compiler: mark FFI functions as ModRef=Ref using TBAA metadata.

whitequark · whitequark · commit 1d8b0d46bc6d · 2016-03-28T02:52:15.000Z
Fascinatingly, the fact that you can mark call instructions with
!tbaa metadata is completely undocumented. Regardless, it is true:
a !tbaa metadata for an "immutable" type will cause
AliasAnalysis::getModRefBehavior to return OnlyReadsMemory for that
call site.

Don't bother marking loads with TBAA yet since we already place
!load.invariant on them (which is as good as the TBAA "immutable"
flag) and after that we're limited by lack of !nonnull anyway.

Also, add TBAA analysis passes in our pipeline to actually engage it.
diff --git a/artiq/compiler/targets.py b/artiq/compiler/targets.py
@@ -89,6 +89,10 @@ def optimize(self, llmodule):
         llpassmgr = llvm.create_module_pass_manager()
         self.target_machine().target_data.add_pass(llpassmgr)
 
+        # Register our alias analysis passes.
+        llpassmgr.add_basic_alias_analysis_pass()
+        llpassmgr.add_type_based_alias_analysis_pass()
+
         # Start by cleaning up after our codegen and exposing as much
         # information to LLVM as possible.
         llpassmgr.add_constant_merge_pass()
diff --git a/artiq/compiler/transforms/llvm_ir_generator.py b/artiq/compiler/transforms/llvm_ir_generator.py
@@ -181,6 +181,14 @@ def __init__(self, engine, module_name, target, function_map, object_map, type_m
         self.phis = []
         self.debug_info_emitter = DebugInfoEmitter(self.llmodule)
         self.empty_metadata = self.llmodule.add_metadata([])
+        self.tbaa_tree = self.llmodule.add_metadata([
+            ll.MetaDataString(self.llmodule, "ARTIQ TBAA")
+        ])
+        self.tbaa_noalias_call = self.llmodule.add_metadata([
+            ll.MetaDataString(self.llmodule, "non-aliasing function call"),
+            self.tbaa_tree,
+            ll.Constant(lli64, 1)
+        ])
 
     def needs_sret(self, lltyp, may_be_large=True):
         if isinstance(lltyp, ll.VoidType):
@@ -1252,17 +1260,18 @@ def ret_error_handler(typ):
         return llret
 
     def process_Call(self, insn):
-        if types.is_rpc_function(insn.target_function().type):
+        functiontyp = insn.target_function().type
+        if types.is_rpc_function(functiontyp):
             return self._build_rpc(insn.target_function().loc,
-                                   insn.target_function().type,
+                                   functiontyp,
                                    insn.arguments(),
                                    llnormalblock=None, llunwindblock=None)
-        elif types.is_c_function(insn.target_function().type):
+        elif types.is_c_function(functiontyp):
             llfun, llargs = self._prepare_ffi_call(insn)
         else:
             llfun, llargs = self._prepare_closure_call(insn)
 
-        if self.has_sret(insn.target_function().type):
+        if self.has_sret(functiontyp):
             llstackptr = self.llbuilder.call(self.llbuiltin("llvm.stacksave"), [])
 
             llresultslot = self.llbuilder.alloca(llfun.type.pointee.args[0].pointee)
@@ -1276,6 +1285,11 @@ def process_Call(self, insn):
         if insn.is_cold:
             llcall.cconv = 'coldcc'
 
+        if types.is_c_function(functiontyp):
+            # All our global state is confined to our compilation unit,
+            # so by definition no FFI call can mutate it.
+            llcall.metadata['tbaa'] = self.tbaa_noalias_call
+
         return llresult
 
     def process_Invoke(self, insn):
