Merge #2635

2635: fix cross-compilation with singlepass compiler r=syrusakbary a=matklad

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# Description

Fix cross-compilation with singlepass compiler

# Review

- [ ] Add a short description of the change to the CHANGELOG.md file


Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>

CHANGELOG.md

@@ -30,6 +30,7 @@ Looking for changes that affect our C API? See the [C API Changelog](lib/c-api/C
 - [#2479](https://github.com/wasmerio/wasmer/pull/2479) Improved `wasmer validate` error message on non-wasm inputs.
 - [#2454](https://github.com/wasmerio/wasmer/issues/2454) Won't set `WASMER_CACHE_DIR` for Windows.
 - [#2426](https://github.com/wasmerio/wasmer/pull/2426) Fix the `wax` script generation.
+- [#2635](https://github.com/wasmerio/wasmer/pull/2635) Fix cross-compilation for singlepass.
 
 ## 2.0.0 - 2021/06/16
 
@@ -193,9 +194,9 @@ This change is unlikely to affect any users of `wasmer`, but if it does please c
 ## 1.0.0-beta2 - 2020-12-16
 
 ### Added
- 
+
 * [#1916](https://github.com/wasmerio/wasmer/pull/1916) Add the `WASMER_VERSION*` constants with the `wasmer_version*` functions in the Wasmer C API
-* [#1867](https://github.com/wasmerio/wasmer/pull/1867) Added `Metering::get_remaining_points` and `Metering::set_remaining_points` 
+* [#1867](https://github.com/wasmerio/wasmer/pull/1867) Added `Metering::get_remaining_points` and `Metering::set_remaining_points`
 * [#1881](https://github.com/wasmerio/wasmer/pull/1881) Added `UnsupportedTarget` error to `CompileError`
 * [#1908](https://github.com/wasmerio/wasmer/pull/1908) Implemented `TryFrom<Value<T>>` for `i32`/`u32`/`i64`/`u64`/`f32`/`f64`
 * [#1927](https://github.com/wasmerio/wasmer/pull/1927) Added mmap support in `Engine::deserialize_from_file` to speed up artifact loading

lib/compiler-singlepass/src/emitter_x64.rs

@@ -2,13 +2,18 @@ pub use crate::x64_decl::{GPR, XMM};
 use dynasm::dynasm;
 use dynasmrt::{x64::Assembler, AssemblyOffset, DynamicLabel, DynasmApi, DynasmLabelApi};
 
-/// Dynasm proc-macro checks for an `.arch` expression in a source file to
-/// determine the architecture it should use.
-fn _dummy(_a: &Assembler) {
-    dynasm!(
-        _a
-        ; .arch x64
-    );
+/// Force `dynasm!` to use the correct arch (x64) when cross-compiling.
+/// `dynasm!` proc-macro tries to auto-detect it by default by looking at the
+/// `target_arch`, but it sees the `target_arch` of the proc-macro itself, which
+/// is always equal to host, even when cross-compiling.
+macro_rules! dynasm {
+    ($a:expr ; $($tt:tt)*) => {
+        dynasm::dynasm!(
+            $a
+            ; .arch x64
+            ; $($tt)*
+        )
+    };
 }
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]