auto merge of #11173 : whitequark/rust/master, r=thestinger

This PR adds `std::unsafe::intrinsics::{volatile_load,volatile_store}`, which map to LLVM's `load volatile` and `store volatile` operations correspondingly.

This would fix #11172.

I have addressed several uncertainties with this PR in the line comments.

Author: bors

src/librustc/lib/llvm.rs

@@ -787,6 +787,10 @@ pub mod llvm {
         pub fn LLVMIsTailCall(CallInst: ValueRef) -> Bool;
         pub fn LLVMSetTailCall(CallInst: ValueRef, IsTailCall: Bool);
 
+        /* Operations on load/store instructions (only) */
+        pub fn LLVMGetVolatile(MemoryAccessInst: ValueRef) -> Bool;
+        pub fn LLVMSetVolatile(MemoryAccessInst: ValueRef, volatile: Bool);
+
         /* Operations on phi nodes */
         pub fn LLVMAddIncoming(PhiNode: ValueRef,
                                IncomingValues: *ValueRef,

src/librustc/middle/trans/build.rs

@@ -349,6 +349,13 @@ pub fn Load(cx: &Block, PointerVal: ValueRef) -> ValueRef {
     }
 }
 
+pub fn VolatileLoad(cx: &Block, PointerVal: ValueRef) -> ValueRef {
+    unsafe {
+        if cx.unreachable.get() { return llvm::LLVMGetUndef(Type::nil().to_ref()); }
+        B(cx).volatile_load(PointerVal)
+    }
+}
+
 pub fn AtomicLoad(cx: &Block, PointerVal: ValueRef, order: AtomicOrdering) -> ValueRef {
     unsafe {
         let ccx = cx.fcx.ccx;
@@ -383,6 +390,11 @@ pub fn Store(cx: &Block, Val: ValueRef, Ptr: ValueRef) {
     B(cx).store(Val, Ptr)
 }
 
+pub fn VolatileStore(cx: &Block, Val: ValueRef, Ptr: ValueRef) {
+    if cx.unreachable.get() { return; }
+    B(cx).volatile_store(Val, Ptr)
+}
+
 pub fn AtomicStore(cx: &Block, Val: ValueRef, Ptr: ValueRef, order: AtomicOrdering) {
     if cx.unreachable.get() { return; }
     B(cx).atomic_store(Val, Ptr, order)

src/librustc/middle/trans/builder.rs

@@ -449,6 +449,15 @@ impl Builder {
         }
     }
 
+    pub fn volatile_load(&self, ptr: ValueRef) -> ValueRef {
+        self.count_insn("load.volatile");
+        unsafe {
+            let insn = llvm::LLVMBuildLoad(self.llbuilder, ptr, noname());
+            llvm::LLVMSetVolatile(insn, lib::llvm::True);
+            insn
+        }
+    }
+
     pub fn atomic_load(&self, ptr: ValueRef, order: AtomicOrdering) -> ValueRef {
         self.count_insn("load.atomic");
         unsafe {
@@ -488,6 +497,18 @@ impl Builder {
         }
     }
 
+    pub fn volatile_store(&self, val: ValueRef, ptr: ValueRef) {
+        debug!("Store {} -> {}",
+               self.ccx.tn.val_to_str(val),
+               self.ccx.tn.val_to_str(ptr));
+        assert!(is_not_null(self.llbuilder));
+        self.count_insn("store.volatile");
+        unsafe {
+            let insn = llvm::LLVMBuildStore(self.llbuilder, val, ptr);
+            llvm::LLVMSetVolatile(insn, lib::llvm::True);
+        }
+    }
+
     pub fn atomic_store(&self, val: ValueRef, ptr: ValueRef, order: AtomicOrdering) {
         debug!("Store {} -> {}",
                self.ccx.tn.val_to_str(val),

