, at 11:09 AM, aparna kotha wrote:
>
> > Hi Guys,
> >
> > I was working on some floating point intensive benchmarks and realize
> that the floating point register allocation in llvm assumes that there are
> only 7 floating point registers in X86, whereas the hardware has 8.
> >
> > Line number
> > 00266   assert(Reg >= X86::FP0 && Reg <= X86::FP6
&& "Expected FP
> register!");
> >
> > of X86FloatingPoint.cpp.
> >
> > Is there any reason for only counting from 0 to 6, when there are
> actually 8  in hardware ?
>
> It has to do with the weird tricks that are needed to generate code for a
> stack machine.
>
> > Is there an assumption somewhere else, that I am missing.
>
> Yes, the default cpu on Linux is i386 which doesn't have SSE support.
>
> Use SSE if you care about floating point performance. I think -mcpu=... is
> all you need.
>
> /jakob
>
>
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