http://d.puremagic.com/issues/show_bug.cgi?id=5546
Summary: Assigning and initializing structs from functions make
more copies than necessary
Product: D
Version: D2
Platform: x86_64
OS/Version: Linux
Status: NEW
Severity: enhancement
Priority: P2
Component: DMD
AssignedTo: nob...@puremagic.com
ReportedBy: akb...@gmail.com
--- Comment #0 from akb...@gmail.com 2011-02-07 23:31:56 PST ---
When initializing a struct from a function that returns by value, more copies
(calling the post blit and destructors) are called than are necessary. For
example, see the attached source file.
The output for TestCopy.d is currently:
Creating temp
Copying temp
Deleting temp
Creating copy
Copying copy
Deleting copy
Forwarding copy
Copying copy
Deleting copy
Returning global
Copying global
Deleting global
Deleting copy
Deleting temp
Ideally, the output should look like this:
Creating temp
Creating copy
Forwarding copy
Returning global
Copying global
Deleting global
Deleting copy
Deleting temp
When a struct is being initialized by the return value of a function, apart
from the memory being blitted over, no post blit or destructor should need to
be called, since semantically it's equivalent to directly initializing the
struct in the called function. This can be achieved by always returning a local
object and not destructing the local object being returned. In the case of
globalFunc(), which is returning a non-local object, a temporary would be made
before returning from globalFunc().
When assigning the returning value of a function to a struct that's already
initialized, additional optimizations can be made if no custom assignment
operator exists.
For example:
Test testVal;
testVal = function();
This will create the copy for the return value of function(), post blit
testVal, destroy the previous value of testVal, then destroy the return value
of function(). If Test or any of its members have an overridden assignment
operator, they must be called. However, in the case where there is no custom
assignment operator, the post blit of testVal and destruction of the return
value of function() can be omitted, since you are semantically moving the value
from the return value to testVal.
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