[llvm-dev] RFC: Disambiguate RegClass->getSize()
Krzysztof Parzyszek via llvm-dev
llvm-dev at lists.llvm.org
Wed Aug 17 08:20:04 PDT 2016
The documentation for function "getSize" in both MCRegisterClass and
/// getSize - Return the size of the register in bytes, which is also
/// of a stack slot allocated to hold a spilled copy of this register.
The problem is that these two values are not always the same. For
example, Hexagon has predicate registers that are 8 bits long and vector
predicate registers that are 64 or 128 bits long, but their spill slots
are of different sizes. None of these registers can be spilled directly,
and they have to be transferred into a spillable register first. These
spillable registers are larger: 32 bits and 512/1024 bits respectively,
and so getSize must return the larger value in order to handle spills
Different users of getSize may be interested in different aspects of the
returned value: register allocator may care about the spill size while
optimizations tracking the predicate values will be interested in the
actual size of the register.
Moreover, if a register for which these two values differ had
subregisters, the SubRegCoveredBits information could not be assumed to
match the return value of getSize.
Replace the getSize function in MCRegisterClass and TargetRegisterClass
with two functions: getRegSize and getSpillSize.
TableGen would need to be modified to generate both of them, and unless
the user specifies otherwise in a .td file, these values would be equal.
All users of getSize would need to be updated to call one of the new
replacement functions. This can be done one step at the time: at first
the old getSize would be present together with the new functions, it
would be removed once all calls to it have been updated.
What does everyone think about it?
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