rustc_trans: don't emit ZST allocas that are only assigned to.

src/librustc_trans/mir/analyze.rs

@@ -18,7 +18,6 @@ use rustc::mir::visit::{Visitor, LvalueContext};
 use rustc::mir::traversal;
 use common;
 use super::MirContext;
-use super::rvalue;
 
 pub fn lvalue_locals<'a, 'tcx>(mircx: &MirContext<'a, 'tcx>) -> BitVector {
     let mir = mircx.mir;
@@ -92,7 +91,7 @@ impl<'mir, 'a, 'tcx> Visitor<'tcx> for LocalAnalyzer<'mir, 'a, 'tcx> {
 
         if let mir::Lvalue::Local(index) = *lvalue {
             self.mark_assigned(index);
-            if !rvalue::rvalue_creates_operand(rvalue) {
+            if !self.cx.rvalue_creates_operand(rvalue) {
                 self.mark_as_lvalue(index);
             }
         } else {

src/librustc_trans/mir/mod.rs

@@ -11,20 +11,18 @@
 use libc::c_uint;
 use llvm::{self, ValueRef, BasicBlockRef};
 use llvm::debuginfo::DIScope;
-use rustc::ty::{self, layout};
+use rustc::ty::{self, layout, Ty, TypeFoldable};
 use rustc::mir::{self, Mir};
 use rustc::mir::tcx::LvalueTy;
 use rustc::ty::subst::Substs;
 use rustc::infer::TransNormalize;
-use rustc::ty::TypeFoldable;
 use session::config::FullDebugInfo;
 use base;
 use builder::Builder;
-use common::{self, CrateContext, C_null, Funclet};
+use common::{self, CrateContext, Funclet};
 use debuginfo::{self, declare_local, VariableAccess, VariableKind, FunctionDebugContext};
 use monomorphize::{self, Instance};
 use abi::FnType;
-use type_of;
 
 use syntax_pos::{DUMMY_SP, NO_EXPANSION, COMMAND_LINE_EXPN, BytePos, Span};
 use syntax::symbol::keywords;
@@ -176,23 +174,12 @@ enum LocalRef<'tcx> {
 
 impl<'tcx> LocalRef<'tcx> {
     fn new_operand<'a>(ccx: &CrateContext<'a, 'tcx>,
-                         ty: ty::Ty<'tcx>) -> LocalRef<'tcx> {
+                       ty: Ty<'tcx>) -> LocalRef<'tcx> {
         if common::type_is_zero_size(ccx, ty) {
             // Zero-size temporaries aren't always initialized, which
             // doesn't matter because they don't contain data, but
             // we need something in the operand.
-            let llty = type_of::type_of(ccx, ty);
-            let val = if common::type_is_imm_pair(ccx, ty) {
-                let fields = llty.field_types();
-                OperandValue::Pair(C_null(fields[0]), C_null(fields[1]))
-            } else {
-                OperandValue::Immediate(C_null(llty))
-            };
-            let op = OperandRef {
-                val: val,
-                ty: ty
-            };
-            LocalRef::Operand(Some(op))
+            LocalRef::Operand(Some(OperandRef::new_zst(ccx, ty)))
         } else {
             LocalRef::Operand(None)
         }

src/librustc_trans/mir/operand.rs

@@ -15,7 +15,7 @@ use rustc::mir;
 use rustc_data_structures::indexed_vec::Idx;
 
 use base;
-use common;
+use common::{self, CrateContext, C_null};
 use builder::Builder;
 use value::Value;
 use type_of;
@@ -77,6 +77,22 @@ impl<'tcx> fmt::Debug for OperandRef<'tcx> {
 }
 