src/librustc/middle/trans/intrinsic.rs

@@ -73,6 +73,23 @@ pub fn trans_intrinsic(ccx: @CrateContext,
         }
     }
 
+    fn volatile_load_intrinsic(bcx: @Block) {
+        let first_real_arg = bcx.fcx.arg_pos(0u);
+        let src = get_param(bcx.fcx.llfn, first_real_arg);
+
+        let val = VolatileLoad(bcx, src);
+        Ret(bcx, val);
+    }
+
+    fn volatile_store_intrinsic(bcx: @Block) {
+        let first_real_arg = bcx.fcx.arg_pos(0u);
+        let dst = get_param(bcx.fcx.llfn, first_real_arg);
+        let val = get_param(bcx.fcx.llfn, first_real_arg + 1);
+
+        VolatileStore(bcx, val, dst);
+        RetVoid(bcx);
+    }
+
     fn copy_intrinsic(bcx: @Block, allow_overlap: bool, tp_ty: ty::t) {
         let ccx = bcx.ccx();
         let lltp_ty = type_of::type_of(ccx, tp_ty);
@@ -480,6 +497,9 @@ pub fn trans_intrinsic(ccx: @CrateContext,
         "bswap32" => simple_llvm_intrinsic(bcx, "llvm.bswap.i32", 1),
         "bswap64" => simple_llvm_intrinsic(bcx, "llvm.bswap.i64", 1),
 
+        "volatile_load" => volatile_load_intrinsic(bcx),
+        "volatile_store" => volatile_store_intrinsic(bcx),
+
         "i8_add_with_overflow" =>
             with_overflow_instrinsic(bcx, "llvm.sadd.with.overflow.i8", output_type),
         "i16_add_with_overflow" =>

src/librustc/middle/typeck/check/mod.rs

@@ -4226,6 +4226,11 @@ pub fn check_intrinsic_type(ccx: @CrateCtxt, it: @ast::foreign_item) {
             "bswap32"      => (0, ~[ ty::mk_i32() ], ty::mk_i32()),
             "bswap64"      => (0, ~[ ty::mk_i64() ], ty::mk_i64()),
 
+            "volatile_load" =>
+                (1, ~[ ty::mk_imm_ptr(tcx, param(ccx, 0)) ], param(ccx, 0)),
+            "volatile_store" =>
+                (1, ~[ ty::mk_mut_ptr(tcx, param(ccx, 0)), param(ccx, 0) ], ty::mk_nil()),
+
             "i8_add_with_overflow" | "i8_sub_with_overflow" | "i8_mul_with_overflow" =>
                 (0, ~[ty::mk_i8(), ty::mk_i8()],
                 ty::mk_tup(tcx, ~[ty::mk_i8(), ty::mk_bool()])),

src/libstd/unstable/intrinsics.rs

@@ -12,6 +12,15 @@
 
 The corresponding definitions are in librustc/middle/trans/foreign.rs.
 
+# Volatiles
+
+The volatile intrinsics provide operations intended to act on I/O
+memory, which are guaranteed to not be reordered by the compiler
+across other volatile intrinsics. See the LLVM documentation on
+[[volatile]].
+
+[volatile]: http://llvm.org/docs/LangRef.html#volatile-memory-accesses
+
 # Atomics
 
 The atomic intrinsics provide common atomic operations on machine
@@ -179,6 +188,9 @@ extern "rust-intrinsic" {
     /// Execute a breakpoint trap, for inspection by a debugger.
     pub fn breakpoint();
 
+    #[cfg(not(stage0))] pub fn volatile_load<T>(src: *T) -> T;
+    #[cfg(not(stage0))] pub fn volatile_store<T>(dst: *mut T, val: T);
+
     /// Atomic compare and exchange, sequentially consistent.
     pub fn atomic_cxchg(dst: &mut int, old: int, src: int) -> int;
     /// Atomic compare and exchange, acquire ordering.

src/test/run-make/volatile-intrinsics/Makefile

@@ -0,0 +1,10 @@
+-include ../tools.mk
+
+all:
+	# The tests must pass...
+	$(RUSTC) main.rs
+	$(call RUN,main)
+	# ... and the loads/stores must not be optimized out.
+	$(RUSTC) main.rs --emit-llvm -S
+	grep "load volatile"  $(TMPDIR)/main.ll
+	grep "store volatile" $(TMPDIR)/main.ll

src/test/run-make/volatile-intrinsics/main.rs

@@ -0,0 +1,19 @@
+// Copyright 2013 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.
+
+use std::unstable::intrinsics::{volatile_load, volatile_store};
+
+pub fn main() {
+    unsafe {
+        let mut i : int = 1;
+        volatile_store(&mut i, 2);
+        assert_eq!(volatile_load(&i), 2);
+    }
+}