[PATCH] Loop Rerolling Pass

[Hal Finkel]

----- Original Message -----
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> On Oct 17, 2013, at 11:33 AM, Hal Finkel < hfinkel at anl.gov > wrote:
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> ----- Original Message -----
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> On Oct 17, 2013, at 10:30 AM, Hal Finkel < hfinkel at anl.gov > wrote:
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> If the loop is testing less-than constant ’n’, I think we already
> handle it (knowing n < INT_MAX-stride). I’m not sure what we do for
> equals ’n’.
> 
> No, that does not currently work. Compiling the following with clang:
> 
> void foo(int n, int *x) {
> for (int i = 0; i < n; i += 3) {
> x[i] = i;
> x[i+1] = i+1;
> x[i+2] = i+2;
> }
> }
> 
> I find that SE->hasLoopInvariantBackedgeTakenCount(L) returns false.
> From what you're saying, this sounds like a bug, no?
> 
> By “constant n” I meant “a constant” ;)
> 
> Right, and the constant case works. The variable case does not, and
> if I understand correctly, this is the 'we ignore the nsw flag'
> case. If we get to assume that i += 3 does not wrap, then we can
> positively return a backedge taken count of n/3-1, right?
> 
> You had also said that we need to make clear what the number returned
> means (because we can't actually guarantee that many iterations in
> the face of functions that might throw, etc.), but I don't see how
> this interpretation concern is affected by whether or not n is a
> constant. Are these two issues related?
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> 
> Ah. When I say the “constant” case is handled, I mean we know that we
> must take the exit at iteration n/3. For a nonconstant, we may exit
> at n/3 or may terminate later via a call, exit later via a different
> loop test, etc. If the client can prove that there’s no other way
> for the loop to terminate or exit, then it can use n/3 as the trip
> count.

Okay; we need to stop overloading our terminology here. How about I'll say that currently SE->hasLoopInvariantBackedgeTakenCount(L) returns false on the aforementioned loop when n is a runtime value, and true when n is a compile-time value. And, assuming that we have nsw on the increment (or nuw for an unsigned latch comparison), this is a bug, correct?

 -Hal
 
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> 
> -Andy

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory