Rollup merge of #46264 - scottmcm:mir-array-len, r=arielb1

InstCombine Len([_; N]) => const N in MIR

A small opportunity I noticed in passing.

Not super exciting on its own, but opens the door for a const propagation pass that could completely remove const bounds checks from arrays at MIR time, for example.

Author: kennytm

src/librustc_mir/transform/instcombine.rs

@@ -10,10 +10,10 @@
 
 //! Performs various peephole optimizations.
 
-use rustc::mir::{Location, Lvalue, Mir, Operand, ProjectionElem, Rvalue, Local};
+use rustc::mir::{Constant, Literal, Location, Lvalue, Mir, Operand, ProjectionElem, Rvalue, Local};
 use rustc::mir::visit::{MutVisitor, Visitor};
-use rustc::ty::TyCtxt;
-use rustc::util::nodemap::FxHashSet;
+use rustc::ty::{TyCtxt, TypeVariants};
+use rustc::util::nodemap::{FxHashMap, FxHashSet};
 use rustc_data_structures::indexed_vec::Idx;
 use std::mem;
 use transform::{MirPass, MirSource};
@@ -44,11 +44,11 @@ impl MirPass for InstCombine {
     }
 }
 
-pub struct InstCombineVisitor {
-    optimizations: OptimizationList,
+pub struct InstCombineVisitor<'tcx> {
+    optimizations: OptimizationList<'tcx>,
 }
 
-impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor {
+impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
     fn visit_rvalue(&mut self, rvalue: &mut Rvalue<'tcx>, location: Location) {
         if self.optimizations.and_stars.remove(&location) {
             debug!("Replacing `&*`: {:?}", rvalue);
@@ -62,6 +62,11 @@ impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor {
             *rvalue = Rvalue::Use(Operand::Consume(new_lvalue))
         }
 
+        if let Some(constant) = self.optimizations.arrays_lengths.remove(&location) {
+            debug!("Replacing `Len([_; N])`: {:?}", rvalue);
+            *rvalue = Rvalue::Use(Operand::Constant(box constant));
+        }
+
         self.super_rvalue(rvalue, location)
     }
 }
@@ -70,7 +75,7 @@ impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor {
 struct OptimizationFinder<'b, 'a, 'tcx:'a+'b> {
     mir: &'b Mir<'tcx>,
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
-    optimizations: OptimizationList,
+    optimizations: OptimizationList<'tcx>,
 }
 
 impl<'b, 'a, 'tcx:'b> OptimizationFinder<'b, 'a, 'tcx> {
@@ -93,11 +98,23 @@ impl<'b, 'a, 'tcx> Visitor<'tcx> for OptimizationFinder<'b, 'a, 'tcx> {
             }
         }
 
+        if let Rvalue::Len(ref lvalue) = *rvalue {
+            let lvalue_ty = lvalue.ty(&self.mir.local_decls, self.tcx).to_ty(self.tcx);
+            if let TypeVariants::TyArray(_, len) = lvalue_ty.sty {
+                let span = self.mir.source_info(location).span;
+                let ty = self.tcx.types.usize;
+                let literal = Literal::Value { value: len };
+                let constant = Constant { span, ty, literal };
+                self.optimizations.arrays_lengths.insert(location, constant);
+            }
+        }
+
         self.super_rvalue(rvalue, location)
     }
 }
 
 #[derive(Default)]
-struct OptimizationList {
+struct OptimizationList<'tcx> {
     and_stars: FxHashSet<Location>,
+    arrays_lengths: FxHashMap<Location, Constant<'tcx>>,
 }

src/test/mir-opt/combine_array_len.rs

@@ -0,0 +1,33 @@
+// Copyright 2017 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.
+
+fn norm2(x: [f32; 2]) -> f32 {
+    let a = x[0];
+    let b = x[1];
+    a*a + b*b
+}
+
+fn main() {
+    assert_eq!(norm2([3.0, 4.0]), 5.0*5.0);
+}
+
+// END RUST SOURCE
+
+// START rustc.norm2.InstCombine.before.mir
+//     _5 = Len(_1);
+//     ...
+//     _10 = Len(_1);
+// END rustc.norm2.InstCombine.before.mir
+
+// START rustc.norm2.InstCombine.after.mir
+//     _5 = const 2usize;
+//     ...
+//     _10 = const 2usize;
+// END rustc.norm2.InstCombine.after.mir