diff --git a/flang/runtime/complex-reduction.h b/flang/runtime/complex-reduction.h
--- a/flang/runtime/complex-reduction.h
+++ b/flang/runtime/complex-reduction.h
@@ -16,6 +16,7 @@
 #define FORTRAN_RUNTIME_COMPLEX_REDUCTION_H_
 
 #include "entry-names.h"
+#include <complex.h>
 
 struct CppDescriptor; /* dummy type name for Fortran::runtime::Descriptor */
 
diff --git a/flang/runtime/complex-reduction.c b/flang/runtime/complex-reduction.c
--- a/flang/runtime/complex-reduction.c
+++ b/flang/runtime/complex-reduction.c
@@ -9,7 +9,6 @@
 
 #include "complex-reduction.h"
 #include "flang/Common/long-double.h"
-#include <complex.h>
 
 struct CppComplexFloat {
   float r, i;
diff --git a/flang/runtime/reduction.h b/flang/runtime/reduction.h
--- a/flang/runtime/reduction.h
+++ b/flang/runtime/reduction.h
@@ -51,8 +51,10 @@
     int line, int dim = 0, const Descriptor *mask = nullptr);
 std::int64_t RTNAME(SumInteger8)(const Descriptor &, const char *source,
     int line, int dim = 0, const Descriptor *mask = nullptr);
+#ifdef __SIZEOF_INT128__
 common::int128_t RTNAME(SumInteger16)(const Descriptor &, const char *source,
     int line, int dim = 0, const Descriptor *mask = nullptr);
+#endif
 
 // REAL/COMPLEX(2 & 3) return 32-bit float results for the caller to downconvert
 float RTNAME(SumReal2)(const Descriptor &, const char *source, int line,
@@ -100,9 +102,11 @@
     int line, int dim = 0, const Descriptor *mask = nullptr);
 std::int64_t RTNAME(ProductInteger8)(const Descriptor &, const char *source,
     int line, int dim = 0, const Descriptor *mask = nullptr);
+#ifdef __SIZEOF_INT128__
 common::int128_t RTNAME(ProductInteger16)(const Descriptor &,
     const char *source, int line, int dim = 0,
     const Descriptor *mask = nullptr);
+#endif
 
 // REAL/COMPLEX(2 & 3) return 32-bit float results for the caller to downconvert
 float RTNAME(ProductReal2)(const Descriptor &, const char *source, int line,
@@ -166,8 +170,10 @@
     int line, int dim = 0, const Descriptor *mask = nullptr);
 std::int64_t RTNAME(MaxvalInteger8)(const Descriptor &, const char *source,
     int line, int dim = 0, const Descriptor *mask = nullptr);
+#ifdef __SIZEOF_INT128__
 common::int128_t RTNAME(MaxvalInteger16)(const Descriptor &, const char *source,
     int line, int dim = 0, const Descriptor *mask = nullptr);
+#endif
 float RTNAME(MaxvalReal2)(const Descriptor &, const char *source, int line,
     int dim = 0, const Descriptor *mask = nullptr);
 float RTNAME(MaxvalReal3)(const Descriptor &, const char *source, int line,
@@ -191,8 +197,10 @@
     int line, int dim = 0, const Descriptor *mask = nullptr);
 std::int64_t RTNAME(MivalInteger8)(const Descriptor &, const char *source,
     int line, int dim = 0, const Descriptor *mask = nullptr);
+#ifdef __SIZEOF_INT128__
 common::int128_t RTNAME(MivalInteger16)(const Descriptor &, const char *source,
     int line, int dim = 0, const Descriptor *mask = nullptr);
+#endif
 float RTNAME(MinvalReal2)(const Descriptor &, const char *source, int line,
     int dim = 0, const Descriptor *mask = nullptr);
 float RTNAME(MinvalReal3)(const Descriptor &, const char *source, int line,
diff --git a/flang/runtime/reduction.cpp b/flang/runtime/reduction.cpp
--- a/flang/runtime/reduction.cpp
+++ b/flang/runtime/reduction.cpp
@@ -253,11 +253,13 @@
           TypeCategory::Integer, 8>(
           result, x, dim, mask, terminator, intrinsic);
       return;
+#ifdef __SIZEOF_INT128__
     case 16:
       PartialReduction<INTEGER_ACCUM<CppTypeFor<TypeCategory::Integer, 16>>,
           TypeCategory::Integer, 16>(
           result, x, dim, mask, terminator, intrinsic);
       return;
+#endif
     }
     break;
   case TypeCategory::Real:
@@ -416,12 +418,14 @@
   return GetTotalReduction<TypeCategory::Integer, 8>(x, source, line, dim, mask,
       IntegerSumAccumulator<CppTypeFor<TypeCategory::Integer, 8>>{x}, "SUM");
 }
+#ifdef __SIZEOF_INT128__
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(SumInteger16)(const Descriptor &x,
     const char *source, int line, int dim, const Descriptor *mask) {
   return GetTotalReduction<TypeCategory::Integer, 16>(x, source, line, dim,
       mask, IntegerSumAccumulator<CppTypeFor<TypeCategory::Integer, 16>>{x},
       "SUM");
 }
+#endif
 
