codegen: improve our argument annotations (#35463)

Reflect that immutable struct parameters are readonly by the time LLVM
can see them, even passed inside boxes.

LLVM seems to require an explicit additional LICM call, to avoid
regressions from this (observing the scheduling of fewer LICM calls).

Author: vtjnash

src/aotcompile.cpp

@@ -676,6 +676,7 @@ void addOptimizationPasses(legacy::PassManagerBase *PM, int opt_level,
     PM->add(createLowerSimdLoopPass()); // Annotate loop marked with "loopinfo" as LLVM parallel loop
     PM->add(createLICMPass());
     PM->add(createLoopUnswitchPass());
+    PM->add(createLICMPass());
     // Subsequent passes not stripping metadata from terminator
     PM->add(createInstSimplifyLegacyPass());
     PM->add(createIndVarSimplifyPass());

src/codegen.cpp

@@ -4997,7 +4997,8 @@ static jl_returninfo_t get_specsig_function(jl_codectx_t &ctx, Module *M, String
         jl_value_t *jt = jl_tparam(sig, i);
         if (is_uniquerep_Type(jt))
             continue;
-        Type *ty = deserves_argbox(jt) ? T_prjlvalue : julia_type_to_llvm(ctx, jt);
+        bool isboxed = deserves_argbox(jt);
+        Type *ty = isboxed ? T_prjlvalue : julia_type_to_llvm(ctx, jt);
         if (type_is_ghost(ty))
             continue;
         unsigned argno = fsig.size();
@@ -5006,6 +5007,9 @@ static jl_returninfo_t get_specsig_function(jl_codectx_t &ctx, Module *M, String
             attributes = attributes.addParamAttribute(jl_LLVMContext, argno, Attribute::ReadOnly);
             ty = PointerType::get(ty, AddressSpace::Derived);
         }
+        else if (isboxed && jl_is_immutable_datatype(jt)) {
+            attributes = attributes.addParamAttribute(jl_LLVMContext, argno, Attribute::ReadOnly);
+        }
         fsig.push_back(ty);
     }
 
@@ -5581,6 +5585,8 @@ static std::pair<std::unique_ptr<Module>, jl_llvm_functions_t>
             if (isboxed) // e.g. is-pointer
                 maybe_mark_argument_dereferenceable(Arg, argType);
             theArg = mark_julia_type(ctx, Arg, isboxed, argType);
+            if (theArg.tbaa == tbaa_immut)
+                theArg.tbaa = tbaa_const;
         }
         return theArg;
     };