[LLVMdev] another SCEV surprise
Hal Finkel
hfinkel at anl.gov
Wed Jun 20 05:50:34 PDT 2012
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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--
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory
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