Fixes issue 39827: ICE in volatile_store intrinsic

- adds handling of zero-sized types for volatile_store.
- adds type size checks and warnigns for other volatile intrinsics.
- adds a test to check warnings emitting.

Cause of the issue

While preparing for trans_intrinsic_call() invoke arguments are
processed with trans_argument() method which excludes zero-sized types
from argument list (to be more correct - all arguments for which
ArgKind is Ignore are filtered out). As result volatile_store() intrinsic
gets one argument instead of expected address and value.

How it is fixed

Modification of the trans_argument() method may cause side effects,
therefore change was implemented in volatile_store() intrinsic building
code itself. Now it checks function signature and if it was specialised
with zero-sized type, then emits C_nil() instead of accessing
non-existing second argument.

Additionally warnings are added for all volatile operations which are
specialised with zero-sized arguments. In fact, those operations are omitted
in LLVM backend if no memory affected at all, e.g. number of elements
is zero or type is zero-sized. This was not explicitly documented before
and could lead to potential issues if developer expects volatile behaviour,
but type has degraded to zero-sized.

src/test/ui/issue-39827.rs

@@ -0,0 +1,37 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+#![feature(core_intrinsics)]
+
+use std::intrinsics::{ volatile_copy_memory, volatile_store, volatile_load,
+                       volatile_copy_nonoverlapping_memory,
+                       volatile_set_memory };
+
+fn main () {
+    let mut dst_pair = (1, 2);
+    let src_pair = (3, 4);
+    let mut dst_empty = ();
+    let src_empty = ();
+
+    const COUNT_0: usize = 0;
+    const COUNT_100: usize = 100;
+
+    unsafe {
+        volatile_copy_memory(&mut dst_pair, &dst_pair, COUNT_0);
+        volatile_copy_nonoverlapping_memory(&mut dst_pair, &src_pair, 0);
+        volatile_copy_memory(&mut dst_empty, &dst_empty, 100);
+        volatile_copy_nonoverlapping_memory(&mut dst_empty, &src_empty,
+                                            COUNT_100);
+        volatile_set_memory(&mut dst_empty, 0, COUNT_100);
+        volatile_set_memory(&mut dst_pair, 0, COUNT_0);
+        volatile_store(&mut dst_empty, ());
+        volatile_store(&mut dst_empty, src_empty);
+        volatile_load(&src_empty);
+    }
+}

src/test/ui/issue-39827.stderr

@@ -0,0 +1,73 @@
+warning: suspicious monomorphization of `volatile_copy_memory` intrinsic
+  --> $DIR/issue-39827.rs:26:9
+   |
+26 |         volatile_copy_memory(&mut dst_pair, &dst_pair, COUNT_0);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: 'volatile_copy_memory' was specialized with type '(i32, i32)', number of elements is 0
+
+warning: suspicious monomorphization of `volatile_copy_nonoverlapping_memory` intrinsic
+  --> $DIR/issue-39827.rs:27:9
+   |
+27 |         volatile_copy_nonoverlapping_memory(&mut dst_pair, &src_pair, 0);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: 'volatile_copy_nonoverlapping_memory' was specialized with type '(i32, i32)', number of elements is 0
+
+warning: suspicious monomorphization of `volatile_copy_memory` intrinsic
+  --> $DIR/issue-39827.rs:28:9
+   |
+28 |         volatile_copy_memory(&mut dst_empty, &dst_empty, 100);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: 'volatile_copy_memory' was specialized with type '()', number of elements is 100
+
+warning: suspicious monomorphization of `volatile_copy_nonoverlapping_memory` intrinsic
+  --> $DIR/issue-39827.rs:29:9
+   |
+29 | /         volatile_copy_nonoverlapping_memory(&mut dst_empty, &src_empty,
+30 | |                                             COUNT_100);
+   | |______________________________________________________^
+   |
+   = note: 'volatile_copy_nonoverlapping_memory' was specialized with type '()', number of elements is 100
+
+warning: suspicious monomorphization of `volatile_set_memory` intrinsic
+  --> $DIR/issue-39827.rs:31:9
+   |
+31 |         volatile_set_memory(&mut dst_empty, 0, COUNT_100);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: 'volatile_set_memory' was specialized with type '()', number of elements is 100
+
+warning: suspicious monomorphization of `volatile_set_memory` intrinsic
+  --> $DIR/issue-39827.rs:32:9
+   |
+32 |         volatile_set_memory(&mut dst_pair, 0, COUNT_0);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: 'volatile_set_memory' was specialized with type '(i32, i32)', number of elements is 0
+
+warning: suspicious monomorphization of `volatile_store` intrinsic
+  --> $DIR/issue-39827.rs:33:9
+   |
+33 |         volatile_store(&mut dst_empty, ());
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: 'volatile_store' was specialized with zero-sized type '()'
+
+warning: suspicious monomorphization of `volatile_store` intrinsic
+  --> $DIR/issue-39827.rs:34:9
+   |
+34 |         volatile_store(&mut dst_empty, src_empty);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: 'volatile_store' was specialized with zero-sized type '()'
+
+warning: suspicious monomorphization of `volatile_load` intrinsic
+  --> $DIR/issue-39827.rs:35:9
+   |
+35 |         volatile_load(&src_empty);
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: 'volatile_load' was specialized with zero-sized type '()'
+