[PATCH] D32674: [LoopIdiom] PR32811 check for safety while expanding

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


================
Comment at: lib/Transforms/Scalar/LoopIdiomRecognize.cpp:823
   Value *NumBytes =
       Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator());
 
----------------
hiraditya wrote:
> efriedma wrote:
> > Do we also need to check whether it's safe to expand NumBytesS?
> I don't think so (IIUC), because the NumBytes is based on the trip count of the loop*storesize, both of them should be safe to expand if the loop has a valid SCEV.
The store size is a constant, so that's obviously fine, but I don't see how the loop's trip count is obviously safe to expand; .  Consider, e.g.:

```
#include <stdio.h>
void f(unsigned a, unsigned b, char* x) {
  for (int j = 0; j < a; j++)
    if (b != 0)
      for (long long i = 0; i < a / b; ++i)
        x[i] = 0;
}
```


https://reviews.llvm.org/D32674