[mir-opt] Turn on the ConstProp pass by default

perf.rlo shows that running the `ConstProp` pass results in
across-the-board wins regardless of debug or opt complilation mode. As a
result, we're turning it on to get the compile time benefits.

`ConstProp` doesn't currently intern the memory used by its `Machine` so
we can't yet propagate allocations which is why
`ConstProp::should_const_prop()` checks if the value being propagated is
a scalar or not.

src/librustc_mir/transform/const_prop.rs

@@ -641,8 +641,21 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
         }
     }
 
-    fn should_const_prop(&self) -> bool {
-        self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2
+    fn should_const_prop(&mut self, op: OpTy<'tcx>) -> bool {
+        if self.tcx.sess.opts.debugging_opts.mir_opt_level >= 2 {
+            return true;
+        } else if self.tcx.sess.opts.debugging_opts.mir_opt_level == 0 {
+            return false;
+        }
+
+        match *op {
+            interpret::Operand::Immediate(Immediate::Scalar(ScalarMaybeUndef::Scalar(s))) =>
+                s.is_bits(),
+            interpret::Operand::Immediate(Immediate::ScalarPair(ScalarMaybeUndef::Scalar(l),
+                                                                ScalarMaybeUndef::Scalar(r))) =>
+                l.is_bits() && r.is_bits(),
+            _ => false
+        }
     }
 }
 
@@ -742,15 +755,15 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
                         if self.can_const_prop[local] {
                             trace!("propagated into {:?}", local);
 
-                            if self.should_const_prop() {
-                                let value =
-                                    self.get_const(local).expect("local was dead/uninitialized");
-                                trace!("replacing {:?} with {:?}", rval, value);
-                                self.replace_with_const(
-                                    rval,
-                                    value,
-                                    statement.source_info,
-                                );
+                            if let Some(value) = self.get_const(local) {
+                                if self.should_const_prop(value) {
+                                    trace!("replacing {:?} with {:?}", rval, value);
+                                    self.replace_with_const(
+                                        rval,
+                                        value,
+                                        statement.source_info,
+                                    );
+                                }
                             }
                         } else {
                             trace!("can't propagate into {:?}", local);
@@ -852,7 +865,7 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
                             &msg,
                         );
                     } else {
-                        if self.should_const_prop() {
+                        if self.should_const_prop(value) {
                             if let ScalarMaybeUndef::Scalar(scalar) = value_const {
                                 *cond = self.operand_from_scalar(
                                     scalar,
@@ -865,8 +878,8 @@ impl<'mir, 'tcx> MutVisitor<'tcx> for ConstPropagator<'mir, 'tcx> {
                 }
             },
             TerminatorKind::SwitchInt { ref mut discr, switch_ty, .. } => {
-                if self.should_const_prop() {
-                    if let Some(value) = self.eval_operand(&discr, source_info) {
+                if let Some(value) = self.eval_operand(&discr, source_info) {
+                    if self.should_const_prop(value) {
                         if let ScalarMaybeUndef::Scalar(scalar) =
                                 self.ecx.read_scalar(value).unwrap() {
                             *discr = self.operand_from_scalar(scalar, switch_ty, source_info.span);

src/test/codegen/optimize-attr-1.rs

@@ -8,7 +8,7 @@
 
 // CHECK-LABEL: define i32 @nothing
 // CHECK-SAME: [[NOTHING_ATTRS:#[0-9]+]]
-// NO-OPT: ret i32 %_1.0
+// NO-OPT: ret i32 4
 // SIZE-OPT: ret i32 4
 // SPEEC-OPT: ret i32 4
 #[no_mangle]
@@ -18,7 +18,7 @@ pub fn nothing() -> i32 {
 
 // CHECK-LABEL: define i32 @size
 // CHECK-SAME: [[SIZE_ATTRS:#[0-9]+]]
-// NO-OPT: ret i32 %_1.0
+// NO-OPT: ret i32 6
 // SIZE-OPT: ret i32 6
 // SPEED-OPT: ret i32 6
 #[optimize(size)]
@@ -31,7 +31,7 @@ pub fn size() -> i32 {
 // NO-OPT-SAME: [[NOTHING_ATTRS]]
 // SPEED-OPT-SAME: [[NOTHING_ATTRS]]
 // SIZE-OPT-SAME: [[SPEED_ATTRS:#[0-9]+]]
-// NO-OPT: ret i32 %_1.0
+// NO-OPT: ret i32 8
 // SIZE-OPT: ret i32 8
 // SPEED-OPT: ret i32 8
 #[optimize(speed)]

src/test/incremental/hashes/while_let_loops.rs

@@ -48,7 +48,7 @@ pub fn change_loop_condition() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_loop_condition() {
     let mut _x = 0;
@@ -70,7 +70,7 @@ pub fn add_break() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir, typeck_tables_of")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, typeck_tables_of")]
 #[rustc_clean(cfg="cfail3")]
 pub fn add_break() {
     let mut _x = 0;
@@ -141,7 +141,7 @@ pub fn change_break_label() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_break_label() {
     let mut _x = 0;
@@ -191,7 +191,7 @@ pub fn change_continue_label() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_continue_label() {
     let mut _x = 0;
@@ -216,7 +216,7 @@ pub fn change_continue_to_break() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_continue_to_break() {
     let mut _x = 0;

src/test/incremental/hashes/while_loops.rs

@@ -48,7 +48,7 @@ pub fn change_loop_condition() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_loop_condition() {
     let mut _x = 0;
@@ -191,7 +191,7 @@ pub fn change_continue_label() {
 }
 
 #[cfg(not(cfail1))]
-#[rustc_clean(cfg="cfail2", except="HirBody, mir_built, optimized_mir")]
+#[rustc_clean(cfg="cfail2", except="HirBody, mir_built")]
 #[rustc_clean(cfg="cfail3")]
 pub fn change_continue_label() {
     let mut _x = 0;