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

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.

artiq/compiler/targets.py

@@ -89,6 +89,10 @@ class Target:
         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()

artiq/compiler/transforms/llvm_ir_generator.py

@@ -181,6 +181,14 @@ class LLVMIRGenerator:
         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 @@ class LLVMIRGenerator:
         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 @@ class LLVMIRGenerator:
         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):