Fix LTO

Author: antoyo

src/back/write.rs

@@ -1,6 +1,6 @@
 use std::{env, fs};
 
-use gccjit::OutputKind;
+use gccjit::{Context, OutputKind};
 use rustc_codegen_ssa::back::link::ensure_removed;
 use rustc_codegen_ssa::back::write::{BitcodeSection, CodegenContext, EmitObj, ModuleConfig};
 use rustc_codegen_ssa::{CompiledModule, ModuleCodegen};
@@ -176,10 +176,45 @@ pub(crate) unsafe fn codegen(
                     context.add_driver_option("-nostdlib");
 
                     // NOTE: this doesn't actually generate an executable. With the above flags, it combines the .o files together in another .o.
-                    context.compile_to_file(
+                    // FIXME FIXME: this produces an object file with GIMPLE IR, but it should
+                    // produce an object file with machine code.
+                    //println!("LTO-ed object file: {:?}", obj_out);
+                    /*context.compile_to_file(
                         OutputKind::Executable,
                         obj_out.to_str().expect("path to str"),
-                    );
+                    );*/
+
+                    let path = obj_out.to_str().expect("path to str");
+
+                    let lto_path = format!("{}.lto", path);
+                    context.compile_to_file(OutputKind::Executable, &lto_path);
+                    //println!("****************************************************************************************************");
+
+                    let context = Context::default(); // TODO: might need to set some other flags from new_context.
+                    //context.add_driver_option("-v");
+                    //println!("*** Arch: {}", cgcx.target_arch);
+                    if cgcx.target_arch == "x86" || cgcx.target_arch == "x86_64" {
+                        //println!("**** Added -masm=intel");
+                        //context.add_command_line_option("-masm=intel");
+                        // NOTE: it seems we need to use add_driver_option instead of
+                        // add_command_line_option here because we use the LTO frontend via gcc.
+                        context.add_driver_option("-masm=intel");
+                    }
+
+                    // NOTE: these two options are needed to invoke LTO to produce an object file.
+                    // We need to initiate a second compilation because the arguments "-x lto"
+                    // needs to be at the very beginning.
+                    // TODO TODO: check that LTO still does the optimizations across different
+                    // object files with this change.
+                    // TODO: this should probably be in a condition `if fat_lto`.
+                    context.add_driver_option("-x");
+                    context.add_driver_option("lto");
+                    context.add_driver_option("-fPIC"); // TODO TODO: only add this flag when necessary.
+                    context.add_driver_option(lto_path);
+
+                    // FIXME FIXME: it seems this object file doesn't contain the main function and
+                    // some other code.
+                    context.compile_to_file(OutputKind::ObjectFile, path);
                 } else {
                     context.compile_to_file(
                         OutputKind::ObjectFile,