APIs for terminator results that forward ownership

[atrick]

Add SILValue::getDefiningInstructionOrTerminator().

This allows code to handle terminator results similar to any other
instruction result. Data flow should generally handle terminator
results like any other instruction that may forward operand ownership
to its results. The fact that it is represented as a block argument is
an implementation detail that gets in the way of conceptual
simplicity.

Add APIs for terminator results that forward ownership.

Add TermInst::forwardedOperand.

Add SILArgument::forwardedTerminatorResultOperand. This API will be
moved into a proper TerminatorResult abstraction.

Remove getSingleTerminatorOperand, which could be misused because it's
not necessarilly forwarding ownership.

Remove the isTransformationTerminator API, which is not useful or well
defined.

Rewrite several instances of complex logic to handle block arguments
with the simple terminator result API. This defines away potential
bugs where we don't detect casts that perform implicit conversion.

Replace uses of the SILPhiArgument type and code that explicitly
handle block arguments. Control flow is irrelevant in these
situations. SILPhiArgument needs to be deleted ASAP. Instead, use
simple APIs like SILArgument::isTerminatorResult(). Eventually this
will be replaced by a TerminatorResult type.

[atrick]

@swift-ci test

[meg-gupta]

I see PhiOperand constructor still works with branch operands. Any reason this had to change ?

[atrick]

The problem with these abstractions is that we still don't have all the necessary SIL restrictions. In particular, OSSA still supports CondBr arguments for trivial types. We deferred doing this work because OSSA utilities generally ignores trivial values anyway. Until we fix this, \p PhiOperand might be invalid for trivial phis.

I started to add support for CondBr arguments everywhere, but reconsidered. I just changed the getBranch method back to returning BranchInst as before.

[meg-gupta]

Same here, should the PhiOperand constructor be updated to handle cond_br in non-ossa?

[atrick]

For now, I fixed PhiOperand to be invalid for CondBr. We'll either need need prohibit CondBr arguments or change the representation of this abstraction. Prohibiting CondBr arguments would be far simpler in the long term.

[meg-gupta]

For the same reason as this, shouldn't we return true in the case there is only a single predecessor block below ? https://github.com/apple/swift/blob/bf84d8dfe0df272b32c9a9c17d9d47c4f844440c/lib/SIL/IR/SILArgument.cpp#L92

[atrick]

When there is a single predecessor, we check for terminator results using
isa<BranchInst>(termInst) || isa<CondBranchInst>(termInst)

[atrick]

@swift-ci smoke test

[meg-gupta]

Thanks. LGTM

[nate-chandler]

nit: pre condition(?)

[atrick]

Well, the post-condition only holds if the a forwarding operand is returned. I can clarify that in the next PR