transpile: revert __builtin_ia32_pause from spin_loop to _mm_pause

`_mm_pause` is the more direct, unstable equivalent of `__builtin_ia2_pause`.

Fixes 2nd half of <#1263 (comment)>.

Author: kkysen

c2rust-transpile/src/translator/builtins.rs

@@ -381,10 +381,10 @@ impl<'c> Translation<'c> {
             }
 
             "__builtin_ia32_pause" => {
-                // `spin_loop()` is implemented as `_mm_pause()` (the `pause` instruction) on `x86`/`x86_64`,
-                // but it's the safe and cross-platform version of it, so prefer it.
-                let spin_loop = mk().abs_path_expr(vec!["core", "hint", "spin_loop"]);
-                let call = mk().call_expr(spin_loop, vec![]);
+                let fn_name = "_mm_pause";
+                self.import_simd_function(fn_name)?;
+                let ident = mk().ident_expr(fn_name);
+                let call = mk().call_expr(ident, vec![]);
                 self.convert_side_effects_expr(
                     ctx,
                     WithStmts::new_val(call),

c2rust-transpile/tests/snapshots/arch-specific/spin.x86_64.rs

@@ -7,7 +7,12 @@
     unused_assignments,
     unused_mut
 )]
+#![feature(stdsimd)]
+#[cfg(target_arch = "x86")]
+pub use core::arch::x86::_mm_pause;
+#[cfg(target_arch = "x86_64")]
+pub use core::arch::x86_64::_mm_pause;
 #[no_mangle]
 pub unsafe extern "C" fn spin() {
-    ::core::hint::spin_loop();
+    _mm_pause();
 }

c2rust-transpile/tests/snapshots/[email protected]

@@ -12,7 +12,12 @@ input_file: c2rust-transpile/tests/snapshots/arch-specific/spin.c
     unused_assignments,
     unused_mut
 )]
+#![feature(stdsimd)]
+#[cfg(target_arch = "x86")]
+pub use core::arch::x86::_mm_pause;
+#[cfg(target_arch = "x86_64")]
+pub use core::arch::x86_64::_mm_pause;
 #[no_mangle]
 pub unsafe extern "C" fn spin() {
-    ::core::hint::spin_loop();
+    _mm_pause();
 }