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<div class="moz-cite-prefix">On 1/26/18 1:04 AM, Linhai Song via
llvm-dev wrote:<br>
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<p>Hello everyone,</p>
<p><br>
</p>
<p>I am writing a pass to instrument program and count how many
basic block executed. What I have tried is to instrument a
local counter inside each function, add 1 to the local counter
inside each basic block, and save the counter value to a
global counter. The current runtime overhead is around 25%. Is
there any way I can try to lower the overhead? Like keeping
the local counter inside a register or applying the path
profiling algorithm? <br>
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<br>
By "local counter," I assume you mean that you created an alloca
instruction that allocates memory and that you increment the value
in this alloca'ed memory using a load, add, and store instruction.
Is that correct?<br>
<br>
If so, have you tried using the mem2reg pass to convert the local
counter into a SSA virtual register? That may speed it up a bit.
After that, other LLVM optimizations may be able to remove redundant
instructions or combine additions.<br>
<br>
If that isn't enough, then you'll probably need to make your
instrumentation smarter. LLVM has passes that you can use to locate
loops; if the loop has the right structure, you can increment the
count at the end of the loop. Likewise, if you can find control
equivalent basic blocks, you only need to increment the counter in
one of them.<br>
<br>
Regards,<br>
<br>
John Criswell<br>
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<p>Thanks a lot!</p>
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<p>Best,</p>
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<p> Linhai</p>
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<pre class="moz-signature" cols="72">--
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
<a class="moz-txt-link-freetext" href="http://www.cs.rochester.edu/u/criswell">http://www.cs.rochester.edu/u/criswell</a></pre>
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