[LoopUnroll] Modify a comment to clarify the usage of TripCount. NFC.

[haicheng]

Summary

Per the request of Evgeny, I add a few sentences in the comments to clarif when TripCount contains the upper bound value.

[haicheng]

Hi Evgeny,

Would you please take a look at what I modified? Please let me know if it is what you want.

Thank you,

Haicheng

[mzolotukhin]

Hi Haicheng,

The comment is still confusing, at least to me. For instance, this sentence:

... Parameter TripCount
may contain the exact number or the upper bound.

repeats what we've said in:

... In other words, control may exit the loop prior to TripCount 
iterations via an early branch, but control may not exit the loop from the
LatchBlock's terminator prior to TripCount iterations.

At least, it looks very much the same.

AFAIU, now we started to handle cases where the loop may exit from the LatchBlock's terminator prior to TripCount iterations - am I right here? If so, can we somehow reflect it in the comment (maybe reword it entirely)?

Thanks,
Michael

[haicheng]

In D26675#609778, @mzolotukhin wrote:

AFAIU, now we started to handle cases where the loop may exit from the LatchBlock's terminator prior to TripCount iterations - am I right here? If so, can we somehow reflect it in the comment (maybe reword it entirely)?

You are absolutely correct. Let me try to rephrase it. Do you have any suggestions?

Thanks,

Haicheng

[haicheng]

How about rewriting the whole paragraph as this?

TripCount is the upper bound of the iteration on which control exits LatchBlock. Control may exit the loop prior to TripCount iterations either via an early branch of other loop block or via LatchBlock terminator. This is relaxed from the general definition of trip count which is the number of times the loop header executes. Note that UnrollLoop assumes that the loop counter test is in LatchBlock in order to remove unnecesssary instances of the test.

[haicheng]

Rewrite the comment paragraph. Hopefully, it is more clear.

[mzolotukhin]

Yes, it looks better now.

Thank you,
Michael

llvm/trunk/lib/Transforms/Utils/LoopUnroll.cpp

 }
 
 /// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
 /// if unrolling was successful, or false if the loop was unmodified. Unrolling
 /// can only fail when the loop's latch block is not terminated by a conditional
 /// branch instruction. However, if the trip count (and multiple) are not known,
 /// loop unrolling will mostly produce more code that is no faster.
 ///
-/// TripCount is generally defined as the number of times the loop header
-/// executes. UnrollLoop relaxes the definition to permit early exits: here
-/// TripCount is the iteration on which control exits LatchBlock if no early
-/// exits were taken. Note that UnrollLoop assumes that the loop counter test
-/// terminates LatchBlock in order to remove unnecesssary instances of the
-/// test. In other words, control may exit the loop prior to TripCount
-/// iterations via an early branch, but control may not exit the loop from the
-/// LatchBlock's terminator prior to TripCount iterations.
+/// TripCount is the upper bound of the iteration on which control exits
+/// LatchBlock. Control may exit the loop prior to TripCount iterations either
+/// via an early branch in other loop block or via LatchBlock terminator. This
+/// is relaxed from the general definition of trip count which is the number of
+/// times the loop header executes. Note that UnrollLoop assumes that the loop
+/// counter test is in LatchBlock in order to remove unnecesssary instances of
+/// the test.  If control can exit the loop from the LatchBlock's terminator
+/// prior to TripCount iterations, flag PreserveCondBr needs to be set.
 ///
 /// PreserveCondBr indicates whether the conditional branch of the LatchBlock
 /// needs to be preserved.  It is needed when we use trip count upper bound to
 /// fully unroll the loop. If PreserveOnlyFirst is also set then only the first
 /// conditional branch needs to be preserved.
 ///
 /// Similarly, TripMultiple divides the number of times that the LatchBlock may
 /// execute without exiting the loop.