[LLVMdev] Floating Point Register Allocation in X86 backend

[Jakob Stoklund Olesen]

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