llvm dev - Apr 2010 - [LLVMdev] Get the loop trip count variable

Hi,

On 04/05/2010 10:51 PM, Eli Friedman wrote:
> On Mon, Apr 5, 2010 at 1:19 PM, Zheng Wang<jason.wangz at gmail.com> 
wrote:
>> Hello,
>>
>> I am wondering whether I can get the variable name of loop trip count
in LLVM?
>>
>> For example,
>>
>> int NUM;
>>
>> NUM=atoi(argv[i]);
>>
>> for (int i=0; i<NUM; i++)
>> {
>>     ...
>> }
>>
>> How can I get the corresponding variable name for "NUM"?
Then, I can
>> instrument something in the source code to record the loop trip count
>> for a given input data set.
>>
>> BasicBlock* b = L->getHeader();
>>
>> returns the basicblock of the loop header, but I don't know how to
>> extract "NUM" from basicblock b.
>
> If you need to instrument the source, you'd probably be better off
> doing this with clang purely at the source level; by the time LLVM
> loop analyzers can tell "NUM" is the trip count, the name is
likely to
> be lost due to optimization.
You can also use the instrumentation pass 
(-insert-optimal-edge-instrumenation) to instrument all your code, this 
adds about 10% runtime overhead.

Also with some tweaking the instrumentation pass could be talked into 
instrumenting only loop headers so that only the loop counts get 
recorded but for a code with a whole lot of loops this would be almost 
equivalent to do the optimal instrumentation. (For a loop with only one 
block in the body only one counter gets added anyway...)

Andi

Hello Andreas,

Thanks! But I could not find -insert-optimal-edge-instrumenation pass
from "opt -help". Did you mean -insert-edge-profiling?

Cheers,
Zheng

On 7 April 2010 12:29, Andreas Neustifter <astifter-llvm at gmx.at>
wrote:
> Hi,
>
> On 04/05/2010 10:51 PM, Eli Friedman wrote:
>>
>> On Mon, Apr 5, 2010 at 1:19 PM, Zheng Wang<jason.wangz at
gmail.com>  wrote:
>>>
>>> Hello,
>>>
>>> I am wondering whether I can get the variable name of loop trip
count in
>>> LLVM?
>>>
>>> For example,
>>>
>>> int NUM;
>>>
>>> NUM=atoi(argv[i]);
>>>
>>> for (int i=0; i<NUM; i++)
>>> {
>>>    ...
>>> }
>>>
>>> How can I get the corresponding variable name for "NUM"?
Then, I can
>>> instrument something in the source code to record the loop trip
count
>>> for a given input data set.
>>>
>>> BasicBlock* b = L->getHeader();
>>>
>>> returns the basicblock of the loop header, but I don't know how
to
>>> extract "NUM" from basicblock b.
>>
>> If you need to instrument the source, you'd probably be better off
>> doing this with clang purely at the source level; by the time LLVM
>> loop analyzers can tell "NUM" is the trip count, the name is
likely to
>> be lost due to optimization.
>
> You can also use the instrumentation pass
> (-insert-optimal-edge-instrumenation) to instrument all your code, this
adds
> about 10% runtime overhead.
>
> Also with some tweaking the instrumentation pass could be talked into
> instrumenting only loop headers so that only the loop counts get recorded
> but for a code with a whole lot of loops this would be almost equivalent to
> do the optimal instrumentation. (For a loop with only one block in the body
> only one counter gets added anyway...)
>
> Andi
>
>


-- 
Best regards,

WANG Zheng

Hi!

On 07.04.2010, at 16:15, Zheng Wang wrote:
> [...] But I could not find -insert-optimal-edge-instrumenation pass
> from "opt -help". Did you mean -insert-edge-profiling?
Yeah, I meant "-insert-optimal-edge-profiling", sorry.

Andi