diff --git a/dmd/mtype.d b/dmd/mtype.d
index 9b210528f7d..cf1e58fde29 100644
--- a/dmd/mtype.d
+++ b/dmd/mtype.d
@@ -892,10 +892,38 @@ version (IN_LLVM)
         twstring = twchar.immutableOf().arrayOf();
         tdstring = tdchar.immutableOf().arrayOf();
 
+version (IN_LLVM)
+{
+        switch (target.ptrsize)
+        {
+            case 1:
+                tsize_t    = basic[Tuns8];
+                tptrdiff_t = basic[Tint8];
+                break;
+            case 2:
+                tsize_t    = basic[Tuns16];
+                tptrdiff_t = basic[Tint16];
+                break;
+            case 4:
+                tsize_t    = basic[Tuns32];
+                tptrdiff_t = basic[Tint32];
+                break;
+            case 8:
+                tsize_t    = basic[Tuns64];
+                tptrdiff_t = basic[Tint64];
+                break;
+            default:
+                assert(0, "Unsupported target pointer size");
+        }
+}
+else
+{
         const isLP64 = target.isLP64;
 
         tsize_t    = basic[isLP64 ? Tuns64 : Tuns32];
         tptrdiff_t = basic[isLP64 ? Tint64 : Tint32];
+}
+
         thash_t = tsize_t;
     }
 
diff --git a/runtime/druntime/src/object.d b/runtime/druntime/src/object.d
index 28c6db1b009..a737ba0ca37 100644
--- a/runtime/druntime/src/object.d
+++ b/runtime/druntime/src/object.d
@@ -2765,12 +2765,12 @@ class Exception : Throwable
      * This constructor does not automatically throw the newly-created
      * Exception; the $(D throw) expression should be used for that purpose.
      */
-    @nogc @safe pure nothrow this(string msg, string file = __FILE__, size_t line = __LINE__, Throwable nextInChain = null)
+    @nogc @safe pure nothrow this(string msg, string file = __FILE__, size_t line = cast(size_t) __LINE__, Throwable nextInChain = null)
     {
         super(msg, file, line, nextInChain);
     }
 
-    @nogc @safe pure nothrow this(string msg, Throwable nextInChain, string file = __FILE__, size_t line = __LINE__)
+    @nogc @safe pure nothrow this(string msg, Throwable nextInChain, string file = __FILE__, size_t line = cast(size_t) __LINE__)
     {
         super(msg, file, line, nextInChain);
     }
@@ -2797,7 +2797,7 @@ class Exception : Throwable
     {
         auto e = new Exception("msg");
         assert(e.file == __FILE__);
-        assert(e.line == __LINE__ - 2);
+        assert(e.line == cast(size_t) (__LINE__ - 2));
         assert(e.nextInChain is null);
         assert(e.msg == "msg");
     }
diff --git a/tests/compilable/size_t_16bit.d b/tests/compilable/size_t_16bit.d
new file mode 100644
index 00000000000..53e8681e63f
--- /dev/null
+++ b/tests/compilable/size_t_16bit.d
@@ -0,0 +1,13 @@
+// A minimal test wrt. size_t on a 16-bit architecture.
+
+// REQUIRES: target_AVR
+// RUN: %ldc -mtriple=avr -betterC -c %s
+
+static assert(size_t.sizeof == 2);
+static assert(ptrdiff_t.sizeof == 2);
+
+// test bounds checks
+int foo(int[] arr)
+{
+    return arr[1];
+}