[LLVMdev] DCE and external function

[Nick Lewycky]

leledumbo wrote:
>
>> No, however it can prove it if the function is in a static library
> consisting
>> of bitcode files (what you get if you compile with -emit-llvm) and you do
>> link-time optimization.  However it's probably simpler to declare the
>> function
>> as having no side-effects using gcc's "pure" or "const" attribute.
>
> Hmm... the library can't be made static so I guess it's OK.
>
>> Only if the right function attributes are added. See
>> http://llvm.org/docs/LangRef.html#fnattrs for a list, but
>> 'readonly'/'readnone' and 'nounwind' will be most interesting for you
>> here. I guess calling abort() probably disqualifies a function for
>> 'readonly' (and it's stronger variant 'readnone').
>> Currently calls to any function that has either of the readnone or
>> readonly attributes as well as the nounwind attribute qualify as
>> "trivially dead" if their results aren't used, meaning lots of
>> optimizations will get rid of them. (in particular, this is pretty
>> much the entire the job of -die and -dce, but many others will delete
>> them rather than change or move them if they get in the way)
>
> Without those attributes, will the optimizer assume something?

It will assume that the function could do absolutely anything, which 
forbids pretty much any optimization.

> Actually, I don't use llvm-gcc here. I'm creating a compiler for a language
> defined by my professor that generates llvm assembly language, for my
> bachelor graduation project. The language is database centric but isn't
> tight to any database implementation. For my implementation, it's decided to
> use postgresql, and to concentrate more in the language rather than the
> database I create a shared library for most commonly database related
> functionality (connect, disconnect, getting error message, query) so I can
> just declare and call it from the generated llvm assembly.
>
> What I'm afraid of is, for example, return value of a call to connect
> function may be ignored, and the optimizer will eliminate the call due to
> this.

No of course not. Nobody ever checks the return value of printf, for 
example. :)

The only special case is if you name your function something that exists 
in the C standard library; if you name your function "malloc" we're 
going to remove calls to it with unused results.

Nick