llvm dev - Mar 2017 - Inserting an external call in FunctionPass

Hi,

how do I insert a call to an external function in a FunctionPass?
My problem is with creation of the declaration for the external function.

According to http://llvm.org/docs/WritingAnLLVMPass.html#the-functionpass-class,
a FunctionPass is not supposed to add or remove functions. I assume
it's also true for function declarations.

A declaration inserted in doInitialization is cleaned up in
GlobalOptLegacyPass::runOnModule, which happens to run before my pass'
runOnFunction().

Do I need a ModulePass for this?

On 3/23/17 8:19 PM, Evgenii Stepanov via llvm-dev wrote:
> Hi,
>
> how do I insert a call to an external function in a FunctionPass?
> My problem is with creation of the declaration for the external function.
>
> According to
http://llvm.org/docs/WritingAnLLVMPass.html#the-functionpass-class,
> a FunctionPass is not supposed to add or remove functions. I assume
> it's also true for function declarations.
>
> A declaration inserted in doInitialization is cleaned up in
> GlobalOptLegacyPass::runOnModule, which happens to run before my pass'
> runOnFunction().
>
> Do I need a ModulePass for this?
I see two possible solutions:

1. Inform the GlobalOptLegacyPass that it should not remove the function 
when you add it in doInitialization().  I believe adding the function to 
the llvm.used or llvm.compiler.used array will prevent the function from 
being removed.  Skim the LLVM Language Reference Manual to see how to do 
this 
(http://llvm.org/docs/LangRef.html#the-llvm-compiler-used-global-variable)

2. Make your pass a ModulePass.

Regards,

John Criswell
> _______________________________________________
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> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

I think the documentation is wrong. Existing function passes frequently
insert new intrinsic function declarations, and we expect that to work. The
loop vectorizer also inserts calls to non-intrinsic library functions.

The intention is that function passes don't introduce new function
definitions, because then an outer pass manager would need to discover them
and add them to its worklist.

On Thu, Mar 23, 2017 at 5:19 PM, Evgenii Stepanov via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> how do I insert a call to an external function in a FunctionPass?
> My problem is with creation of the declaration for the external function.
>
> According to http://llvm.org/docs/WritingAnLLVMPass.html#the-
> functionpass-class,
> a FunctionPass is not supposed to add or remove functions. I assume
> it's also true for function declarations.
>
> A declaration inserted in doInitialization is cleaned up in
> GlobalOptLegacyPass::runOnModule, which happens to run before my pass'
> runOnFunction().
>
> Do I need a ModulePass for this?
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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