Improve usage of lifetime intrinsics in match expressions

The allocas used in match expression currently don't get good lifetime
markers, in fact they only get lifetime start markers, because their
lifetimes don't match to cleanup scopes.

While the bindings themselves are bog standard and just need a matching
pair of start and end markers, they might need them twice, once for a
guard clause and once for the match body.

The __llmatch alloca OTOH needs a single lifetime start marker, but
when there's a guard clause, it needs two end markers, because its
lifetime ends either when the guard doesn't match or after the match
body.

With these intrinsics in place, LLVM can now, for example, optimize
code like this:

````rust
enum E {
  A1(int),
  A2(int),
  A3(int),
  A4(int),
}

pub fn variants(x: E) {
  match x {
    A1(m) => bar(&m),
    A2(m) => bar(&m),
    A3(m) => bar(&m),
    A4(m) => bar(&m),
  }
}
````

To a single call to bar, using only a single stack slot. It still fails
to eliminate some of checks.

````gas
.Ltmp5:
	.cfi_def_cfa_offset 16
	movb	(%rdi), %al
	testb	%al, %al
	je	.LBB3_5
	movzbl	%al, %eax
	cmpl	$1, %eax
	je	.LBB3_5
	cmpl	$2, %eax
.LBB3_5:
	movq	8(%rdi), %rax
	movq	%rax, (%rsp)
	leaq	(%rsp), %rdi
	callq	_ZN3bar20hcb7a0d8be8e17e37daaE@PLT
	popq	%rax
	retq
````

src/librustc/middle/trans/_match.rs

@@ -892,7 +892,12 @@ fn insert_lllocals<'a>(mut bcx: &'a Block<'a>, bindings_map: &BindingsMap,
             TrByCopy(llbinding) => {
                 let llval = Load(bcx, binding_info.llmatch);
                 let datum = Datum::new(llval, binding_info.ty, Lvalue);
+                call_lifetime_start(bcx, llbinding);
                 bcx = datum.store_to(bcx, llbinding);
+                match cs {
+                    Some(cs) => bcx.fcx.schedule_lifetime_end(cs, llbinding),
+                    _ => {}
+                }
 
                 llbinding
             },
@@ -906,7 +911,10 @@ fn insert_lllocals<'a>(mut bcx: &'a Block<'a>, bindings_map: &BindingsMap,
 
         let datum = Datum::new(llval, binding_info.ty, Lvalue);
         match cs {
-            Some(cs) => bcx.fcx.schedule_drop_and_zero_mem(cs, llval, binding_info.ty),
+            Some(cs) => {
+                bcx.fcx.schedule_drop_and_zero_mem(cs, llval, binding_info.ty);
+                bcx.fcx.schedule_lifetime_end(cs, binding_info.llmatch);
+            }
             _ => {}
         }
 
@@ -945,9 +953,17 @@ fn compile_guard<'a, 'b>(
     let val = unpack_datum!(bcx, expr::trans(bcx, guard_expr));
     let val = val.to_llbool(bcx);
 
