big-allocs benchmark: use up less of the address space on 32bit

Author: RalfJung

bench-cargo-miri/big-allocs/src/main.rs

@@ -7,7 +7,10 @@ fn main() {
     // We can't use too big of an allocation or this code will encounter an allocation failure in
     // CI. Since the allocation can't be huge, we need to do a few iterations so that the effect
     // we're trying to measure is clearly visible above the interpreter's startup time.
-    for _ in 0..10 {
+    // FIXME (https://github.com/rust-lang/miri/issues/4253): On 32bit targets, we can run out of
+    // usable addresses if we don't reuse, leading to random test failures.
+    let count = if cfg!(target_pointer_width = "32") { 8 } else { 12 };
+    for _ in 0..count {
         drop(Vec::<u8>::with_capacity(512 * 1024 * 1024));
     }
 }