1.7 introduced a significant regression compared to 1.6:

SqrtIndirect-4  2.32ns ± 0%  7.86ns ± 0%  +238.79%        (p=0.000 n=20+18)

This is caused by sqrtsd preserving upper part of destination register.
Which introduces dependency on previous  value of X0.
In 1.6 benchmark loop didn't use X0 immediately after call:

callq  *%rbx
movsd  0x8(%rsp),%xmm2
movsd  0x20(%rsp),%xmm1
addsd  %xmm2,%xmm1
mov    0x18(%rsp),%rax
inc    %rax
jmp    loop

In 1.7 however xmm0 is used just after call:

callq  *%rbx
mov    0x10(%rsp),%rcx
lea    0x1(%rcx),%rax
movsd  0x8(%rsp),%xmm0
movsd  0x18(%rsp),%xmm1

I've  verified that this is caused by dependency, by inserting
XORPS X0,X0 in the beginning of math.Sqrt, which puts performance back on 1.6 level.

Splitting SQRTSD mem,reg into:
MOVSD mem,reg
SQRTSD reg,reg

Removes dependency, because MOVSD (load version)
doesn't need to preserve upper part of a register.
And reg,reg operation is solved by renamer in CPU.

As a result of this change regression is gone:
SqrtIndirect-4  7.86ns ± 0%  2.33ns ± 0%  -70.36%  (p=0.000 n=18+17)

This also removes old Sqrt benchmarks, in favor of benchmarks measuring latency.
Only SqrtIndirect is kept, to show impact of this patch.

Change-Id: Ic7eebe8866445adff5bc38192fa8d64c9a6b8872
Reviewed-on: https://go-review.googlesource.com/28392
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Keith Randall <khr@golang.org>