chacha20: SSE2 autodetection support (#270)

i586 targets do not have SSE2 enabled by default.

This commit adds autodetection for SSE2 in addition to the existing
autodetection for AVX2, and falls back to a portable implementation if
SSE2 is not available.

Author: tarcieri

chacha20/src/backend/autodetect.rs

@@ -1,8 +1,7 @@
-//! Autodetection support for AVX2 CPU intrinsics on x86 CPUs, with fallback
-//! to the SSE2 backend when it's unavailable (the `sse2` target feature is
-//! enabled-by-default on all x86(_64) CPUs)
+//! Autodetection support for AVX2 CPU and SSE2 intrinsics on x86 CPUs, with
+//! fallback to a portable version when they're unavailable.
 
-use super::{avx2, sse2};
+use super::{avx2, soft, sse2};
 use crate::{rounds::Rounds, BLOCK_SIZE, IV_SIZE, KEY_SIZE};
 use core::mem::ManuallyDrop;
 
@@ -13,75 +12,99 @@ use core::mem::ManuallyDrop;
 pub(crate) const BUFFER_SIZE: usize = BLOCK_SIZE * 4;
 
 cpufeatures::new!(avx2_cpuid, "avx2");
+cpufeatures::new!(sse2_cpuid, "sse2");
 
 /// The ChaCha20 core function.
 pub struct Core<R: Rounds> {
     inner: Inner<R>,
-    token: avx2_cpuid::InitToken,
+    avx2_token: avx2_cpuid::InitToken,
+    sse2_token: sse2_cpuid::InitToken,
 }
 
 union Inner<R: Rounds> {
     avx2: ManuallyDrop<avx2::Core<R>>,
     sse2: ManuallyDrop<sse2::Core<R>>,
+    soft: ManuallyDrop<soft::Core<R>>,
 }
 
 impl<R: Rounds> Core<R> {
     /// Initialize ChaCha core function with the given key size, IV, and
     /// number of rounds.
+    ///
+    /// Attempts to use AVX2 if present, followed by SSE2, with fallback to a
+    /// portable software implementation if neither are available.
     #[inline]
     pub fn new(key: &[u8; KEY_SIZE], iv: [u8; IV_SIZE]) -> Self {
-        let (token, avx2_present) = avx2_cpuid::init_get();
+        let (avx2_token, avx2_present) = avx2_cpuid::init_get();
+        let (sse2_token, sse2_present) = sse2_cpuid::init_get();
 
         let inner = if avx2_present {
             Inner {
                 avx2: ManuallyDrop::new(avx2::Core::new(key, iv)),
             }
-        } else {
+        } else if sse2_present {
             Inner {
                 sse2: ManuallyDrop::new(sse2::Core::new(key, iv)),
             }
+        } else {
+            Inner {
+                soft: ManuallyDrop::new(soft::Core::new(key, iv)),
+            }
         };
 
-        Self { inner, token }
+        Self {
+            inner,
+            avx2_token,
+            sse2_token,
+        }
     }
 
-    /// Generate output, overwriting data already in the buffer
+    /// Generate output, overwriting data already in the buffer.
     #[inline]
-    pub fn generate(&self, counter: u64, output: &mut [u8]) {
-        if self.token.get() {
+    pub fn generate(&mut self, counter: u64, output: &mut [u8]) {
+        if self.avx2_token.get() {
             unsafe { (*self.inner.avx2).generate(counter, output) }
-        } else {
+        } else if self.sse2_token.get() {
             unsafe { (*self.inner.sse2).generate(counter, output) }
+        } else {
+            unsafe { (*self.inner.soft).generate(counter, output) }
         }
     }
 
-    /// Apply generated keystream to the output buffer
+    /// Apply generated keystream to the output buffer.
     #[inline]
     #[cfg(feature = "cipher")]
-    pub fn apply_keystream(&self, counter: u64, output: &mut [u8]) {
-        if self.token.get() {
+    pub fn apply_keystream(&mut self, counter: u64, output: &mut [u8]) {
+        if self.avx2_token.get() {
             unsafe { (*self.inner.avx2).apply_keystream(counter, output) }
-        } else {
+        } else if self.sse2_token.get() {
             unsafe { (*self.inner.sse2).apply_keystream(counter, output) }
+        } else {
+            unsafe { (*self.inner.soft).apply_keystream(counter, output) }
         }
     }
 }
 
 impl<R: Rounds> Clone for Core<R> {
     fn clone(&self) -> Self {
-        let inner = if self.token.get() {
+        let inner = if self.avx2_token.get() {
             Inner {
                 avx2: ManuallyDrop::new(unsafe { (*self.inner.avx2).clone() }),
             }
-        } else {
+        } else if self.sse2_token.get() {
             Inner {
                 sse2: ManuallyDrop::new(unsafe { (*self.inner.sse2).clone() }),
             }
+        } else {
+            Inner {
+                soft: ManuallyDrop::new(unsafe { (*self.inner.soft).clone() }),
+            }
         };
 
         Self {
             inner,
-            token: self.token,
+            avx2_token: self.avx2_token,
+            sse2_token: self.sse2_token,
         }
     }
 }