Elide unnecessary ret slot allocas

src/librustc/lib/llvm.rs

@@ -1553,6 +1553,8 @@ pub mod llvm {
         /* Selected entries from the downcasts. */
         #[fast_ffi]
         pub fn LLVMIsATerminatorInst(Inst: ValueRef) -> ValueRef;
+        #[fast_ffi]
+        pub fn LLVMIsAStoreInst(Inst: ValueRef) -> ValueRef;
 
         /** Writes a module to the specified path. Returns 0 on success. */
         #[fast_ffi]

src/librustc/middle/trans/base.rs

@@ -59,6 +59,7 @@ use middle::trans::monomorphize;
 use middle::trans::tvec;
 use middle::trans::type_of;
 use middle::trans::type_of::*;
+use middle::trans::value::Value;
 use middle::ty;
 use util::common::indenter;
 use util::ppaux::{Repr, ty_to_str};
@@ -1791,11 +1792,30 @@ pub fn finish_fn(fcx: @mut FunctionContext, last_bcx: @mut Block) {
 // Builds the return block for a function.
 pub fn build_return_block(fcx: &FunctionContext, ret_cx: @mut Block) {
     // Return the value if this function immediate; otherwise, return void.
-    if fcx.llretptr.is_some() && fcx.has_immediate_return_value {
-        Ret(ret_cx, Load(ret_cx, fcx.llretptr.unwrap()))
-    } else {
-        RetVoid(ret_cx)
+    if fcx.llretptr.is_none() || !fcx.has_immediate_return_value {
+        return RetVoid(ret_cx);
     }
+
+    let retptr = Value(fcx.llretptr.unwrap());
+    let retval = match retptr.get_dominating_store(ret_cx) {
+        // If there's only a single store to the ret slot, we can directly return
+        // the value that was stored and omit the store and the alloca
+        Some(s) => {
+            let retval = *s.get_operand(0).unwrap();
+            s.erase_from_parent();
+
+            if retptr.has_no_uses() {
+                retptr.erase_from_parent();
+            }
+
+            retval
+        }
+        // Otherwise, load the return value from the ret slot
+        None => Load(ret_cx, fcx.llretptr.unwrap())
+    };
+
+
+    Ret(ret_cx, retval);
 }
 
 pub enum self_arg { impl_self(ty::t, ty::SelfMode), no_self, }

src/librustc/middle/trans/basic_block.rs

@@ -0,0 +1,42 @@
+// 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 lib::llvm::{llvm, BasicBlockRef};
+use middle::trans::value::{UserIterator, Value};
+use std::iterator::{Filter, Map};
+
+pub struct BasicBlock(BasicBlockRef);
+
+pub type PredIterator<'self> = Map<'self, Value, BasicBlock, Filter<'self, Value, UserIterator>>;
+
+/**
+ * Wrapper for LLVM BasicBlockRef
+ */
+impl BasicBlock {
+    pub fn as_value(self) -> Value {
+        unsafe {
+            Value(llvm::LLVMBasicBlockAsValue(*self))
+        }
+    }
+
+    pub fn pred_iter(self) -> PredIterator {
+        self.as_value().user_iter()
+            .filter(|user| user.is_a_terminator_inst())
+            .transform(|user| user.get_parent().unwrap())
+    }
+
+    pub fn get_single_predecessor(self) -> Option<BasicBlock> {
+        let mut iter = self.pred_iter();
+        match (iter.next(), iter.next()) {
+            (Some(first), None) => Some(first),
+            _ => None
+        }
+    }
+}

src/librustc/middle/trans/mod.rs

@@ -42,3 +42,5 @@ pub mod machine;
 pub mod adt;
 pub mod asm;
 pub mod type_;
+pub mod value;
+pub mod basic_block;

