[Issue 21065] New: the new operator should lower to a template function call in object.d

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21065

          Issue ID: 21065
           Summary: the new operator should lower to a template function
                    call in object.d
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: andrei at erdani.com

Currently the new expression lowers to a nontemplate call in druntime:

    extern (C) Object _d_newclass(const ClassInfo ci);

followed by a call to the appropriate __ctor() if any. This approach has the
usual rigidity disadvantages of a runtime solution: loss of compile-time type
information in the implementation, awkward to instrument, bulkier code due to
indirections.

Calls to "new C(a1, a2, a3) should lower to ".object.__makeClassObject!(C)(a1,
a2, a3)". That way a function such as:

T __makeClassObject(T, A...)(auto ref A);

in object.d would carry the allocation and initialization.

Such a function would be trivial to instrument, see
https://github.com/dlang/dmd/pull/11381. In contrast, the current new
instrumentation code is unbearably slow because it uses hash tables.

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