[llvm] [DA] Rewrite the formula in the Strong SIV test (PR #179665)

[Ryotaro Kasuga via llvm-commits]

================
@@ -1353,38 +1331,55 @@ bool DependenceInfo::strongSIVtest(const SCEV *Coeff, const SCEV *SrcConst,
   assert(0 < Level && Level <= CommonLevels && "level out of range");
   Level--;
 
-  const SCEV *Delta = minusSCEVNoSignedOverflow(SrcConst, DstConst, *SE);
-  if (!Delta) {
-    Result.Consistent = false;
-    return false;
-  }
-  LLVM_DEBUG(dbgs() << "\t    Delta = " << *Delta);
-  LLVM_DEBUG(dbgs() << ", " << *Delta->getType() << "\n");
-
-  // check that |Delta| < iteration count
-  bool IsDeltaLarge = [&] {
-    const SCEV *UpperBound = collectUpperBound(CurSrcLoop, Delta->getType());
+  // Src: [a*0 + c1], [a*1 + c1], [a*2 + c1], ..., [a*BTC + c1]
+  // Dst: [a*0 + c2], [a*1 + c2], [a*2 + c2], ..., [a*BTC + c2]
+  //
+  // When 0 <=s a, we can prove independence if:
+  //
+  //   (a*BTC + c1 <s a*0 + c2) or (a*BTC + c2 <s a*0 + c1)
+  //
+  // When a <s 0, we can prove independence if:
+  //
+  //   (a*0 + c1 <s a*BTC + c2) or (a*0 + c2 <s a*BTC + c1)
----------------
kasuga-fj wrote:

Sorry, but I don't really understand what you are talking about. It's already possible for nowrap flags to be set on expressions that are not guaranteed to be actually executed.

https://github.com/llvm/llvm-project/pull/179665