[PATCH] Allow CodeGen to take advantage of functions that always return arguments

[Stephen Lin]

Hi,

This patch adds a target-independent parameter attribute 'returned'
(I'm very open to renaming it, just couldn't think of anything
better...), which indicates to the caller that a function's return
value is always equal to (a bitcast of) the parameter value, allowing
tail call optimization and possibly save/restore elision. It is the
callee's responsibility to ensure that this contract is fulfilled.

The initial intended use case is for C++ constructors and destructors
that are ABI-guaranteed to return 'this'; however, I've made it
flexible enough to apply to any parameter position or even multiple
parameters, since there's no real technical reason not to (although
obviously the contract can only be fulfilled in the latter case if
multiple arguments are always equal for some reason...)

For now, the attribute is taken advantage of in two places:

1. SelectionDAGBuilder::LowerCallTo will detect a tail call
opportunity through 'returned' arguments (subject to target-dependent
requirements checked later).

2. The ARM backend, in ARMTargetLowering::LowerCall, will avoid
save/restoring R0 when 'returned' is on the first argument and the
argument's type lowers to MVT::i32, as R0 is used for both the first
argument and the return value in that case.

Eventually, support can be added to the register allocator to take
advantage of the 'returned' guarantee to do better register allocation
on all targets, although apparently this is non-trivial.

Please kindly review.

Stephen

P.S. This patch depends on two previous patches, which have yet to be committed:

    http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20130415/171948.html
(only the first patch)

and

    http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20130415/171945.html
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