| commit | 6e703ae7093b8921ce8e64a08e600d94ea1f9f28 | [log] [tgz] |
|---|---|---|
| author | Ilya Tocar <ilya.tocar@intel.com> | Thu Sep 01 21:04:08 2016 +0300 |
| committer | Ilya Tocar <ilya.tocar@intel.com> | Tue Sep 06 15:45:02 2016 +0000 |
| tree | b650c318e4e0a327b49ea2a24f7816acd5421ff0 | |
| parent | 6bcca5e91836cae42fc9a88c4ac9a6a5ffed67f6 [diff] |
math: fix sqrt regression on AMD64 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>
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