compiler: the len builtin is not polymorphic, coerce the result.

Fixes #659.

artiq/compiler/transforms/artiq_ir_generator.py

@@ -482,8 +482,9 @@ class ARTIQIRGenerator(algorithm.Visitor):
                 name = None
             else:
                 name = "{}.len".format(value.name)
-            return self.append(ir.Builtin("len", [value], typ,
+            len = self.append(ir.Builtin("len", [value], builtins.TInt32(),
                                          name=name))
+            return self.append(ir.Coerce(len, typ))
         elif builtins.is_range(value.type):
             start  = self.append(ir.GetAttr(value, "start"))
             stop   = self.append(ir.GetAttr(value, "stop"))

artiq/test/lit/regression/bug_659.py

@@ -0,0 +1,17 @@
+# RUN: %python -m artiq.compiler.testbench.embedding %s
+
+from artiq.language.core import *
+from artiq.language.types import *
+import numpy as np
+
+@kernel
+def rotate(array):
+    '''Rotates an array, deleting the oldest value'''
+    length = len(array)
+    for i in range(np.int64(len(array)) - 1):
+        array[length - i - 1] = array[length - i - 2]
+    array[0] = 0
+
+@kernel
+def entrypoint():
+    rotate([1,2,3,4])

artiq/test/lit/regression/device_db.pyon

@@ -0,0 +1,14 @@
+{
+    "comm": {
+        "type": "local",
+        "module": "artiq.coredevice.comm_dummy",
+        "class": "Comm",
+        "arguments": {}
+    },
+    "core": {
+        "type": "local",
+        "module": "artiq.coredevice.core",
+        "class": "Core",
+        "arguments": {"ref_period": 1e-9}
+    }
+}