All:
I am working on changing the definition of assignment in Chapel to use a
signature that conforms with that used for the recently-introduced assign-op
operators (+=, *=, etc.). Specifically, the default implementation of
assignment will change from
proc =(a, b) return b;
to
proc =(ref a, b) { __primitive("=", a, b); }
The intent of the change is to avoid the copy operations associated with
returning a value. The compiler currently auto-generates a MOVE primitive to
store the result of an assignment back into the left operand. This
implementation entails one copy into the return-value temporary and one copy
from that temporary into the left-hand argument. Removing these copies is
expected to reduce execution times as well as the size of the generated code.
Since Chapel now supports 'ref' intents, it has become possible to use the new
signature. In many cases, the value of the left operand can be updated
directly without using a return statement. That renders the added MOVE
primitive moot, and it is removed by expression folding. There are a few cases
in the module code where the return statement is still required. Automatic
insertion of the MOVE primitive will be left in-place until these cases are
brought into conformance with the normal case.
A new primitive "=" (called PRIM_ASSIGN internally) is being introduced to
perform the required data transfer on fundamental types. Updating a left
operand of class or record type can be done using field selection, as in:
record R { var i:int, r:real; }
proc =(ref a:R, b:R) { a.i = b.i; a.r = b.r; }
As part of this change, the old signature for assignment will be deprecated.
The 'ref' intent will be required on the left operand of assignment and
assign-op operators. A warning message will be issued by the compiler if the
'ref' intent is absent.
A consequence of this change is that the error message issued when an attempt
is made to supply a non-lvalue as the left operand of assignment will change
from
error: illegal lvalue in assignment
to
error: non-lvalue actual is passed to a 'ref' formal of =
Suggestions for further improvements in the error message would be welcome.
This change should be ready to commit some time next week. Please let me know
if you have objections, recommendations or schedule constraints that I can
accommodate.
Tom Hildebrandt
Chapel Team
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