!fixup also generate umax if the value may be poison.

Author: fhahn

llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp

@@ -684,9 +684,15 @@ Value *SCEVExpander::visitUDivExpr(const SCEVUDivExpr *S) {
   const SCEV *RHSExpr = S->getRHS();
   Value *RHS = expand(RHSExpr);
   if (SafeUDivMode) {
-    if (!ScalarEvolution::isGuaranteedNotToBePoison(RHSExpr))
+    bool GuaranteedNotPoison =
+        ScalarEvolution::isGuaranteedNotToBePoison(RHSExpr);
+    if (!GuaranteedNotPoison)
       RHS = Builder.CreateFreeze(RHS);
-    if (!SE.isKnownNonZero(RHSExpr))
+
+    // 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)
       RHS = Builder.CreateIntrinsic(RHS->getType(), Intrinsic::umax,
                                     {RHS, ConstantInt::get(RHS->getType(), 1)});
   }

llvm/test/Transforms/LoopVectorize/trip-count-expansion-may-introduce-ub.ll

@@ -1271,7 +1271,8 @@ define i64 @multi_exit_count_with_udiv_by_value_in_latch_different_bounds_diviso
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[M_1:%.*]] = call i64 @llvm.umax.i64(i64 [[M]], i64 1)
 ; CHECK-NEXT:    [[TMP9:%.*]] = freeze i64 [[M_1]]
-; CHECK-NEXT:    [[TMP0:%.*]] = udiv i64 42, [[TMP9]]
+; CHECK-NEXT:    [[TMP10:%.*]] = call i64 @llvm.umax.i64(i64 [[TMP9]], i64 1)
+; CHECK-NEXT:    [[TMP0:%.*]] = udiv i64 42, [[TMP10]]
 ; CHECK-NEXT:    [[TMP1:%.*]] = freeze i64 [[TMP0]]
 ; CHECK-NEXT:    [[SMAX:%.*]] = call i64 @llvm.smax.i64(i64 [[N]], i64 0)
 ; CHECK-NEXT:    [[UMIN:%.*]] = call i64 @llvm.umin.i64(i64 [[TMP1]], i64 [[SMAX]])