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?)
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.)
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
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.
"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