llvm dev - Feb 2017 - Can we keep debug information of local variable when in the optimization condition?

Hi, I'm curious that whether we can keep debug information of local variable
when in the optimization condition (for example -O2, -O3) in LLVM.


For example this simple C source code:


int main()
{
    int i = 5;
    if(i > 5) {
        return 1;
    } else {
        return 0;
    }
}


If we compile clang -g a.c, we can get our expected result. We can check and
update the local variable i in the debugger.

However, when we compile it using clang -O3 -g a.c, we can only check the
variable i but we can not  update the local variable i.

I know, the design of LLVM opt-debug is that debug can not affect optimization,
so this case llvm-opt will turn the llvm.dbg.declare and related alloc / store
instruction to llvm.dbg.value, we don't have symbol of local variable i. So
we can only check local variable i but we can not update the value of local
variable i like set variable i = 7 in the gdb.

I think keep debug information of local variable in some condition is very
useful and I want to explore it. However, I find there are many places do this
work of local variable debug info. I want to know whether I have one simple way
like disabling some passes or modify some llvm code to continue this way.

Thanks in advance. 
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I don't think there's any simple approach to this - there's been
some idea
that -O1 (or -Og, or that -O1 shuold be -Og) could be made to be this mode,
where values are still findable/etc. (though I'm not sure
"modifiable" is a
goal there - the ability to see the behavior of the program correctly is
one thing, but making sure all values are modifiable would be a larger goal
and probably really limit the ability to optimize the program)

- Dave

On Thu, Feb 16, 2017 at 7:39 AM Wu Zhao via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi, I'm curious that whether we can keep debug information of local
> variable when in the optimization condition (for example -O2, -O3) in LLVM.
>
> For example this simple C source code:
>
> int main()
> {
>     int i = 5;
>     if(i > 5) {
>         return 1;
>     } else {
>         return 0;
>     }
> }
>
> If we compile clang -g a.c, we can get our expected result. We can check
> and update the local variable i in the debugger.
>
> However, when we compile it using clang -O3 -g a.c, we can only check the
> variable i but we can not  update the local variable i.
>
> I know, the design of LLVM opt-debug is that debug can not affect
> optimization, so this case llvm-opt will turn the llvm.dbg.declare and
> related alloc / store instruction to llvm.dbg.value, we don't have
symbol
> of local variable i. So we can only check local variable i but we can not
> update the value of local variable i like set variable i = 7 in the gdb.
>
> I think keep debug information of local variable in some condition is very
> useful and I want to explore it. However, I find there are many places do
> this work of local variable debug info. I want to know whether I have one
> simple way like disabling some passes or modify some llvm code to continue
> this way.
>
> Thanks in advance.
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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In this specific example, with optimization, the compiler sees that in the
source code, the value of the variable 'i' is set once and does not
change afterward.  Therefore the compiler can substitute the known value into
the expressions that use the variable, and when it is done optimizing, in fact
there is no need for a register or memory location for the variable.  That is
why you cannot modify the value in the debugger.  The debug information
correctly reflects that the variable has the value '5' and that is as
much as we can do with the optimized code.
If you have a particular function that you want to debug, you can use the
`optnone` attribute or `#pragma clang optimize` pragma to suppress optimization.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of David
Blaikie via llvm-dev
Sent: Thursday, February 16, 2017 8:21 AM
To: Wu Zhao; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Can we keep debug information of local variable when in
the optimization condition?

I don't think there's any simple approach to this - there's been
some idea that -O1 (or -Og, or that -O1 shuold be -Og) could be made to be this
mode, where values are still findable/etc. (though I'm not sure
"modifiable" is a goal there - the ability to see the behavior of the
program correctly is one thing, but making sure all values are modifiable would
be a larger goal and probably really limit the ability to optimize the program)

- Dave

On Thu, Feb 16, 2017 at 7:39 AM Wu Zhao via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi, I'm curious that whether we can keep debug information of local variable
when in the optimization condition (for example -O2, -O3) in LLVM.
For example this simple C source code:

int main()
{
    int i = 5;
    if(i > 5) {
        return 1;
    } else {
        return 0;
    }
}

If we compile clang -g a.c, we can get our expected result. We can check and
update the local variable i in the debugger.

However, when we compile it using clang -O3 -g a.c, we can only check the
variable i but we can not  update the local variable i.

I know, the design of LLVM opt-debug is that debug can not affect optimization,
so this case llvm-opt will turn the llvm.dbg.declare and related alloc / store
instruction to llvm.dbg.value, we don't have symbol of local variable i. So
we can only check local variable i but we can not update the value of local
variable i like set variable i = 7 in the gdb.

I think keep debug information of local variable in some condition is very
useful and I want to explore it. However, I find there are many places do this
work of local variable debug info. I want to know whether I have one simple way
like disabling some passes or modify some llvm code to continue this way.

Thanks in advance.



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llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
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