Get the loop trip count variable

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#1

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.

Cheers,
Zheng

#2

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.

-Eli

#3

Hi Eli,

Could you tell me how to extract the variable name?
The reason I want to do it at the IR level is because I have more
information at this stage, such as, constant propagation and back
edges.

Cheers,
Zheng

#4

You'll have to look at the debug information, if it's still present.
See http://llvm.org/docs/SourceLevelDebugging.html. This is probably
not what you want to do though. The loop trip count may not even be a
simple variable. You might be able to transform the IR to record the
trip count somewhere, but the source is harder.

#5

Thanks a lot for your guys' help!!!

I guess once I am able to get *V* (which probably is a pointer to a
Value object), then, I can instrument some code at the IR level to
dump V. As long as I maintain V at this pass stage, I should be able
to dump the loop trip count. This is true, isn't it?

Basically, what I am going to do is to add a function call before the
loop body, such as:

dumpInfo(FILE_NAME, LINE_NUMBER, VALUE_TYPE, Value)
...
for (int i=0; i<num; i++)

of course, this may have some side effects. If the trip count is a
function call, and I need to rewrite the loop header to avoid calling
the function twice. For example, if the loop header is:

for (int i=0; i<foo(a,b,c); i++)

then I should rewrite it as:

tmp_var = foo(a,b,c);
dumpInfo(...)

for (int i=0; i<tmp_var; i++)
{
   ...
    tmp_var=foo(a,b,c);
}

This looks quite complicated!

Does anyone has any ideal about how easy to insert some basic blocks
and to rewrite the loop header in LLVM?

Cheers,
Zheng

#6

Sorry, I could not the the loop trip count with getTripCount(). I used
a simple program as a test case:

#7

You're likely to get bad results if you don't run mem2reg.

-Eli

#8

Hi,

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

#9

Hello Andreas,

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

Cheers,
Zheng

#10

Hi!