core/char: Speed up to_digit() for radix <= 10

src/libcore/benches/char/methods.rs

@@ -0,0 +1,42 @@
+// 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.
+
+use test::Bencher;
+
+const CHARS: [char; 9] = ['0', 'x', '2', '5', 'A', 'f', '7', '8', '9'];
+const RADIX: [u32; 5] = [2, 8, 10, 16, 32];
+
+#[bench]
+fn bench_to_digit_radix_2(b: &mut Bencher) {
+    b.iter(|| CHARS.iter().cycle().take(10_000).map(|c| c.to_digit(2)).min())
+}
+
+#[bench]
+fn bench_to_digit_radix_10(b: &mut Bencher) {
+    b.iter(|| CHARS.iter().cycle().take(10_000).map(|c| c.to_digit(10)).min())
+}
+
+#[bench]
+fn bench_to_digit_radix_16(b: &mut Bencher) {
+    b.iter(|| CHARS.iter().cycle().take(10_000).map(|c| c.to_digit(16)).min())
+}
+
+#[bench]
+fn bench_to_digit_radix_36(b: &mut Bencher) {
+    b.iter(|| CHARS.iter().cycle().take(10_000).map(|c| c.to_digit(36)).min())
+}
+
+#[bench]
+fn bench_to_digit_radix_var(b: &mut Bencher) {
+    b.iter(|| CHARS.iter().cycle()
+        .zip(RADIX.iter().cycle())
+        .take(10_000)
+        .map(|(c, radix)| c.to_digit(*radix)).min())
+}

src/libcore/benches/char/mod.rs

@@ -0,0 +1,11 @@
+// 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.
+
+mod methods;

src/libcore/benches/lib.rs

@@ -15,6 +15,7 @@ extern crate core;
 extern crate test;
 
 mod any;
+mod char;
 mod hash;
 mod iter;
 mod num;

src/libcore/char/methods.rs

@@ -121,15 +121,24 @@ impl char {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn to_digit(self, radix: u32) -> Option<u32> {
-        if radix > 36 {
-            panic!("to_digit: radix is too high (maximum 36)");
-        }
-        let val = match self {
-          '0' ..= '9' => self as u32 - '0' as u32,
-          'a' ..= 'z' => self as u32 - 'a' as u32 + 10,
-          'A' ..= 'Z' => self as u32 - 'A' as u32 + 10,
-          _ => return None,
+        assert!(radix <= 36, "to_digit: radix is too high (maximum 36)");
+
+        // the code is split up here to improve execution speed for cases where
+        // the `radix` is constant and 10 or smaller
+        let val = if radix <= 10  {
+            match self {
+                '0' ..= '9' => self as u32 - '0' as u32,
+                _ => return None,
+            }
+        } else {
+            match self {
+                '0'..='9' => self as u32 - '0' as u32,
+                'a'..='z' => self as u32 - 'a' as u32 + 10,
+                'A'..='Z' => self as u32 - 'A' as u32 + 10,
+                _ => return None,
+            }
         };
+
         if val < radix { Some(val) }
         else { None }
     }