MaterializeTemporaryExpr captures a reference binding to a temporary
value, making explicit that the temporary value (a prvalue) needs to
be materialized into memory so that its address can be used. The
intended AST invariant here is that a reference will always bind to a
glvalue, and MaterializeTemporaryExpr will be used to convert prvalues
into glvalues for that binding to happen. For example, given
const int& r = 1.0;
The initializer of "r" will be a MaterializeTemporaryExpr whose
subexpression is an implicit conversion from the double literal "1.0"
to an integer value.
IR generation benefits most from this new node, since it was
previously guessing (badly) when to materialize temporaries for the
purposes of reference binding. There are likely more refactoring and
cleanups we could perform there, but the introduction of
MaterializeTemporaryExpr fixes PR9565, a case where IR generation
would effectively bind a const reference directly to a bitfield in a
struct. Addresses <rdar://problem/9552231>.
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temporaries. There are actually several interrelated fixes here:
- When converting an object to a base class, it's only an lvalue
cast when the original object was an lvalue and we aren't casting
pointer-to-derived to pointer-to-base. Previously, we were
misclassifying derived-to-base casts of class rvalues as lvalues,
causing various oddities (including problems with reference binding
not extending the lifetimes of some temporaries).
- Teach the code for emitting a reference binding how to look
through no-op casts and parentheses directly, since
Expr::IgnoreParenNoOpCasts is just plain wrong for this. Also, make
sure that we properly look through multiple levels of indirection
from the temporary object, but destroy the actual temporary object;
this fixes the reference-binding issue mentioned above.
- Teach Objective-C message sends to bind the result as a temporary
when needed. This is actually John's change, but it triggered the
reference-binding problem above, so it's included here. Now John
can actually test his return-slot improvements.
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subobject. Previously, we could only properly bind to a base class
subobject while extending the lifetime of the complete object (of a
derived type); for non-static data member subobjects, we could memcpy
(!) the result and bind to that, which is rather broken.
Now, we pull apart the expression that we're binding to, to figure out
which subobject we're accessing, then construct the temporary object
(adding a destruction if needed) and, finally, dig out the subobject
we actually meant to access.
This fixes yet another instance where we were memcpy'ing rather than
doing the right thing. However, note the FIXME in references.cpp:
there's more work to be done for binding to subobjects, since the AST
is incorrectly modeling some member accesses in base classes as
lvalues when they are really rvalues.
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initializer, don't fold paramters. Their initializers are just default
arguments which can be overridden. This fixes some spectacular regressions due
to more things making it into the constant folding.
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EmitReferenceBindingToExpr() rather than assuming we have an
lvalue. This is just the lowest hanging fruit for PR6024, which still
requires a bit of work.
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- This is designed to make it obvious that %clang_cc1 is a "test variable"
which is substituted. It is '%clang_cc1' instead of '%clang -cc1' because it
can be useful to redefine what gets run as 'clang -cc1' (for example, to set
a default target).
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new notion of an "initialization sequence", which encapsulates the
computation of the initialization sequence along with diagnostic
information and the capability to turn the computed sequence into an
expression. At present, I've only switched one CheckReferenceInit
callers over to this new mechanism; more will follow.
Aside from (hopefully) being much more true to the standard, the
diagnostics provided by this reference-initialization code are a bit
better than before. Some examples:
p5-var.cpp:54:12: error: non-const lvalue reference to type 'struct
Derived'
cannot bind to a value of unrelated type 'struct Base'
Derived &dr2 = b; // expected-error{{non-const lvalue reference to
...
^ ~
p5-var.cpp:55:9: error: binding of reference to type 'struct Base' to
a value of
type 'struct Base const' drops qualifiers
Base &br3 = bc; // expected-error{{drops qualifiers}}
^ ~~
p5-var.cpp:57:15: error: ambiguous conversion from derived class
'struct Diamond' to base class 'struct Base':
struct Diamond -> struct Derived -> struct Base
struct Diamond -> struct Derived2 -> struct Base
Base &br5 = diamond; // expected-error{{ambiguous conversion from
...
^~~~~~~
p5-var.cpp:59:9: error: non-const lvalue reference to type 'long'
cannot bind to
a value of unrelated type 'int'
long &lr = i; // expected-error{{non-const lvalue reference to type
...
^ ~
p5-var.cpp:74:9: error: non-const lvalue reference to type 'struct
Base' cannot
bind to a temporary of type 'struct Base'
Base &br1 = Base(); // expected-error{{non-const lvalue reference to
...
^ ~~~~~~
p5-var.cpp:102:9: error: non-const reference cannot bind to bit-field
'i'
int & ir1 = (ib.i); // expected-error{{non-const reference cannot
...
^ ~~~~~~
p5-var.cpp:98:7: note: bit-field is declared here
int i : 17; // expected-note{{bit-field is declared here}}
^
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struct A { };
struct B : A { };
void f() {
const A& a = B();
}
correctly. (This now does the offset conversion if necessary and calls the destructor when a goes out of scope).
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It would be nice if someone could write an ObjC++ testcase for the case
of passing a property returning a struct to a function taking a const
reference.
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Eli Friedman
Chris Lattner
Douglas Gregor
Anders Carlsson
Chandler Carruth
Daniel Dunbar
Sebastian Redl