[SCEV] Check whether the start is non-zero in `ScalarEvolution::howFarToZero` (#131522)

https://github.com/llvm/llvm-project/pull/94525 assumes that the loop
will be infinite when the stride is zero. However, it doesn't hold when
the start value of addrec is also zero.

Closes https://github.com/llvm/llvm-project/issues/131465.

1 files changed, 5 insertions, 4 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 8f74c1c..314baa7 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10635,10 +10635,11 @@ ScalarEvolution::ExitLimit ScalarEvolution::howFarToZero(const SCEV *V,
   if (ControlsOnlyExit && AddRec->hasNoSelfWrap() &&
       loopHasNoAbnormalExits(AddRec->getLoop())) {
 
-    // If the stride is zero, the loop must be infinite.  In C++, most loops
-    // are finite by assumption, in which case the step being zero implies
-    // UB must execute if the loop is entered.
-    if (!loopIsFiniteByAssumption(L) && !isKnownNonZero(StepWLG))
+    // If the stride is zero and the start is non-zero, the loop must be
+    // infinite. In C++, most loops are finite by assumption, in which case the
+    // step being zero implies UB must execute if the loop is entered.
+    if (!(loopIsFiniteByAssumption(L) && isKnownNonZero(Start)) &&
+        !isKnownNonZero(StepWLG))
       return getCouldNotCompute();
 
     const SCEV *Exact =