compiler: Emit all-kernel_invariant objects as LLVM constants

This enables constant propagation optimisations, as verified by
the included test case. This is only a first stop-gap measure, though;
we should support optimisation based on kernel invariants on a more
fine-grained level.

artiq/compiler/transforms/llvm_ir_generator.py

@@ -1384,6 +1384,7 @@ class LLVMIRGenerator:
         def _quote_attributes():
             llglobal = None
             llfields = []
+            emit_as_constant = True
             for attr in typ.attributes:
                 if attr == "__objectid__":
                     objectid = self.embedding_map.store_object(value)
@@ -1404,6 +1405,8 @@ class LLVMIRGenerator:
                                          not types.is_c_function(typ.attributes[attr]))
                     if is_class_function:
                         attrvalue = self.embedding_map.specialize_function(typ.instance, attrvalue)
+                    if not (types.is_instance(typ) and attr in typ.constant_attributes):
+                        emit_as_constant = False
                     llattrvalue = self._quote(attrvalue, typ.attributes[attr],
                                               lambda: path() + [attr])
                     llfields.append(llattrvalue)
@@ -1411,6 +1414,7 @@ class LLVMIRGenerator:
                         llclosureptr = self.get_global_closure_ptr(typ, attr)
                         llclosureptr.initializer = llattrvalue
 
+            llglobal.global_constant = emit_as_constant
             llglobal.initializer = ll.Constant(llty.pointee, llfields)
             llglobal.linkage = "private"
             return llglobal

artiq/test/lit/embedding/invariant_propagation.py

@@ -0,0 +1,26 @@
+# RUN: env ARTIQ_DUMP_LLVM=%t %python -m artiq.compiler.testbench.embedding +compile %s
+# RUN: OutputCheck %s --file-to-check=%t.ll
+
+from artiq.language.core import *
+from artiq.language.types import *
+
+class Class:
+    kernel_invariants = {"foo"}
+
+    def __init__(self):
+        self.foo = True
+
+    @kernel
+    def run(self):
+        if self.foo:
+            print("bar")
+        else:
+            # Make sure all the code for this branch will be completely elided:
+            # CHECK-NOT: baz
+            print("baz")
+
+obj = Class()
+
+@kernel
+def entrypoint():
+    obj.run()