rollup merge of #20463: amaranth/black_box_output

By returning the passed value black_box can be used on data being
passed to a function being benchmarked. This ensures the compiler
does not optimize the function for the input which could result in
the entire function being optimized away.

Author: alexcrichton

src/libcollections/bit.rs

@@ -2526,7 +2526,7 @@ mod bitv_bench {
             for _ in range(0u, 100) {
                 bitv |= 1 << ((r.next_u32() as uint) % u32::BITS);
             }
-            black_box(&bitv)
+            black_box(&bitv);
         });
     }
 
@@ -2538,7 +2538,7 @@ mod bitv_bench {
             for _ in range(0u, 100) {
                 bitv.set((r.next_u32() as uint) % BENCH_BITS, true);
             }
-            black_box(&bitv)
+            black_box(&bitv);
         });
     }
 

src/libcollections/str.rs

@@ -2841,7 +2841,7 @@ mod bench {
         let s = "ศไทย中华Việt Nam; Mary had a little lamb, Little lamb";
 
         b.iter(|| {
-            for ch in s.chars() { black_box(ch) }
+            for ch in s.chars() { black_box(ch); }
         });
     }
 
@@ -2869,7 +2869,7 @@ mod bench {
         let s = "ศไทย中华Việt Nam; Mary had a little lamb, Little lamb";
 
         b.iter(|| {
-            for ch in s.chars().rev() { black_box(ch) }
+            for ch in s.chars().rev() { black_box(ch); }
         });
     }
 

src/libtest/lib.rs

@@ -1332,10 +1332,11 @@ impl MetricMap {
 /// elimination.
 ///
 /// This function is a no-op, and does not even read from `dummy`.
-pub fn black_box<T>(dummy: T) {
+pub fn black_box<T>(dummy: T) -> T {
     // we need to "use" the argument in some way LLVM can't
     // introspect.
     unsafe {asm!("" : : "r"(&dummy))}
+    dummy
 }