New upstream version 1.48.0~beta.8+dfsg1

[rustc.git] / library / core / src / hint.rs

diff --git a/library/core/src/hint.rs b/library/core/src/hint.rs
index 461b4c79a1d1c12b8faf03978f2619d19b91f50b..4eb47dd1378c56562b4de07e84f29d924fc6e99a 100644 (file)
--- a/library/core/src/hint.rs
+++ b/library/core/src/hint.rs
@@ -101,17 +101,18 @@ pub fn spin_loop() {
 /// [`std::convert::identity`]: https://doc.rust-lang.org/core/convert/fn.identity.html
 ///
 /// Unlike [`std::convert::identity`], a Rust compiler is encouraged to assume that `black_box` can
-/// use `x` in any possible valid way that Rust code is allowed to without introducing undefined
+/// use `dummy` in any possible valid way that Rust code is allowed to without introducing undefined
 /// behavior in the calling code. This property makes `black_box` useful for writing code in which
 /// certain optimizations are not desired, such as benchmarks.
 ///
 /// Note however, that `black_box` is only (and can only be) provided on a "best-effort" basis. The
 /// extent to which it can block optimisations may vary depending upon the platform and code-gen
 /// backend used. Programs cannot rely on `black_box` for *correctness* in any way.
-#[inline]
+#[cfg_attr(not(miri), inline)]
+#[cfg_attr(miri, inline(never))]
 #[unstable(feature = "test", issue = "50297")]
-#[allow(unreachable_code)] // this makes #[cfg] a bit easier below.
-pub fn black_box<T>(dummy: T) -> T {
+#[cfg_attr(miri, allow(unused_mut))]
+pub fn black_box<T>(mut dummy: T) -> T {
     // We need to "use" the argument in some way LLVM can't introspect, and on
     // targets that support it we can typically leverage inline assembly to do
     // this. LLVM's interpretation of inline assembly is that it's, well, a black
@@ -121,7 +122,8 @@ pub fn black_box<T>(dummy: T) -> T {
     #[cfg(not(miri))] // This is just a hint, so it is fine to skip in Miri.
     // SAFETY: the inline assembly is a no-op.
     unsafe {
-        llvm_asm!("" : : "r"(&dummy));
+        // FIXME: Cannot use `asm!` because it doesn't support MIPS and other architectures.
+        llvm_asm!("" : : "r"(&mut dummy) : "memory" : "volatile");
     }
 
     dummy