src/librustc/middle/trans/value.rs

@@ -0,0 +1,157 @@
+// 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 lib::llvm::{llvm, UseRef, ValueRef};
+use middle::trans::basic_block::BasicBlock;
+use middle::trans::common::Block;
+use std::libc::c_uint;
+
+pub struct Value(ValueRef);
+
+macro_rules! opt_val ( ($e:expr) => (
+    unsafe {
+        match $e {
+            p if p.is_not_null() => Some(Value(p)),
+            _ => None
+        }
+    }
+))
+
+/**
+ * Wrapper for LLVM ValueRef
+ */
+impl Value {
+    /// Returns the BasicBlock that contains this value
+    pub fn get_parent(self) -> Option<BasicBlock> {
+        unsafe {
+            match llvm::LLVMGetInstructionParent(*self) {
+                p if p.is_not_null() => Some(BasicBlock(p)),
+                _ => None
+            }
+        }
+    }
+
+    /// Removes this value from its containing BasicBlock
+    pub fn erase_from_parent(self) {
+        unsafe {
+            llvm::LLVMInstructionEraseFromParent(*self);
+        }
+    }
+
+    /// Returns the single dominating store to this value, if any
+    /// This only performs a search for a trivially dominating store. The store
+    /// must be the only user of this value, and there must not be any conditional
+    /// branches between the store and the given block.
+    pub fn get_dominating_store(self, bcx: @mut Block) -> Option<Value> {
+        match self.get_single_user().chain(|user| user.as_store_inst()) {
+            Some(store) => {
+                do store.get_parent().chain |store_bb| {
+                    let mut bb = BasicBlock(bcx.llbb);
+                    let mut ret = Some(store);
+                    while *bb != *store_bb {
+                        match bb.get_single_predecessor() {
+                            Some(pred) => bb = pred,
+                            None => { ret = None; break }
+                        }
+                    }
+                    ret
+                }
+            }
+            _ => None
+        }
+    }
+
+    /// Returns the first use of this value, if any
+    pub fn get_first_use(self) -> Option<Use> {
+        unsafe {
+            match llvm::LLVMGetFirstUse(*self) {
+                u if u.is_not_null() => Some(Use(u)),
+                _ => None
+            }
+        }
+    }
+
+    /// Tests if there are no uses of this value
+    pub fn has_no_uses(self) -> bool {
+        self.get_first_use().is_none()
+    }
+
+    /// Returns the single user of this value
+    /// If there are no users or multiple users, this returns None
+    pub fn get_single_user(self) -> Option<Value> {
+        let mut iter = self.user_iter();
+        match (iter.next(), iter.next()) {
+            (Some(first), None) => Some(first),
+            _ => None
+        }
+    }
+
+    /// Returns an iterator for the users of this value
+    pub fn user_iter(self) -> UserIterator {
+        UserIterator {
+            next: self.get_first_use()
+        }
+    }
+
+    /// Returns the requested operand of this instruction
+    /// Returns None, if there's no operand at the given index
+    pub fn get_operand(self, i: uint) -> Option<Value> {
+        opt_val!(llvm::LLVMGetOperand(*self, i as c_uint))
+    }
+
+    /// Returns the Store represent by this value, if any
+    pub fn as_store_inst(self) -> Option<Value> {
+        opt_val!(llvm::LLVMIsAStoreInst(*self))
+    }
+
+    /// Tests if this value is a terminator instruction
+    pub fn is_a_terminator_inst(self) -> bool {
+        unsafe {
+            llvm::LLVMIsATerminatorInst(*self).is_not_null()
+        }
+    }
+}
+
+pub struct Use(UseRef);
+
+/**
+ * Wrapper for LLVM UseRef
+ */
+impl Use {
+    pub fn get_user(self) -> Value {
+        unsafe {
+            Value(llvm::LLVMGetUser(*self))
+        }
+    }
+
+    pub fn get_next_use(self) -> Option<Use> {
+        unsafe {
+            match llvm::LLVMGetNextUse(*self) {
+                u if u.is_not_null() => Some(Use(u)),
+                _ => None
+            }
+        }
+    }
+}
+
+/// Iterator for the users of a value
+pub struct UserIterator {
+    priv next: Option<Use>
+}
+
+impl Iterator<Value> for UserIterator {
+    fn next(&mut self) -> Option<Value> {
+        let current = self.next;
+
+        self.next = do current.chain |u| { u.get_next_use() };
+
+        do current.map |u| { u.get_user() }
+    }
+}

src/test/codegen/single-return-value.cc

@@ -0,0 +1,14 @@
+// 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.
+
+extern "C"
+int test() {
+  return 5;
+}

src/test/codegen/single-return-value.rs

@@ -0,0 +1,14 @@
+// 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.
+
+#[no_mangle]
+fn test() -> int {
+    5
+}