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Muhui Jiang via llvm-dev

2018-Jun-02 05:30 UTC

[llvm-dev] Function start address

Hi Paulr

Thanks for your very useful and quick reply. Below is my response.

Are the functions emitted to the final binary?  If a function is not used,
there might not be any object code for it in the final binary.  Naturally
there would be no entry in the line table in this case.

=============
Yes, I think so. At least, I use IDAPro to analysis the binary and I am
pretty sure that many function are in the binary but its source line
mapping table are not in the dwarf mapping table.


If the function does exist in the binary, it is entirely possible (I think)
to have no instruction specifically associated with the function
definition's source line, even though other instructions are associated
with other lines in the function.  I (or someone) would need to look at a
specific example before being able to say one way or the other if that is
what you are running into.

=============
I don't understand what "no instruction specifically associated with
the
function definition's source line,"  this sentence mean. Could you
please
explain more? If you are interested, I am glad to give you a specific
example. Please tell me whether you need it.


Have you considered building a static array of function addresses?  If you
used weak references it would not interfere with optimizing away entire
functions, which I mentioned above.  Or would that be too intrusive into
your use case?  Apologies if this suggestion has come up before.

=============
To be honest, No.  Since I am using llvm IR to do the code analysis.
Function start address is just a part of the whole evaluation. I would
prefer to combine all the tools into one tool with llvm IR and dwarf debug
information. Thus, I may not evaluate the  function addresses with a static
table. Thank you very much


Regards

Muhui



<paul.robinson at sony.com>于2018年6月1日 周五下午10:45写道：
> Hi Muhui,
>
> Are the functions emitted to the final binary?  If a function is not used,
> there might not be any object code for it in the final binary.  Naturally
> there would be no entry in the line table in this case.
>
> If the function does exist in the binary, it is entirely possible (I
> think) to have no instruction specifically associated with the function
> definition's source line, even though other instructions are associated
> with other lines in the function.  I (or someone) would need to look at a
> specific example before being able to say one way or the other if that is
> what you are running into.
>
>
>
> Have you considered building a static array of function addresses?  If you
> used weak references it would not interfere with optimizing away entire
> functions, which I mentioned above.  Or would that be too intrusive into
> your use case?  Apologies if this suggestion has come up before.
>
> --paulr
>
>
>
>
>
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of
*Muhui
> Jiang via llvm-dev
> *Sent:* Friday, June 01, 2018 6:36 AM
> *To:* llvm-dev
> *Subject:* [llvm-dev] Function start address
>
>
>
> Hi
>
>
>
> I am using LLVM Pass combined with dwarf debug information to get all the
> function's start address. My steps are below:
>
>
>
> First, I write the function pass to get the start line of each function,
> which is finished.
>
>
>
> Then, based on the start line of every single function, I try to query the
> specific line from the dwarf's line binary table, which is generated
with
> llvm-dwarfdump -debug-line.
>
>
>
> However, About one third of the whole functions' start line is not
found
> in the mapping table. Thus, I can not get the start binary address. I know
> that the mapping between source locations and binary addresses is not
> bijective. I am using O1 optimization option. I know that some of the
> information might be lost legitimately because of optimization. But I
don't
> think dwarf will miss so many functions' start addresses. Am I right?
Any
> useful comments and suggestions are welcomed. Many Thanks
>
>
>
> Regards
>
> Muhui
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via llvm-dev

2018-Jun-03 18:57 UTC

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[llvm-dev] Function start address

Hi Muhui,

If the function does exist in the binary, it is entirely possible (I think) to
have no instruction specifically associated with the function definition's
source line, even though other instructions are associated with other lines in
the function.  I (or someone) would need to look at a specific example before
being able to say one way or the other if that is what you are running into.
=============I don't understand what "no instruction specifically
associated with the function definition's source line,"  this sentence
mean. Could you please explain more? If you are interested, I am glad to give
you a specific example. Please tell me whether you need it.

I tried a simple example like this:

int f(int a, int b)
{
 return a + b;
}

In the IR, it looks like the function definition is given as being on line 1
(the "int f" part), however in the DWARF line table the first
instruction is associated with line 2 (the opening brace).  That's what I
meant by "no instruction specifically associated with the function
definition's source line"; even though the function is defined
(according to C/C++) on line 1, there are no instructions for that line; the
first instruction of the function is instead associated with the line where the
scope starts.

I see that there is a different attribute in the IR metadata, called scopeLine,
which is 2.  That might be what you want.

Have you considered building a static array of function addresses?  If you used
weak references it would not interfere with optimizing away entire functions,
which I mentioned above.  Or would that be too intrusive into your use case? 
Apologies if this suggestion has come up before.
=============To be honest, No.  Since I am using llvm IR to do the code
analysis. Function start address is just a part of the whole evaluation. I would
prefer to combine all the tools into one tool with llvm IR and dwarf debug
information. Thus, I may not evaluate the  function addresses with a static
table. Thank you very much

Hmm. I am curious what sort of analysis would be done within the IR, while still
depending on the address of the functions in the final object file.  The IR is
long gone by the time the compiler is emitting the final instruction stream.

Hope this helps,
--paulr

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llvm dev - Jun 2018 - Function start address

[llvm-dev] Function start address

[llvm-dev] Function start address