Re: behavior-preserving optimization in C, was compiler bugs

George Neuner schrieb:


> A problem occurs when there are multiple definitions in separate
> files. Technically, by the standards, there can be only 1 definition
> of a name at top level ... all other references to the name, including
> any forward references to the name in the same file, *must* be
> "extern" declarations.
[...]
> Every compiler complained about multiple definitions of 'x' when all
> the definitions were strong - ie. when 'x' was initialized in both
> files.
5B
How does a compiler know about other files?


I assume that the definitions were in the same compilation unit, i.e. in
the header files included by both source files. Otherwise it's a matter
of object file formats and linkers, what will happen with multiple
definitions of the same symbol in multiple object files.




> However, if either definition was weak, I was able to build an
> executable. When both definitions were weak, however, some of the
> compilers produced optimized executables with NO storage at all
> allocated for the name 'x'.


What's "NO" storage? A NULL address output by printf?


> I know this isn't a C forum and nobody (including me) cares much to
> debate what is right or wrong about C. But for the sake of a
> discussion about compilers, how would you handle the situation if it
> were up to you?


IMO this topic is an excellent exercise in the semantics of programming
languages. When an executable file can be linked from multiple
compilation units, the language specification also should include the
expected behaviour of the linker. When no type information is stored in
the object files, a linker only can overlay multiple definitions of the
same symbol, with the maximum allocation request.


I dunno about specific C++ specs, but when there also the names of
variables are mangled, the symbol definitions for variables of different
type would look different to the linker. Also namespace names should be
preserved, somehow.


In Delphi (OPL) or other languages a (public) symbol can have an
qualified name, consisting of the unit name and identifier, and the
compiler will only produce accordingly qualified references to external
symbols.


DoDi

Return to the comp.compilers page.
Search the comp.compilers archives again.