Emit aliases into the system image

- Put the interposer in llvm.compiler.used.
- Injecting the aliases after optimization:
  Our multiversioning pass interacts badly with the llvm.compiler.used gvar.

Co-authored-by: Tim Besard <tim.besard@gmail.com>
Co-authored-by: Valentin Churavy <v.churavy@gmail.com>

src/aotcompile.cpp

@@ -20,6 +20,7 @@
 #include <llvm/Analysis/BasicAliasAnalysis.h>
 #include <llvm/Analysis/TypeBasedAliasAnalysis.h>
 #include <llvm/Analysis/ScopedNoAliasAA.h>
+#include <llvm/IR/IRBuilder.h>
 #include <llvm/IR/PassManager.h>
 #include <llvm/IR/Verifier.h>
 #include <llvm/Transforms/IPO.h>
@@ -32,6 +33,7 @@
 #include <llvm/Transforms/InstCombine/InstCombine.h>
 #include <llvm/Transforms/Scalar/InstSimplifyPass.h>
 #include <llvm/Transforms/Utils/SimplifyCFGOptions.h>
+#include <llvm/Transforms/Utils/ModuleUtils.h>
 #include <llvm/Passes/PassBuilder.h>
 #include <llvm/Passes/PassPlugin.h>
 #if defined(USE_POLLY)
@@ -434,6 +436,23 @@ static void reportWriterError(const ErrorInfoBase &E)
     jl_safe_printf("ERROR: failed to emit output file %s\n", err.c_str());
 }
 
+static void injectCRTAlias(Module &M, StringRef name, StringRef alias, FunctionType *FT)
+{
+    Function *target = M.getFunction(alias);
+    if (!target) {
+        target = Function::Create(FT, Function::ExternalLinkage, alias, M);
+    }
+    Function *interposer = Function::Create(FT, Function::WeakAnyLinkage, name, M);
+    appendToCompilerUsed(M, {interposer});
+
+    llvm::IRBuilder<> builder(BasicBlock::Create(M.getContext(), "top", interposer));
+    SmallVector<Value *, 4> CallArgs;
+    for (auto &arg : interposer->args())
+        CallArgs.push_back(&arg);
+    auto val = builder.CreateCall(target, CallArgs);
+    builder.CreateRet(val);
+}
+
 
 // takes the running content that has collected in the shadow module and dump it to disk
 // this builds the object file portion of the sysimage files for fast startup
@@ -550,7 +569,24 @@ void jl_dump_native_impl(void *native_code,
     auto add_output = [&] (Module &M, StringRef unopt_bc_Name, StringRef bc_Name, StringRef obj_Name, StringRef asm_Name) {
         preopt.run(M);
         optimizer.run(M);
+
+        // We would like to emit an alias or an weakref alias to redirect these symbols
+        // but LLVM doesn't let us emit a GlobalAlias to a declaration...
+        // So for now we inject a definition of these functions that calls our runtime
+        // functions. We do so after optimization to avoid cloning these functions.
+        injectCRTAlias(M, "__gnu_h2f_ieee", "julia__gnu_h2f_ieee",
+                FunctionType::get(Type::getFloatTy(Context), { Type::getHalfTy(Context) }, false));
+        injectCRTAlias(M, "__extendhfsf2", "julia__gnu_h2f_ieee",
+                FunctionType::get(Type::getFloatTy(Context), { Type::getHalfTy(Context) }, false));
+        injectCRTAlias(M, "__gnu_f2h_ieee", "julia__gnu_f2h_ieee",
+                FunctionType::get(Type::getHalfTy(Context), { Type::getFloatTy(Context) }, false));
+        injectCRTAlias(M, "__truncsfhf2", "julia__gnu_f2h_ieee",
+                FunctionType::get(Type::getHalfTy(Context), { Type::getFloatTy(Context) }, false));
+        injectCRTAlias(M, "__truncdfhf2", "julia__truncdfhf2",
+                FunctionType::get(Type::getHalfTy(Context), { Type::getDoubleTy(Context) }, false));
+
         postopt.run(M);
+
         if (unopt_bc_fname)
             emit_result(unopt_bc_Archive, unopt_bc_Buffer, unopt_bc_Name, outputs);
         if (bc_fname)