transforms.cfg_simplifier: implement.

This allows error reporting in LocalAccessValidator to proceed
even when the predecessor is an empty block with only a branch.

artiq/compiler/module.py

@@ -55,6 +55,7 @@ class Module:
         artiq_ir_generator = transforms.ARTIQIRGenerator(engine=self.engine,
                                                          module_name=src.name,
                                                          ref_period=ref_period)
+        cfg_simplifier = transforms.CFGSimplifier(engine=self.engine)
         dead_code_eliminator = transforms.DeadCodeEliminator(engine=self.engine)
         local_access_validator = validators.LocalAccessValidator(engine=self.engine)
         devirtualization = analyses.Devirtualization()
@@ -70,6 +71,7 @@ class Module:
         devirtualization.visit(src.typedtree)
         self.artiq_ir = artiq_ir_generator.visit(src.typedtree)
         artiq_ir_generator.annotate_calls(devirtualization)
+        cfg_simplifier.process(self.artiq_ir)
         local_access_validator.process(self.artiq_ir)
         dead_code_eliminator.process(self.artiq_ir)
         interleaver.process(self.artiq_ir)

artiq/compiler/transforms/__init__.py

@@ -4,5 +4,6 @@ from .int_monomorphizer import IntMonomorphizer
 from .iodelay_estimator import IODelayEstimator
 from .artiq_ir_generator import ARTIQIRGenerator
 from .dead_code_eliminator import DeadCodeEliminator
+from .cfg_simplifier import CFGSimplifier
 from .llvm_ir_generator import LLVMIRGenerator
 from .interleaver import Interleaver

artiq/compiler/transforms/cfg_simplifier.py

@@ -0,0 +1,22 @@
+"""
+:class:`CFGSimplifier` is a simple control flow graph
+simplification transform: it removes empty basic blocks.
+"""
+
+from .. import ir
+
+class CFGSimplifier:
+    def __init__(self, engine):
+        self.engine = engine
+
+    def process(self, functions):
+        for func in functions:
+            self.process_function(func)
+
+    def process_function(self, func):
+        for block in list(func.basic_blocks):
+            if len(block.instructions) == 1 and \
+                    isinstance(block.terminator(), ir.Branch):
+                successor, = block.successors()
+                block.replace_all_uses_with(successor)
+                block.erase()