simplify Linux futex impl a bit

Author: RalfJung

src/shims/unix/linux/sync.rs

@@ -1,7 +1,7 @@
+use std::time::SystemTime;
+
 use crate::concurrency::thread::{MachineCallback, Time};
 use crate::*;
-use rustc_target::abi::{Align, Size};
-use std::time::SystemTime;
 
 /// Implementation of the SYS_futex syscall.
 /// `args` is the arguments *after* the syscall number.
@@ -28,13 +28,14 @@ pub fn futex<'tcx>(
     // The first three arguments (after the syscall number itself) are the same to all futex operations:
     //     (int *addr, int op, int val).
     // We checked above that these definitely exist.
-    let addr = this.read_immediate(&args[0])?;
+    let addr = this.read_pointer(&args[0])?;
     let op = this.read_scalar(&args[1])?.to_i32()?;
     let val = this.read_scalar(&args[2])?.to_i32()?;
 
     let thread = this.get_active_thread();
-    let addr_scalar = addr.to_scalar();
-    let addr_usize = addr_scalar.to_machine_usize(this)?;
+    // This is a vararg function so we have to bring our own type for this pointer.
+    let addr = MPlaceTy::from_aligned_ptr(addr, this.machine.layouts.i32);
+    let addr_usize = addr.ptr.addr().bytes();
 
     let futex_private = this.eval_libc_i32("FUTEX_PRIVATE_FLAG")?;
     let futex_wait = this.eval_libc_i32("FUTEX_WAIT")?;
@@ -117,15 +118,6 @@ pub fn futex<'tcx>(
                     }
                 })
             };
-            // Check the pointer for alignment and validity.
-            // The API requires `addr` to be a 4-byte aligned pointer, and will
-            // use the 4 bytes at the given address as an (atomic) i32.
-            this.check_ptr_access_align(
-                addr_scalar.to_pointer(this)?,
-                Size::from_bytes(4),
-                Align::from_bytes(4).unwrap(),
-                CheckInAllocMsg::MemoryAccessTest,
-            )?;
             // There may be a concurrent thread changing the value of addr
             // and then invoking the FUTEX_WAKE syscall. It is critical that the
             // effects of this and the other thread are correctly observed,
@@ -172,14 +164,7 @@ pub fn futex<'tcx>(
             this.atomic_fence(AtomicFenceOrd::SeqCst)?;
             // Read an `i32` through the pointer, regardless of any wrapper types.
             // It's not uncommon for `addr` to be passed as another type than `*mut i32`, such as `*const AtomicI32`.
-            let futex_val = this
-                .read_scalar_at_offset_atomic(
-                    &addr.into(),
-                    0,
-                    this.machine.layouts.i32,
-                    AtomicReadOrd::Relaxed,
-                )?
-                .to_i32()?;
+            let futex_val = this.read_scalar_atomic(&addr, AtomicReadOrd::Relaxed)?.to_i32()?;
             if val == futex_val {
                 // The value still matches, so we block the thread make it wait for FUTEX_WAKE.
                 this.block_thread(thread);