Floating Point Register Allocation in X86 backend

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1

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
2

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

3

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

4

Yes. My guess it that the code converting from FP to ST registers sometimes needs the extra stack slot.

/jakob

5

In case we want to have an unsafe register allocator that allocated FP0 to FP6 to include FP7 as well, where in the code should we add this?

–Aparna