Enable conditional build of SIMD features

The packed_simd features enables SIMD.
If the feature isn't enabled, ssimd is used, which uses plain structs to emulate simd vectors.
When optimizations are enabled and the target architecture supports it, the compiler should still
emit SIMD instructions for the ssimd code.

Author: lovasoa

Cargo.toml

@@ -13,7 +13,8 @@ exclude = ["tests/*"]
 [dependencies]
 byteorder = "1.0"
 rayon = { version = "1.0", optional = true }
-packed_simd = "0.3"
+ssimd = { git = "https://github.com/lovasoa/ssimd.git" }
+packed_simd = { version = "0.3", optional = true }
 
 [dev-dependencies]
 png = "0.16"

src/idct.rs

@@ -4,7 +4,19 @@
 use crate::parser::Dimensions;
 use std::num::Wrapping;
 
-use packed_simd::{
+#[cfg(feature = "packed_simd")]
+extern crate packed_simd;
+
+#[cfg(feature = "packed_simd")]
+use self::packed_simd as simd;
+
+#[cfg(not(feature = "packed_simd"))]
+extern crate ssimd;
+
+#[cfg(not(feature = "packed_simd"))]
+use self::ssimd as simd;
+
+use self::simd::{
     i32x8, i16x8, u16x8, u8x8,
     i32x4, i16x4, u16x4, u8x4,
     FromCast,

src/lib.rs

@@ -32,7 +32,6 @@
 extern crate byteorder;
 #[cfg(feature="rayon")]
 extern crate rayon;
-extern crate packed_simd;
 
 pub use decoder::{Decoder, ImageInfo, PixelFormat};
 pub use error::{Error, UnsupportedFeature};