www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.bugs - [Issue 9655] New: Two functions with identical implementations are allowed to have the same address

reply d-bugmail puremagic.com writes:
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



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.

-- 
Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
Mar 05 2013
parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=9655


bearophile_hugs eml.cc changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |bearophile_hugs eml.cc





 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.

-- 
Configure issuemail: http://d.puremagic.com/issues/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
Mar 06 2013