Extracting the global variables accessed by individual function with function pass

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1

Hi All,

Is there a way to dump the global variables, which are accessed (read/written to) by each function within the LLVM function pass?
Any pointers on how that info can be extracted in LLVM for each function in the application?

Thanks.

Regards,
Yan Lin Aung

2

Hi,

Is there a way to dump the global variables, which are accessed (read/written
to) by each function within the LLVM function pass?
Any pointers on how that info can be extracted in LLVM for each function in the
application?

you will have to write your own pass to do this. For each global, visit each
of its uses (via use_begin, use_end). If the use is a constant (isa<Constant>),
visit each of its uses too (do this recursively until you aren't visiting
constants any more). If a use isn't a constant, check whether it's parent
(getParent) is the function you are interested in. The point of recursing on
constant uses is that the global may be used in a constant expression (eg a
constant bitcast) before actually being used by an instruction.

Ciao, Duncan.

3

Hi,

Is there a way to dump the global variables, which are accessed (read/written
to) by each function within the LLVM function pass?
Any pointers on how that info can be extracted in LLVM for each function in the
application?

you will have to write your own pass to do this. For each global, visit each
of its uses (via use_begin, use_end). If the use is a constant (isa<Constant>),
visit each of its uses too (do this recursively until you aren't visiting
constants any more). If a use isn't a constant, check whether it's parent
(getParent) is the function you are interested in. The point of recursing on
constant uses is that the global may be used in a constant expression (eg a
constant bitcast) before actually being used by an instruction.

Note that this only finds globals that are directly accessed by the function. It won't find globals that are accessed by the function due to the global being passed into the function as an argument or loaded from memory via an LLVM load instruction. To find those, you'll need to use alias analysis or a points-to analysis.

-- John T.

4

Hi Duncan and John,

Thank your very much for the advice and help.

Yes, I have got the first part working following Duncan’s advice.

I think I can also incorporate Alias Analysis (AA) in my code.

A bit of guidance on how the LLVM provided methods in AA have to be invoked so as to get the info on
the global being passed into the function as an argument or loaded from memory via an LLVM load instruction as John pointed out?

Thanks again.

Regards,
Yan Lin Aung