NCG lowering of sqrt

[Kavon Farvardin]

Given a Cmm expression such as

    (_c8Gq::F64) = call MO_F64_Sqrt(_s8oX::F64);   // CmmUnsafeForeignCall

the native code generator produces an actual call to the sqrt C function, which has the side-effect of causing all floating-point registers to be dumped as they are caller-saved. In the nbody benchmark, this is particularly bad for a rather hot piece of code (see below).

Ideally the NCG would recognize this foreign call and instead use the `sqrtsd` SSE instruction when targeting x86-64.  

Does anyone know if the NCG can produce this instruction? I think it would be beneficial, as the below would turn into one or two instructions.

Other math functions such as sin/cos require x87 FPU instructions, which as far as I know we're not using.


;;;;;;;;;;;
; NCG generates this in parts of the nbody benchmark
; to compute the sqrt
;
    subq $8,%rsp
    movsd %xmm9,176(%rsp)  ; all floating-point registers
    movsd %xmm1,184(%rsp)  ; are caller-saved in SysV ABI
    movsd %xmm2,192(%rsp)
    movsd %xmm3,200(%rsp)
    movq %rdi,208(%rsp) 
    movq %rcx,216(%rsp)
    movq %rsi,224(%rsp)
    movsd %xmm4,232(%rsp)
    movsd %xmm5,240(%rsp)
    movsd %xmm6,248(%rsp)
    movsd %xmm7,256(%rsp)
    movsd %xmm8,264(%rsp)
    movsd %xmm11,272(%rsp)
    call _sqrt
    ;; the loads
    ;; below are interleaved 
    ;; with computations 
    addq $8,%rsp
    movsd 264(%rsp),%xmm1
    movsd 240(%rsp),%xmm2
    movsd 224(%rsp),%xmm2
    movsd 232(%rsp),%xmm4
    movq 200(%rsp),%rax
    movsd 248(%rsp),%xmm4
    movsd 256(%rsp),%xmm4
    movq 216(%rsp),%rcx
    movsd 192(%rsp),%xmm2


~kavon