diff --git a/src/intptrcast.rs b/src/intptrcast.rs
index f51c8a7ce74..c908bdf24eb 100644
--- a/src/intptrcast.rs
+++ b/src/intptrcast.rs
@@ -9,7 +9,7 @@
 use rustc_mir::interpret::{AllocCheck, AllocId, InterpResult, Memory, Machine, Pointer, PointerArithmetic};
 use rustc_target::abi::{Size, HasDataLayout};
 
-use crate::{Evaluator, Tag, STACK_ADDR, CheckInAllocMsg};
+use crate::*;
 
 pub type MemoryExtra = RefCell<GlobalState>;
 
diff --git a/src/lib.rs b/src/lib.rs
index e0da83840ed..fa357eb9b13 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -34,6 +34,8 @@
 mod sync;
 mod thread;
 
+// Establish a "crate-wide prelude": we often import `crate::*`.
+
 // Make all those symbols available in the same place as our own.
 pub use rustc_mir::interpret::*;
 // Resolve ambiguity.
@@ -42,13 +44,10 @@
 pub use crate::shims::dlsym::{Dlsym, EvalContextExt as _};
 pub use crate::shims::env::{EnvVars, EvalContextExt as _};
 pub use crate::shims::foreign_items::EvalContextExt as _;
-pub use crate::shims::fs::{DirHandler, EvalContextExt as _, FileHandler};
 pub use crate::shims::intrinsics::EvalContextExt as _;
 pub use crate::shims::os_str::EvalContextExt as _;
-pub use crate::shims::panic::{CatchUnwindData, EvalContextExt as _};
-pub use crate::shims::posix_sync::{EvalContextExt as _};
-pub use crate::shims::thread::EvalContextExt as _;
 pub use crate::shims::time::EvalContextExt as _;
+pub use crate::shims::panic::{CatchUnwindData, EvalContextExt as _};
 pub use crate::shims::tls::{EvalContextExt as _, TlsData};
 pub use crate::shims::EvalContextExt as _;
 
diff --git a/src/machine.rs b/src/machine.rs
index 4f45b4f93f1..6233222c004 100644
--- a/src/machine.rs
+++ b/src/machine.rs
@@ -241,8 +241,8 @@ pub struct Evaluator<'mir, 'tcx> {
     /// Whether to enforce the validity invariant.
     pub(crate) validate: bool,
 
-    pub(crate) file_handler: FileHandler,
-    pub(crate) dir_handler: DirHandler,
+    pub(crate) file_handler: shims::posix::FileHandler,
+    pub(crate) dir_handler: shims::posix::DirHandler,
 
     /// The temporary used for storing the argument of
     /// the call to `miri_start_panic` (the panic payload) when unwinding.
diff --git a/src/shims/foreign_items.rs b/src/shims/foreign_items.rs
index 8a75fb03a53..14c5aac4899 100644
--- a/src/shims/foreign_items.rs
+++ b/src/shims/foreign_items.rs
@@ -1,6 +1,3 @@
-mod windows;
-mod posix;
-
 use std::{convert::{TryInto, TryFrom}, iter};
 
 use rustc_hir::def_id::DefId;
@@ -455,13 +452,13 @@ fn emulate_foreign_item_by_name(
             // Architecture-specific shims
             "llvm.x86.sse2.pause" if this.tcx.sess.target.target.arch == "x86" || this.tcx.sess.target.target.arch == "x86_64" => {
                 let &[] = check_arg_count(args)?;
-                this.sched_yield()?;
+                this.yield_active_thread();
             }
 
