XMM in X86 Backend

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1

Hi all,

I am observing an excessive use of xmm registers in the output assembly produced by x86 backend. Basically, for a code like this

double test(double a, double b) {
 double c;

 c = 1.0 + sin (a + b*b);

 return c;
}

llc produced somthing like…

movsd   16(%ebp), %xmm0
       mulsd   %xmm0, %xmm0
       addsd   8(%ebp), %xmm0
       movsd   %xmm0, (%esp)
       .......
       fstpl   -8(%ebp

       movsd   -8(%ebp), %xmm0
       addsd   .LC1, %xmm0
       movsd   %xmm0, -8(%ebp)
       fldl    -8(%ebp)

LLVM Backend is using xmms it involves a lot of register moves. llc has one option -mcpu=686, where output does not use xmm but it disables some other instructions also. Is there any fine grain optimization flag like ( -mfpmath = 387 which is present in gcc) which would just instruction backend to use fpstack instead of SSE instructions and still not limit the instructions

Thanks

–Kapil

.

2

-mattr=-sse

Note that this doesn't change the calling convention; in particular, the only supported calling convention for 64-bit x86 uses SSE, so you'll get an error if you use the above for a function returning double in 64-bit mode.