 // TODO: real/complex(2 & 3)
 CppTypeFor<TypeCategory::Real, 4> RTNAME(SumReal4)(const Descriptor &x,
@@ -549,6 +553,7 @@
       NonComplexProductAccumulator<CppTypeFor<TypeCategory::Integer, 8>>{x},
       "PRODUCT");
 }
+#ifdef __SIZEOF_INT128__
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(ProductInteger16)(
     const Descriptor &x, const char *source, int line, int dim,
     const Descriptor *mask) {
@@ -557,6 +562,7 @@
       NonComplexProductAccumulator<CppTypeFor<TypeCategory::Integer, 16>>{x},
       "PRODUCT");
 }
+#endif
 
 // TODO: real/complex(2 & 3)
 CppTypeFor<TypeCategory::Real, 4> RTNAME(ProductReal4)(const Descriptor &x,
@@ -725,10 +731,12 @@
     accumulator.GetResult(
         result.OffsetElement<CppTypeFor<TypeCategory::Integer, 8>>());
     break;
+#ifdef __SIZEOF_INT128__
   case 16:
     accumulator.GetResult(
         result.OffsetElement<CppTypeFor<TypeCategory::Integer, 16>>());
     break;
+#endif
   default:
     terminator.Crash("%s: bad KIND=%d", intrinsic, kind);
   }
@@ -786,10 +794,12 @@
       DoMaxOrMinLoc<TypeCategory::Integer, 8, IS_MAX, NumericCompare>(
           intrinsic, result, x, kind, source, line, mask, back);
       return;
+#ifdef __SIZEOF_INT128__
     case 16:
       DoMaxOrMinLoc<TypeCategory::Integer, 16, IS_MAX, NumericCompare>(
           intrinsic, result, x, kind, source, line, mask, back);
       return;
+#endif
     }
     break;
   case TypeCategory::Real:
@@ -875,10 +885,12 @@
     PartialReduction<ExtremumLocAccumulator<COMPARE<CppType, IS_MAX, BACK>>,
         TypeCategory::Integer, 8>(result, x, dim, mask, terminator, intrinsic);
     break;
+#ifdef __SIZEOF_INT128__
   case 16:
     PartialReduction<ExtremumLocAccumulator<COMPARE<CppType, IS_MAX, BACK>>,
         TypeCategory::Integer, 16>(result, x, dim, mask, terminator, intrinsic);
     break;
+#endif
   default:
     terminator.Crash("%s: bad KIND=%d", intrinsic, kind);
   }
@@ -925,10 +937,12 @@
       DoPartialMaxOrMinLoc<TypeCategory::Integer, 8, IS_MAX, NumericCompare>(
           intrinsic, result, x, kind, dim, mask, back, terminator);
       return;
+#ifdef __SIZEOF_INT128__
     case 16:
       DoPartialMaxOrMinLoc<TypeCategory::Integer, 16, IS_MAX, NumericCompare>(
           intrinsic, result, x, kind, dim, mask, back, terminator);
       return;
+#endif
     }
     break;
   case TypeCategory::Real:
@@ -1101,11 +1115,13 @@
           NumericExtremumAccumulator>(
           result, x, dim, mask, intrinsic, terminator);
       return;
+#ifdef __SIZEOF_INT128__
     case 16:
       DoMaxOrMin<TypeCategory::Integer, 16, IS_MAXVAL,
           NumericExtremumAccumulator>(
           result, x, dim, mask, intrinsic, terminator);
       return;
+#endif
     }
     break;
   case TypeCategory::Real:
@@ -1223,12 +1239,14 @@
   return TotalNumericMaxOrMin<TypeCategory::Integer, 8, true>(
       x, source, line, dim, mask, "MAXVAL");
 }
+#ifdef __SIZEOF_INT128__
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(MaxvalInteger16)(
     const Descriptor &x, const char *source, int line, int dim,
     const Descriptor *mask) {
   return TotalNumericMaxOrMin<TypeCategory::Integer, 16, true>(
       x, source, line, dim, mask, "MAXVAL");
 }
+#endif
 
 // TODO: REAL(2 & 3)
 CppTypeFor<TypeCategory::Real, 4> RTNAME(MaxvalReal4)(const Descriptor &x,
@@ -1280,12 +1298,14 @@
   return TotalNumericMaxOrMin<TypeCategory::Integer, 8, false>(
       x, source, line, dim, mask, "MINVAL");
 }
+#ifdef __SIZEOF_INT128__
 CppTypeFor<TypeCategory::Integer, 16> RTNAME(MinvalInteger16)(
     const Descriptor &x, const char *source, int line, int dim,
     const Descriptor *mask) {
   return TotalNumericMaxOrMin<TypeCategory::Integer, 16, false>(
       x, source, line, dim, mask, "MINVAL");
 }
+#endif
 
 // TODO: REAL(2 & 3)
 CppTypeFor<TypeCategory::Real, 4> RTNAME(MinvalReal4)(const Descriptor &x,
@@ -1513,9 +1533,11 @@
   case 8:
     CountDimension<8>(result, x, dim, terminator);
     break;
+#ifdef __SIZEOF_INT128__
   case 16:
     CountDimension<16>(result, x, dim, terminator);
     break;
+#endif
   default:
     terminator.Crash("COUNT: bad KIND=%d", kind);
   }