http://d.puremagic.com/issues/show_bug.cgi?id=4357

           Summary: Stack allocation for small scope dynamic arrays
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: bearophile_hugs@eml.cc


--- Comment #0 from bearophile_hugs@eml.cc 2010-06-21 04:52:09 PDT ---
Dynamic arrays are used very often in D, much more often than malloc-allocated arrays in C programs.

A scope dynamic array lives only inside the function it's allocated. So with this program:


int[] foo(int n) {
    scope arr = new int[n];
    return arr;
}
void main() {}


DMD v2.047 generates this error:
test.d(3): Error: escaping reference to scope local arr

So the compiler can add an inlined runtime test: if the amount of memory necessary to allocate 'arr' is "small" (like few hundred bytes) (or better if it is small compared to the free space currently available on the stack), then the memory for 'arr' can be allocated on the stack instead of the heap, with an alloca. This can improve performance a little if foo() is called many times.

Compared to C programs (or D programs written in C-like style), this optimization can make the D idiom of using dynamic arrays often less costly, and avoids most of the need of the Variable Length Arrays of C99.

This enhancement request is marked as relative to the DMD component (instead of being just a runtime thing) because the test and the optional stack allocation need to be inlined.

A D compiler can perform the same optimization on dynamic array allocation even if the 'scope' attribute is missing if it is able to perform some escape analysis and it is able to verify that 'arr' never escapes foo().

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nfxjfg@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |nfxjfg@gmail.com
         Resolution|                            |WONTFIX


--- Comment #1 from nfxjfg@gmail.com 2010-06-21 05:03:49 PDT ---
Andrei said this use of scope is gone.
Sorry pal, WONTFIX *bamm*

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--- Comment #2 from bearophile_hugs@eml.cc 2010-06-21 11:23:19 PDT ---
Sorry, I didn't know that. I have thought that the removal of the scope attribute was only for class instances.

Even if the scope attribute can't be used on dynamic arrays, the last point I have expressed applies still: if the compiler is able to perform escape analysis (LLVM has some of such capability) and it can statically verify a dynamic array never escapes, then if a runtime test shows such dynamic arrays is small it can be allocated on the stack.

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