Michael Gottesman afe402ff86 [silgen] Begin splitting emitRValue into two different APIs: SILGenFunction::emitPlus{One,Zero}RValue(...). ...

Today, SILGenFunction::emitRValue assumes the caller will create any cleanup
scopes that are needed to cleanup side-effects relating to the rvalue
evaluation.  The API also provides the ability for the caller to specify that a
+0 rvalues is an "ok" result. The API then tries to produce a +0 rvalue and
returns a +1 rvalue otherwise. These two properties create conflicting
requirements on the caller since the caller does not know whether or not it
should create a scope (if a +1 rvalue will be returned) or not (if a +0 rvalue
would be returned).

The key issue here is the optionality of returning a +0 rvalue. This change
begins to resolve this difference by creating two separate APIs that guarantee
to the caller whether or not a +0 or a +1 rvalue is returned and also creates
local scopes for the caller as appropriate. So by using these APIs, the caller
knows that the +0 or +1 rvalue that is returned has properly been put into the
caller scope. So the caller no longer needs to create its own scopes anymore.

emitPlusOneRValue is emitRValue except that it scopes the rvalue emission and
then *pushes* the produced rvalue through the scope. emitPlusZeroRValue is
currently a stub implementation that just calls emitPlusOneRValue and then
borrows the resulting +1 RValue in the outer scope, creating the +0 RValue that
was requested by the caller.

rdar://33358110

2017-08-14 13:24:43 -07:00

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