[llvm] [polly] [SCEV] Introduce SDiv expressions (PR #216862)

[Antonio Frighetto via llvm-commits]

================
@@ -729,20 +730,29 @@ Value *SCEVExpander::visitUDivExpr(SCEVUseT<const SCEVUDivExpr *> S) {
   const SCEV *RHSExpr = S->getRHS();
   Value *RHS = expand(RHSExpr);
   if (SafeUDivMode) {
-    bool GuaranteedNotPoison =
-        ScalarEvolution::isGuaranteedNotToBePoison(RHSExpr);
-    if (!GuaranteedNotPoison)
+    if (!ScalarEvolution::isGuaranteedNotToBePoison(RHSExpr))
       RHS = Builder.CreateFreeze(RHS);
 
     // We need an umax if either RHSExpr is not known to be zero, or if it is
     // not guaranteed to be non-poison. In the later case, the frozen poison may
     // be 0.
-    if (!SE.isKnownNonZero(RHSExpr) || !GuaranteedNotPoison)
+    if (S->mayTriggerUB(SE))
       RHS = Builder.CreateIntrinsic(RHS->getType(), Intrinsic::umax,
                                     {RHS, ConstantInt::get(RHS->getType(), 1)});
   }
   return InsertBinop(Instruction::UDiv, LHS, RHS, SCEV::FlagAnyWrap,
-                     /*IsSafeToHoist*/ SE.isKnownNonZero(S->getRHS()));
+                     /*IsSafeToHoist=*/!S->mayTriggerUB(SE));
+}
+
+Value *SCEVExpander::visitSDivExpr(SCEVUseT<const SCEVSDivExpr *> S) {
+  Value *LHS = expand(S->getLHS());
+  Value *RHS = expand(S->getRHS());
+  if (S->mayTriggerUB(SE)) {
+    LHS = Builder.CreateFreeze(LHS);
+    RHS = Builder.CreateFreeze(RHS);
+  }
----------------
antoniofrighetto wrote:

The expansion logic still seems to be incorrect: https://alive2.llvm.org/ce/z/D6KTDh (the entry branch in `tgt` is simplified to skip the vectorized path and prevent Alive2 from timing out). Freeze stops propagating poison, though it does not prevent arithmetic UB. So, if you hoist `sdiv freeze(LHS), freeze(RHS)`, where freeze(0) is still 0, you have immediate UB (when the divisor is zero).

https://github.com/llvm/llvm-project/pull/216862