             // Platform-specific shims
             _ => match this.tcx.sess.target.target.target_os.as_str() {
-                "linux" | "macos" => return posix::EvalContextExt::emulate_foreign_item_by_name(this, link_name, args, dest, ret),
-                "windows" => return windows::EvalContextExt::emulate_foreign_item_by_name(this, link_name, args, dest, ret),
+                "linux" | "macos" => return shims::posix::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, args, dest, ret),
+                "windows" => return shims::windows::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, args, dest, ret),
                 target => throw_unsup_format!("the target `{}` is not supported", target),
             }
         };
diff --git a/src/shims/mod.rs b/src/shims/mod.rs
index cd525e173ed..37e7b8c4046 100644
--- a/src/shims/mod.rs
+++ b/src/shims/mod.rs
@@ -1,15 +1,18 @@
+
+pub mod foreign_items;
+pub mod intrinsics;
+pub mod posix;
+pub mod windows;
+
 pub mod dlsym;
 pub mod env;
-pub mod foreign_items;
-pub mod fs;
-pub mod intrinsics;
 pub mod os_str;
 pub mod panic;
-pub mod sync;
-pub mod thread;
 pub mod time;
 pub mod tls;
 
+// End module management, begin local code
+
 use std::convert::TryFrom;
 
 use log::trace;
diff --git a/src/shims/foreign_items/posix.rs b/src/shims/posix/foreign_items.rs
similarity index 98%
rename from src/shims/foreign_items/posix.rs
rename to src/shims/posix/foreign_items.rs
index 8e4d140b06c..bbda40def62 100644
--- a/src/shims/foreign_items/posix.rs
+++ b/src/shims/posix/foreign_items.rs
@@ -1,15 +1,16 @@
-mod linux;
-mod macos;
-
 use std::convert::TryFrom;
 
 use log::trace;
 
-use crate::*;
-use helpers::check_arg_count;
 use rustc_middle::mir;
 use rustc_target::abi::{Align, LayoutOf, Size};
 
+use crate::*;
+use helpers::check_arg_count;
+use shims::posix::fs::EvalContextExt as _;
+use shims::posix::sync::EvalContextExt as _;
+use shims::posix::thread::EvalContextExt as _;
+
 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
     fn emulate_foreign_item_by_name(
@@ -476,8 +477,8 @@ fn emulate_foreign_item_by_name(
             // Platform-specific shims
             _ => {
                 match this.tcx.sess.target.target.target_os.as_str() {
-                    "linux" => return linux::EvalContextExt::emulate_foreign_item_by_name(this, link_name, args, dest, ret),
-                    "macos" => return macos::EvalContextExt::emulate_foreign_item_by_name(this, link_name, args, dest, ret),
+                    "linux" => return shims::posix::linux::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, args, dest, ret),
+                    "macos" => return shims::posix::macos::foreign_items::EvalContextExt::emulate_foreign_item_by_name(this, link_name, args, dest, ret),
                     _ => unreachable!(),
                 }
             }
diff --git a/src/shims/fs.rs b/src/shims/posix/fs.rs
similarity index 99%
rename from src/shims/fs.rs
rename to src/shims/posix/fs.rs
index 29e0da14ff1..87aa28120c2 100644
--- a/src/shims/fs.rs
+++ b/src/shims/posix/fs.rs
@@ -10,13 +10,13 @@
 use rustc_data_structures::fx::FxHashMap;
 use rustc_target::abi::{Align, LayoutOf, Size};
 
-use crate::stacked_borrows::Tag;
 use crate::*;
+use stacked_borrows::Tag;
 use helpers::{check_arg_count, immty_from_int_checked, immty_from_uint_checked};
 use shims::time::system_time_to_duration;
 
 #[derive(Debug)]
-pub struct FileHandle {
+struct FileHandle {
     file: File,
     writable: bool,
 }
diff --git a/src/shims/foreign_items/posix/linux.rs b/src/shims/posix/linux/foreign_items.rs
similarity index 97%
rename from src/shims/foreign_items/posix/linux.rs
rename to src/shims/posix/linux/foreign_items.rs
index 2d124f9d8c6..ff30609d9ab 100644
--- a/src/shims/foreign_items/posix/linux.rs
+++ b/src/shims/posix/linux/foreign_items.rs
@@ -1,7 +1,11 @@
-use crate::*;
-use helpers::check_arg_count;
 use rustc_middle::mir;
 