+    for (_, &binding_info) in data.bindings_map.iter() {
+        match binding_info.trmode {
+            TrByCopy(llbinding) => call_lifetime_end(bcx, llbinding),
+            _ => {}
+        }
+    }
+
     return with_cond(bcx, Not(bcx, val), |bcx| {
         // Guard does not match: remove all bindings from the lllocals table
         for (_, &binding_info) in data.bindings_map.iter() {
+            call_lifetime_end(bcx, binding_info.llmatch);
             bcx.fcx.lllocals.borrow_mut().remove(&binding_info.id);
         }
         match chk {
@@ -988,6 +1004,7 @@ fn compile_submatch<'a, 'b>(
         let data = &m[0].data;
         for &(ref ident, ref value_ptr) in m[0].bound_ptrs.iter() {
             let llmatch = data.bindings_map.get(ident).llmatch;
+            call_lifetime_start(bcx, llmatch);
             Store(bcx, *value_ptr, llmatch);
         }
         match data.arm.guard {
@@ -1294,10 +1311,10 @@ fn create_bindings_map(bcx: &Block, pat: Gc<ast::Pat>) -> BindingsMap {
         match bm {
             ast::BindByValue(_)
                 if !ty::type_moves_by_default(tcx, variable_ty) => {
-                llmatch = alloca(bcx,
+                llmatch = alloca_no_lifetime(bcx,
                                  llvariable_ty.ptr_to(),
                                  "__llmatch");
-                trmode = TrByCopy(alloca(bcx,
+                trmode = TrByCopy(alloca_no_lifetime(bcx,
                                          llvariable_ty,
                                          bcx.ident(ident).as_slice()));
             }
@@ -1305,13 +1322,13 @@ fn create_bindings_map(bcx: &Block, pat: Gc<ast::Pat>) -> BindingsMap {
                 // in this case, the final type of the variable will be T,
                 // but during matching we need to store a *T as explained
                 // above
-                llmatch = alloca(bcx,
+                llmatch = alloca_no_lifetime(bcx,
                                  llvariable_ty.ptr_to(),
                                  bcx.ident(ident).as_slice());
                 trmode = TrByMove;
             }
             ast::BindByRef(_) => {
-                llmatch = alloca(bcx,
+                llmatch = alloca_no_lifetime(bcx,
                                  llvariable_ty,
                                  bcx.ident(ident).as_slice());
                 trmode = TrByRef;

src/librustc/middle/trans/base.rs

@@ -1169,10 +1169,12 @@ pub fn alloc_ty(bcx: &Block, t: ty::t, name: &str) -> ValueRef {
 }
 
 pub fn alloca(cx: &Block, ty: Type, name: &str) -> ValueRef {
-    alloca_maybe_zeroed(cx, ty, name, false)
+    let p = alloca_no_lifetime(cx, ty, name);
+    call_lifetime_start(cx, p);
+    p
 }
 
-pub fn alloca_maybe_zeroed(cx: &Block, ty: Type, name: &str, zero: bool) -> ValueRef {
+pub fn alloca_no_lifetime(cx: &Block, ty: Type, name: &str) -> ValueRef {
     let _icx = push_ctxt("alloca");
     if cx.unreachable.get() {
         unsafe {
@@ -1180,14 +1182,19 @@ pub fn alloca_maybe_zeroed(cx: &Block, ty: Type, name: &str, zero: bool) -> Valu
         }
     }
     debuginfo::clear_source_location(cx.fcx);
-    let p = Alloca(cx, ty, name);
-    if zero {
-        let b = cx.fcx.ccx.builder();
-        b.position_before(cx.fcx.alloca_insert_pt.get().unwrap());
-        memzero(&b, p, ty);
-    } else {
-        call_lifetime_start(cx, p);
+    Alloca(cx, ty, name)
+}
+
+pub fn alloca_zeroed(cx: &Block, ty: Type, name: &str) -> ValueRef {
+    if cx.unreachable.get() {
+        unsafe {
+            return llvm::LLVMGetUndef(ty.ptr_to().to_ref());
+        }
     }
+    let p = alloca_no_lifetime(cx, ty, name);
+    let b = cx.fcx.ccx.builder();
+    b.position_before(cx.fcx.alloca_insert_pt.get().unwrap());
+    memzero(&b, p, ty);
     p
 }
 

src/librustc/middle/trans/datum.rs

@@ -120,7 +120,11 @@ pub fn lvalue_scratch_datum<'a, A>(bcx: &'a Block<'a>,
      */
 
     let llty = type_of::type_of(bcx.ccx(), ty);
-    let scratch = alloca_maybe_zeroed(bcx, llty, name, zero);
+    let scratch = if zero {
+        alloca_zeroed(bcx, llty, name)
+    } else {
+        alloca(bcx, llty, name)
+    };
 
     // Subtle. Populate the scratch memory *before* scheduling cleanup.
     let bcx = populate(arg, bcx, scratch);
@@ -145,7 +149,7 @@ pub fn rvalue_scratch_datum(bcx: &Block,
      */
 
     let llty = type_of::type_of(bcx.ccx(), ty);
-    let scratch = alloca_maybe_zeroed(bcx, llty, name, false);
+    let scratch = alloca(bcx, llty, name);
     Datum::new(scratch, ty, Rvalue::new(ByRef))
 }