Support no_std compile-time SIMD (#214)

Author: CathalMullan

src/lib.rs

@@ -15,7 +15,7 @@
 //! parsing internals use an `Iterator` instead of direct indexing, while
 //! skipping bounds checks.
 //!
-//! With Rust 1.27.0 or later, support for SIMD is enabled automatically.
+//! SIMD optimizations are enabled automatically when available.
 //! If building an executable to be run on multiple platforms, and thus
 //! not passing `target_feature` or `target_cpu` flags to the compiler,
 //! runtime detection can still detect SSE4.2 or AVX2 support to provide

src/simd/mod.rs

@@ -3,21 +3,34 @@ mod swar;
 #[cfg(any(
     httparse_disable_simd,
     miri,
-    not(feature = "std"),
     not(any(
         target_arch = "x86",
         target_arch = "x86_64",
         all(
             target_arch = "aarch64",
             target_feature = "neon",
         )
-    ))
+    )),
+    all(
+        not(feature = "std"),
+        not(any(
+            target_feature = "sse4.2",
+            target_feature = "avx2",
+        )),
+        any(
+            target_arch = "x86",
+            target_arch = "x86_64",
+        ),
+    )
 ))]
 pub use self::swar::*;
 
 #[cfg(all(
     not(any(httparse_disable_simd, miri)),
-    feature = "std",
+    any(
+        feature = "std",
+        target_feature = "sse4.2",
+    ),
     not(target_feature = "avx2"),
     any(
         target_arch = "x86",
@@ -28,10 +41,12 @@ mod sse42;
 
 #[cfg(all(
     not(any(httparse_disable_simd, miri)),
-    feature = "std",
     any(
         target_feature = "avx2",
-        not(target_feature = "sse4.2"),
+        all(
+            feature = "std",
+            not(target_feature = "sse4.2"),
+        ),
     ),
     any(
         target_arch = "x86",
@@ -70,7 +85,6 @@ pub use self::runtime::*;
 
 #[cfg(all(
     not(any(httparse_disable_simd, miri)),
-    feature = "std",
     target_feature = "sse4.2",
     not(target_feature = "avx2"),
     any(
@@ -97,7 +111,6 @@ mod sse42_compile_time {
 
 #[cfg(all(
     not(any(httparse_disable_simd, miri)),
-    feature = "std",
     target_feature = "sse4.2",
     not(target_feature = "avx2"),
     any(
@@ -109,7 +122,6 @@ pub use self::sse42_compile_time::*;
 
 #[cfg(all(
     not(any(httparse_disable_simd, miri)),
-    feature = "std",
     target_feature = "avx2",
     any(
         target_arch = "x86",
@@ -135,7 +147,6 @@ mod avx2_compile_time {
 
 #[cfg(all(
     not(any(httparse_disable_simd, miri)),
-    feature = "std",
     target_feature = "avx2",
     any(
         target_arch = "x86",
@@ -146,15 +157,13 @@ pub use self::avx2_compile_time::*;
 
 #[cfg(all(
     not(any(httparse_disable_simd, miri)),
-    feature = "std",
     target_arch = "aarch64",
     target_feature = "neon",
 ))]
 mod neon;
 
 #[cfg(all(
     not(any(httparse_disable_simd, miri)),
-    feature = "std",
     target_arch = "aarch64",
     target_feature = "neon",
 ))]