[LLVMdev] another SCEV surprise

[Preston Briggs]

Thanks Eli.

Hal, yes, I agree. I imagine producing an listing with all sorts
of annotations concerning the success/failure of vectorization
and other loop xforms. Seems pretty essential as a tool to
help the performance-oriented programmer understand what's
happening in the compiler. Serves as a feedback mechanism,
teaching programmers what sorts of code can/can't be analyzed
and transformed.

Preston


On Wed, Jun 20, 2012 at 5:50 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> On Wed, 20 Jun 2012 02:18:49 -0700
> Eli Friedman <eli.friedman at gmail.com> wrote:
>
> > On Tue, Jun 19, 2012 at 10:21 PM, Preston Briggs
> > <preston.briggs at gmail.com> wrote:
> > > When compile the following case and look at the SCEV analysis, I
> > > notice that the first two loops don't have a
> > > LoopInvariantBackedgeTakenCount (surprising) and the last one does
> > > (not surprising, except in the context of the first two examples).
> > >
> > > void p4(int *A, int *B, long int n) {
> > >   for (char i = 0; i < n; i++) {
> > >     A[i + 2] = i;
> > >     *B++ = A[i];
> > >   }
> > > }
> > >
> > > void p5(int *A, int *B, long int n) {
> > >   for (short i = 0; i < n; i++) {
> > >     A[i + 2] = i;
> > >     *B++ = A[i];
> > >   }
> > > }
> > >
> > > void p6(int *A, int *B, long int n) {
> > >   for (int i = 0; i < n; i++) {
> > >     A[i + 2] = i;
> > >     *B++ = A[i];
> > >   }
> > > }
> > >
> > >
> > > Why the difference? Is there a reason we don't implement all the
> > > loops using 64-bit values?
> >
> > The third loop is provably not an infinite loop, thanks to C's signed
> > overflow rules; we can't tell whether the first two loops might be
> > infinite.
>
> This seems like a perfect example of where LLVM might want to emit a
> note to the user (especially when we have loop vectorization and other
> transformations).
>
>  -Hal
>
> >
> > -Eli
> >
> > _______________________________________________
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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
> --
> Hal Finkel
> Postdoctoral Appointee
> Leadership Computing Facility
> Argonne National Laboratory
>
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