[LLVMdev] another SCEV surprise

[Eli Friedman]

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.

-Eli