[flang] Don't fold REPEAT() when the result would be too large

[klausler]

Summary

A test program shouldn't be able to crash the compiler by getting it to
fold REPEAT() with an absurdly large repetition count.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Evaluate/character.h

for (; j >= 1; --j) {
       }
     }
     return static_cast<ConstantSubscript>(j);
   }
 
   static Character REPEAT(const Character &str, ConstantSubscript ncopies) {
     Character result;
     if (!str.empty()) {
+      result.reserve(ncopies * str.size());
       while (ncopies-- > 0) {
         result += str;
       }
     }
     return result;
   }
 
   static Character TRIM(const Character &str) {

flang/lib/Evaluate/fold-character.cpp

if (auto identity{Identity<T>(
       return FoldMaxvalMinval<T>(
           context, std::move(funcRef), RelationalOperator::LT, *identity);
     }
   } else if (name == "new_line") {
     return Expr<T>{Constant<T>{CharacterUtils<KIND>::NEW_LINE()}};
   } else if (name == "repeat") { // not elemental
     if (auto scalars{GetScalarConstantArguments<T, SubscriptInteger>(
             context, funcRef.arguments())}) {
-      return Expr<T>{Constant<T>{
-          CharacterUtils<KIND>::REPEAT(std::get<Scalar<T>>(*scalars),
-              std::get<Scalar<SubscriptInteger>>(*scalars).ToInt64())}};
+      auto str{std::get<Scalar<T>>(*scalars)};
+      auto n{std::get<Scalar<SubscriptInteger>>(*scalars).ToInt64()};
+      if (static_cast<double>(n) * str.size() >
+          (1 << 20)) { // sanity limit of 1MiB
+        context.messages().Say(
+            "Result of REPEAT() is too large to compute at compilation time (%g characters)"_port_en_US,
+            static_cast<double>(n) * str.size());
+      } else {
+        return Expr<T>{Constant<T>{CharacterUtils<KIND>::REPEAT(str, n)}};
+      }
     }
   } else if (name == "trim") { // not elemental
     if (auto scalar{
             GetScalarConstantArguments<T>(context, funcRef.arguments())}) {
       return Expr<T>{Constant<T>{
           CharacterUtils<KIND>::TRIM(std::get<Scalar<T>>(*scalar))}};
     }
   }

flang/test/Evaluate/errors01.f90

contains
   subroutine s12(x,y)
     class(t), intent(in) :: x
     class(*), intent(in) :: y
     !CHERK: error: Must be a constant value
     integer, parameter :: bad1 = storage_size(x)
     !CHERK: error: Must be a constant value
     integer, parameter :: bad2 = storage_size(y)
   end subroutine
+  subroutine s13
+    !CHECK: portability: Result of REPEAT() is too large to compute at compilation time (1.1259e+15 characters)
+    print *, repeat(repeat(' ', 2**20), 2**30)
+  end subroutine
   subroutine warnings
     real, parameter :: ok1 = scale(0.0, 99999) ! 0.0
     real, parameter :: ok2 = scale(1.0, -99999) ! 0.0
     !CHECK: SCALE intrinsic folding overflow
     real, parameter :: bad1 = scale(1.0, 99999)
     !CHECK: complex ABS intrinsic folding overflow
     real, parameter :: bad2 = abs(cmplx(huge(0.),huge(0.)))
     !CHECK: warning: overflow on REAL(8) to REAL(4) conversion
     x = 1.D40
   end subroutine
 end module