问题
Well, they bring (should bring at least) great increase in performance, isn’t it?
So, I haven’t seen any Linux kernel sources, but ‘d love to ask: are they used somehow? (In this case – there must be some special “code-cap” for system that has no such instructions?)
回答1:
The SSE and MMX instruction sets have limited value outside of audio/video and gaming work. You might find a few explicit uses in dark corners of the kernel, but I wouldn't count on it. The answer in the general case is "no, they are not used", nor are they used in most non-kernel/userspace applications.
The kernel does sometimes optionally use certain x86 instructions that are specific to certain CPUs (e.g. present on some AMD or Intel models but not all, nor vice-versa), such as syscall, but these are different from the SIMD instruction sets you're referring to, and are not part of some wider set of similarly-themed extensions.
After Mark's answer, I went looking. The only place I could easily identify them being used is in the RAID 6 library (which also has support for AltiVec, which is the PowerPC SIMD instruction set).
(Be wary just grepping the tree, there are a lot of spots where the kernel "knows" about SSE/MMX to support user-space applications, but isn't actually using it. Also a couple cases of unfortunate variable names that have absolutely nothing to do with SSE, e.g. in the SCTP implementation.)
回答2:
There are severe restrictions on using vector registers and floating point registers in kernel code. See e.g. chapter 6.3 of "Calling conventions for different C++ compilers and operating systems". http://www.agner.org/optimize/#manuals
回答3:
They are used in the kernel for a few things, such as
- Software RAID
- Encryption (possibly)
However, I believe it always checks their presence first.
回答4:
"cpu simd instructions use FPU"
erm, no, not as I understand it. They're in part a modern and (much) more efficient replacement for FPU instructions, but a large part of the SIMD instruction set deals with integer operations. I've never looked very hard at it, but I suppose (ok, hope) that SIMD code generated by a recent gcc version will not clobber any registers or state.
来源：https://stackoverflow.com/questions/6227603/are-extended-instruction-sets-sse-mmx-used-in-linux-kernel