http://d.puremagic.com/issues/show_bug.cgi?id=9655

           Summary: Two functions with identical implementations are
                    allowed to have the same address
           Product: D
           Version: D2
          Platform: All
               URL: http://www.informit.com/guides/content.aspx?g=cplusplu
                    s&seqNum=561
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: websites
        AssignedTo: nobody@puremagic.com
        ReportedBy: bugzilla@digitalmars.com


--- Comment #0 from Walter Bright <bugzilla@digitalmars.com> 2013-03-05 21:54:35 PST ---
In regards to:

http://d.puremagic.com/issues/show_bug.cgi?id=9623

Related C++ article "Can Two Functions Have the Same Address?" http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=561

This quote pretty much sells me:

--
Additionally, Google's compiler team have experimented with Identical Code Folding (ICF) and reported that "[d]etailed experiments on the x86 platform show that ICF can reduce the text size [the program section in which functions' code is stored, DK] of some Google binaries, whose average text size is 50 MB, by up to 7%."
--

We should settle the issue by updating the D spec to explicitly allow functions to have the same address.

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bearophile_hugs@eml.cc changed:

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--- Comment #1 from bearophile_hugs@eml.cc 2013-03-06 02:05:52 PST ---
(In reply to comment #0)
> In regards to:
> 
> http://d.puremagic.com/issues/show_bug.cgi?id=9623
> 
> Related C++ article "Can Two Functions Have the Same Address?" http://www.informit.com/guides/content.aspx?g=cplusplus&seqNum=561
> 
> This quote pretty much sells me:
> 
> --
> Additionally, Google's compiler team have experimented with Identical Code Folding (ICF) and reported that "[d]etailed experiments on the x86 platform show that ICF can reduce the text size [the program section in which functions' code is stored, DK] of some Google binaries, whose average text size is 50 MB, by up to 7%."
> --
> 
> We should settle the issue by updating the D spec to explicitly allow functions to have the same address.

1) What are the downsides of such folding in D?

2) Since recently, LLVM (used by LDC) folds identical functions if you use a compiler switch. IF you use such switch, where the compiler sees two identical functions, replaces one of them with just a jump to the other. So their address is distinct, but the amount of wasted space in the binary is minimal.

3) Often templates generate not just identical functions, but functions that differ only in a small part, for only few asm instructions. So a good D compiler could try to split those functions in virtual chunks (maybe if the parts are not inside a loop), keep only one copy of the shared part. I presume this is not easy to do in general.

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