Added --target option for overriding target triple (#1)

* Added --target option for overriding target triple

Helps with cross-compilation when the target triple does not match the
machine you're building on.

* Updated README.md with flags

* Implement target auto-detection detection

Based on llvm-config --host-target

* Dropped manual check for host target, llvm does this for us be default

Author: geky

README.md

@@ -19,10 +19,10 @@ FLAGS:
 
 OPTIONS:
     -o, --output <output>    Output bc file
+        --target <target>    Set compilation target
 
 ARGS:
     <input>    Input wasm file
-
 ```
 
 # Installation from Source
@@ -46,7 +46,6 @@ sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-$LLVM_VER
 sudo update-alternatives --install /usr/bin/llvm-config llvm-config /usr/bin/llvm-config-$LLVM_VERSION 100
 ```
 4. [Install the C++ Standard Library for LLVM](https://libcxx.llvm.org/)
-​
 5. Clone and build silverfish
 ```sh
 git clone https://github.com/gwsystems/silverfish.git

src/codegen/mod.rs

@@ -53,10 +53,11 @@ pub fn process_to_llvm(
     let llvm_ctx = &*LLVMCtx::new();
     let llvm_module = &*LLVMModule::new(&wasm_module.source_name, llvm_ctx);
 
-    // Setup to compile to the local target
-    // FIXME: Should be able to use the local default target, but that doesn't work properly on OSX
-    llvm_module.set_target("x86_64-apple-macosx10.14.0");
-    //    llvm_module.set_target("i686-pc-linux-gnu");
+    // Accept --target to compile for specific target, otherwise omit target
+    // triple from bytecode, this defaults to the host target in LLVM
+    if let Some(ref target) = opt.target {
+        llvm_module.set_target(target);
+    }
 
     // Remap WASM generated names to exported names
     for e in wasm_module.exports {

src/main.rs

@@ -42,6 +42,10 @@ pub struct Opt {
     /// Don't generate native globals, let the runtime handle it
     #[structopt(long = "runtime-globals")]
     use_runtime_global_handling: bool,
+
+    /// Set compilation target
+    #[structopt(long = "target")]
+    target: Option<String>,
 }
 
 fn main() -> io::Result<()> {