llvm dev - Jan 2019 - LLVM Pass to count reachable BB

Hello,
I have code containing conditions and loops. Hence some BB execution are
determined at run time depending on condition. Now I want to count only
those BB that are always executed irrespective of condition result means
reachable. and their execution is evident at compile time.

How to do this?

Please help

Thank You
Regards
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Hello Hameeza,

If you have access to the dominator analysis (and the data is up-to-date)
you can query if each block is dominated by the entry block of a function.
I used this method to locate (and avoid) unreachable blocks at the
beginning of the JumpThreading pass:

https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/Scalar/JumpThreading.cpp#L362

// JumpThreading must not processes blocks unreachable from entry. It's a
// waste of compute time and can potentially lead to hangs.
SmallPtrSet<BasicBlock *, 16> Unreachable;
assert(DTU && "DTU isn't passed into JumpThreading before using
it.");
assert(DTU->hasDomTree() && "JumpThreading relies on DomTree to
proceed.");
DominatorTree &DT = DTU->getDomTree();
for (auto &BB : F)
  if (!DT.isReachableFromEntry(&BB))
    Unreachable.insert(&BB);

And later in the main for loop below (line 378):
if (Unreachable.count(&BB))
  continue;

On Fri, Jan 11, 2019 at 12:29 AM hameeza ahmed via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello,
> I have code containing conditions and loops. Hence some BB execution are
> determined at run time depending on condition. Now I want to count only
> those BB that are always executed irrespective of condition result means
> reachable. and their execution is evident at compile time.
>
> How to do this?
>
> Please help
>
> Thank You
> Regards
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On 1/11/2019 12:28 AM, hameeza ahmed via llvm-dev wrote:
> Hello,
> I have code containing conditions and loops. Hence some BB execution are 
> determined at run time depending on condition. Now I want to count only 
> those BB that are always executed irrespective of condition result means 
> reachable. and their execution is evident at compile time.
> 
> How to do this?
You can find the set of all dominators of the exit block.  Would that 
give you what you want?

-Krzysztof

-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation

Thank You..
I want to implement following algorithm in LLVM pass..
1. Scan BB of CFG
2. If BB is not 1st entry block then
3.   If BB.predecessor block has instruction "conditional br" then
4.     dont print BB
5.   else
6.     print BB

Please help me...How to implement this algorithm in LLVM pass?
Thank You
Regards

On Fri, Jan 11, 2019 at 9:12 PM Brian M. Rzycki <brzycki at gmail.com>
wrote:
> Hello Hameeza,
>
> If you have access to the dominator analysis (and the data is up-to-date)
> you can query if each block is dominated by the entry block of a function.
> I used this method to locate (and avoid) unreachable blocks at the
> beginning of the JumpThreading pass:
>
>
>
https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/Scalar/JumpThreading.cpp#L362
>
> // JumpThreading must not processes blocks unreachable from entry. It's
a
> // waste of compute time and can potentially lead to hangs.
> SmallPtrSet<BasicBlock *, 16> Unreachable;
> assert(DTU && "DTU isn't passed into JumpThreading before
using it.");
> assert(DTU->hasDomTree() && "JumpThreading relies on
DomTree to proceed.");
> DominatorTree &DT = DTU->getDomTree();
> for (auto &BB : F)
>   if (!DT.isReachableFromEntry(&BB))
>     Unreachable.insert(&BB);
>
> And later in the main for loop below (line 378):
> if (Unreachable.count(&BB))
>   continue;
>
> On Fri, Jan 11, 2019 at 12:29 AM hameeza ahmed via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hello,
>> I have code containing conditions and loops. Hence some BB execution
are
>> determined at run time depending on condition. Now I want to count only
>> those BB that are always executed irrespective of condition result
means
>> reachable. and their execution is evident at compile time.
>>
>> How to do this?
>>
>> Please help
>>
>> Thank You
>> Regards
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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