[llvm-dev] Purpose of various register classes in X86 target

[Arvind via llvm-dev]

Hello everyone,
I noticed that there are several register classes defined in X86 target and
many of them are overlapping. Is there a list of all X86 register classes
documented somewhere? I found many listed in
X86GenRegisterInfo.inc(generated by tablegen) but unsure if that is the
complete list. Also, is there documentation on the role and purpose of
these classes and how the X86 backend decides which class to choose when
generating machine code? The comments in X86RegisterInfo.td didn't help
much to fully understand and I wasn't sure where to start reading the
RegisterAllocator source code to understand how it uses the register
classes.
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