Fix infinite recursion in FindIntrinsics (#9109)

Author: abadams

src/FindIntrinsics.cpp

@@ -128,6 +128,13 @@ class FindIntrinsics : public IRMutator {
             Expr a = c->args[0];
             Expr b = c->args[1];
 
+            if (Call::as_intrinsic(b, {Call::signed_integer_overflow})) {
+                // Letting signed integer overflow through here would result in
+                // infinite recursion when we try to construct the rounding
+                // terms.
+                return Expr{};
+            }
+
             // Helper to make the appropriate shift.
             auto rounding_shift = [&](const Expr &a, const Expr &b) {
                 if (c->is_intrinsic(Call::shift_right)) {
@@ -203,7 +210,7 @@ class FindIntrinsics : public IRMutator {
             }
         }
 
-        return Expr();
+        return Expr{};
     }
 
     template<typename LetOrLetStmt>

test/correctness/intrinsics.cpp

@@ -346,6 +346,11 @@ int main(int argc, char **argv) {
     check_intrinsics_over_range<int32_t>();
     check_intrinsics_over_range<uint32_t>();
 
+    // Signed integer overflow on the RHS of a shift used to cause infinite
+    // recursion (issue #9108)
+    Expr e = simplify(i32x << (i32x << 67));
+    check(e, e);
+
     // The intrinsics-matching pass substitutes in widening lets. At
     // one point this caused a missing symbol bug for the code below
     // due to a subexpression not getting mutated.