 impl<'a, 'tcx> OperandRef<'tcx> {
+    pub fn new_zst(ccx: &CrateContext<'a, 'tcx>,
+                   ty: Ty<'tcx>) -> OperandRef<'tcx> {
+        assert!(common::type_is_zero_size(ccx, ty));
+        let llty = type_of::type_of(ccx, ty);
+        let val = if common::type_is_imm_pair(ccx, ty) {
+            let fields = llty.field_types();
+            OperandValue::Pair(C_null(fields[0]), C_null(fields[1]))
+        } else {
+            OperandValue::Immediate(C_null(llty))
+        };
+        OperandRef {
+            val: val,
+            ty: ty
+        }
+    }
+
     /// Asserts that this operand refers to a scalar and returns
     /// a reference to its value.
     pub fn immediate(self) -> ValueRef {

src/librustc_trans/mir/rvalue.rs

@@ -157,7 +157,7 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
             }
 
             _ => {
-                assert!(rvalue_creates_operand(rvalue));
+                assert!(self.rvalue_creates_operand(rvalue));
                 let (bcx, temp) = self.trans_rvalue_operand(bcx, rvalue);
                 self.store_operand(&bcx, dest.llval, dest.alignment.to_align(), temp);
                 bcx
@@ -170,7 +170,7 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
                                 rvalue: &mir::Rvalue<'tcx>)
                                 -> (Builder<'a, 'tcx>, OperandRef<'tcx>)
     {
-        assert!(rvalue_creates_operand(rvalue), "cannot trans {:?} to operand", rvalue);
+        assert!(self.rvalue_creates_operand(rvalue), "cannot trans {:?} to operand", rvalue);
 
         match *rvalue {
             mir::Rvalue::Cast(ref kind, ref source, cast_ty) => {
@@ -478,8 +478,10 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
             }
             mir::Rvalue::Repeat(..) |
             mir::Rvalue::Aggregate(..) => {
-                bug!("cannot generate operand from rvalue {:?}", rvalue);
-
+                // According to `rvalue_creates_operand`, only ZST
+                // aggregate rvalues are allowed to be operands.
+                let ty = rvalue.ty(self.mir, self.ccx.tcx());
+                (bcx, OperandRef::new_zst(self.ccx, self.monomorphize(&ty)))
             }
         }
     }
@@ -662,26 +664,29 @@ impl<'a, 'tcx> MirContext<'a, 'tcx> {
 
         OperandValue::Pair(val, of)
     }
-}
 
-pub fn rvalue_creates_operand(rvalue: &mir::Rvalue) -> bool {
-    match *rvalue {
-        mir::Rvalue::Ref(..) |
-        mir::Rvalue::Len(..) |
-        mir::Rvalue::Cast(..) | // (*)
-        mir::Rvalue::BinaryOp(..) |
-        mir::Rvalue::CheckedBinaryOp(..) |
-        mir::Rvalue::UnaryOp(..) |
-        mir::Rvalue::Discriminant(..) |
-        mir::Rvalue::Box(..) |
-        mir::Rvalue::Use(..) =>
-            true,
-        mir::Rvalue::Repeat(..) |
-        mir::Rvalue::Aggregate(..) =>
-            false,
-    }
+    pub fn rvalue_creates_operand(&self, rvalue: &mir::Rvalue<'tcx>) -> bool {
+        match *rvalue {
+            mir::Rvalue::Ref(..) |
+            mir::Rvalue::Len(..) |
+            mir::Rvalue::Cast(..) | // (*)
+            mir::Rvalue::BinaryOp(..) |
+            mir::Rvalue::CheckedBinaryOp(..) |
+            mir::Rvalue::UnaryOp(..) |
+            mir::Rvalue::Discriminant(..) |
+            mir::Rvalue::Box(..) |
+            mir::Rvalue::Use(..) =>
+                true,
+            mir::Rvalue::Repeat(..) |
+            mir::Rvalue::Aggregate(..) => {
+                let ty = rvalue.ty(self.mir, self.ccx.tcx());
+                let ty = self.monomorphize(&ty);
+                common::type_is_zero_size(self.ccx, ty)
+            }
+        }
 
-    // (*) this is only true if the type is suitable
+        // (*) this is only true if the type is suitable
+    }
 }
 
 #[derive(Copy, Clone)]