Rollup merge of #46030 - Zoxc:asm-volatile, r=nikomatsakis

Make inline assembly volatile if it has no outputs. Fixes #46026

src/libsyntax_ext/asm.rs

@@ -239,6 +239,12 @@ pub fn expand_asm<'cx>(cx: &'cx mut ExtCtxt,
         }
     }
 
+    // If there are no outputs, the inline assembly is executed just for its side effects,
+    // so ensure that it is volatile
+    if outputs.is_empty() {
+        volatile = true;
+    }
+
     MacEager::expr(P(ast::Expr {
         id: ast::DUMMY_NODE_ID,
         node: ast::ExprKind::InlineAsm(P(ast::InlineAsm {

src/test/codegen/no-output-asm-is-volatile.rs

@@ -0,0 +1,26 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -O
+
+// ignore-asmjs
+
+#![feature(asm)]
+#![crate_type = "lib"]
+
+// Check that inline assembly expressions without any outputs
+// are marked as having side effects / being volatile
+
+// CHECK-LABEL: @assembly
+#[no_mangle]
+pub fn assembly() {
+    unsafe { asm!("") }
+// CHECK: tail call void asm sideeffect "", {{.*}}
+}