+use crate::*;
+use crate::helpers::check_arg_count;
+use shims::posix::fs::EvalContextExt as _;
+use shims::posix::sync::EvalContextExt as _;
+use shims::posix::thread::EvalContextExt as _;
+
 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
     fn emulate_foreign_item_by_name(
diff --git a/src/shims/posix/linux/mod.rs b/src/shims/posix/linux/mod.rs
new file mode 100644
index 00000000000..09c6507b24f
--- /dev/null
+++ b/src/shims/posix/linux/mod.rs
@@ -0,0 +1 @@
+pub mod foreign_items;
diff --git a/src/shims/foreign_items/posix/macos.rs b/src/shims/posix/macos/foreign_items.rs
similarity index 98%
rename from src/shims/foreign_items/posix/macos.rs
rename to src/shims/posix/macos/foreign_items.rs
index fb50e4d9181..ef649c3e840 100644
--- a/src/shims/foreign_items/posix/macos.rs
+++ b/src/shims/posix/macos/foreign_items.rs
@@ -1,6 +1,9 @@
+use rustc_middle::mir;
+
 use crate::*;
 use helpers::check_arg_count;
-use rustc_middle::mir;
+use shims::posix::fs::EvalContextExt as _;
+use shims::posix::thread::EvalContextExt as _;
 
 impl<'mir, 'tcx: 'mir> EvalContextExt<'mir, 'tcx> for crate::MiriEvalContext<'mir, 'tcx> {}
 pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx> {
diff --git a/src/shims/posix/macos/mod.rs b/src/shims/posix/macos/mod.rs
new file mode 100644
index 00000000000..09c6507b24f
--- /dev/null
+++ b/src/shims/posix/macos/mod.rs
@@ -0,0 +1 @@
+pub mod foreign_items;
diff --git a/src/shims/posix/mod.rs b/src/shims/posix/mod.rs
new file mode 100644
index 00000000000..2f505cfb9c0
--- /dev/null
+++ b/src/shims/posix/mod.rs
@@ -0,0 +1,10 @@
+pub mod foreign_items;
+
+mod fs;
+mod sync;
+mod thread;
+
+mod linux;
+mod macos;
+
+pub use fs::{DirHandler, FileHandler};
diff --git a/src/shims/posix_sync.rs b/src/shims/posix/sync.rs
similarity index 99%
rename from src/shims/posix_sync.rs
rename to src/shims/posix/sync.rs
index cdc1f8cc763..a61c80d5118 100644
--- a/src/shims/posix_sync.rs
+++ b/src/shims/posix/sync.rs
@@ -5,10 +5,10 @@
 use rustc_middle::ty::{layout::TyAndLayout, TyKind, TypeAndMut};
 use rustc_target::abi::{LayoutOf, Size};
 
-use crate::stacked_borrows::Tag;
-use crate::thread::Time;
-
 use crate::*;
+use stacked_borrows::Tag;
+use thread::Time;
+
 
 fn assert_ptr_target_min_size<'mir, 'tcx: 'mir>(
     ecx: &MiriEvalContext<'mir, 'tcx>,
diff --git a/src/shims/thread.rs b/src/shims/posix/thread.rs
similarity index 100%
rename from src/shims/thread.rs
rename to src/shims/posix/thread.rs
diff --git a/src/shims/tls.rs b/src/shims/tls.rs
index 69561463368..704598ef2c6 100644
--- a/src/shims/tls.rs
+++ b/src/shims/tls.rs
@@ -10,10 +10,7 @@
 use rustc_middle::ty;
 use rustc_target::abi::{Size, HasDataLayout};
 
-use crate::{
-    HelpersEvalContextExt, InterpResult, MPlaceTy, Scalar, StackPopCleanup, Tag, ThreadId,
-    ThreadsEvalContextExt,
-};
+use crate::*;
 
 pub type TlsKey = u128;
 
diff --git a/src/shims/foreign_items/windows.rs b/src/shims/windows/foreign_items.rs
similarity index 100%
rename from src/shims/foreign_items/windows.rs
rename to src/shims/windows/foreign_items.rs
diff --git a/src/shims/windows/mod.rs b/src/shims/windows/mod.rs
new file mode 100644
index 00000000000..09c6507b24f
--- /dev/null
+++ b/src/shims/windows/mod.rs
@@ -0,0 +1 @@
+pub mod foreign_items;