Replace avx2_backend with simd_backend (autoselects AVX2/IFMA)

Cargo.toml

@@ -69,10 +69,10 @@ yolocrypto = []
 u32_backend = []
 # The u64 backend uses u64s with u128 products.
 u64_backend = []
-# The AVX2 backend uses u32x8s with u64x4 products.
-# It uses the u64 code for serial operations.
-avx2_backend = ["nightly", "u64_backend", "packed_simd"]
-ifma_backend = ["nightly", "u64_backend", "packed_simd", "yolocrypto"]
+# The SIMD backend uses parallel formulas, using either AVX2 or AVX512-IFMA.
+simd_backend = ["nightly", "u64_backend", "packed_simd"]
+# Old name for the SIMD backend, preserved for compatibility
+avx2_backend = ["simd_backend"]
 
 # Signals that we're in the main build stage.  This is off by default,
 # to signal stage 1 of the build, where build.rs loads the library

build.rs

@@ -1,5 +1,11 @@
-#![cfg_attr(feature = "ifma_backend", feature(simd_ffi))]
-#![cfg_attr(feature = "ifma_backend", feature(link_llvm_intrinsics))]
+#![cfg_attr(
+    all(feature = "simd_backend", target_feature = "avx512ifma"),
+    feature(simd_ffi)
+)]
+#![cfg_attr(
+    all(feature = "simd_backend", target_feature = "avx512ifma"),
+    feature(link_llvm_intrinsics)
+)]
 #![cfg_attr(all(feature = "alloc", not(feature = "std")), feature(alloc))]
 #![cfg_attr(feature = "nightly", feature(cfg_target_feature))]
 #![allow(unused_variables)]
@@ -15,7 +21,7 @@ extern crate digest;
 extern crate rand;
 extern crate subtle;
 
-#[cfg(all(feature = "nightly", any(feature = "avx2_backend", feature = "ifma_backend")))]
+#[cfg(all(feature = "nightly", feature = "packed_simd"))]
 extern crate packed_simd;
 
 use std::env;

src/backend/mod.rs

@@ -24,17 +24,17 @@
 #[cfg(not(any(
     feature = "u32_backend",
     feature = "u64_backend",
-    feature = "avx2_backend"
+    feature = "simd_backend",
 )))]
 compile_error!(
     "no curve25519-dalek backend cargo feature enabled! \
-     please enable one of: u32_backend, u64_backend, avx2_backend"
+     please enable one of: u32_backend, u64_backend, simd_backend"
 );
 
 pub mod serial;
 
-#[cfg(any(
-    all(feature = "ifma_backend", target_feature = "avx512ifma"),
-    all(feature = "avx2_backend", target_feature = "avx2"),
+#[cfg(all(
+    feature = "simd_backend",
+    any(target_feature = "avx2", target_feature = "avx512ifma")
 ))]
 pub mod vector;

src/backend/vector/mod.rs

@@ -21,15 +21,17 @@
 //! `32bit` since identifiers can't start with letters, and the backends
 //! do use `u32`/`u64`, so this seems like a least-bad option.
 
-#[cfg(all(feature = "avx2_backend", target_feature = "avx2"))]
+#[cfg(not(any(target_feature = "avx2", target_feature = "avx512ifma",)))]
+compile_error!("simd_backend selected without target_feature=+avx2 or +avx512ifma");
+
+#[cfg(all(target_feature = "avx2", not(target_feature = "avx512ifma")))]
 pub mod avx2;
-#[cfg(all(feature = "avx2_backend", target_feature = "avx2"))]
+#[cfg(all(target_feature = "avx2", not(target_feature = "avx512ifma")))]
 pub(crate) use self::avx2::{
     constants::BASEPOINT_ODD_LOOKUP_TABLE, edwards::CachedPoint, edwards::ExtendedPoint,
 };
 
-#[cfg(all(feature = "ifma_backend", target_feature = "avx512ifma"))]
+#[cfg(all(target_feature = "avx512ifma"))]
 pub mod ifma;
 
 pub mod scalar_mul;
-

src/edwards.rs

@@ -130,6 +130,17 @@ use traits::MultiscalarMul;
 #[cfg(any(feature = "alloc", feature = "std"))]
 use traits::VartimeMultiscalarMul;
 
+#[cfg(not(all(
+    feature = "simd_backend",
+    any(target_feature = "avx2", target_feature = "avx512ifma")
+)))]
+use backend::serial::scalar_mul;
+#[cfg(all(
+    feature = "simd_backend",
+    any(target_feature = "avx2", target_feature = "avx512ifma")
+))]
+use backend::vector::scalar_mul;
+
 // ------------------------------------------------------------------------
 // Compressed points
 // ------------------------------------------------------------------------
@@ -572,18 +583,7 @@ impl<'a, 'b> Mul<&'b Scalar> for &'a EdwardsPoint {
     /// For scalar multiplication of a basepoint,
     /// `EdwardsBasepointTable` is approximately 4x faster.
     fn mul(self, scalar: &'b Scalar) -> EdwardsPoint {
-        // If we built with AVX2, use the AVX2 backend.
-        #[cfg(all(feature="avx2_backend", target_feature="avx2"))]
-        {
-            use backend::vector::scalar_mul::variable_base::mul;
-            mul(self, scalar)
-        }
-        // Otherwise, use the serial backend:
-        #[cfg(not(all(feature="avx2_backend", target_feature="avx2")))]
-        {
-            use backend::serial::scalar_mul::variable_base::mul;
-            mul(self, scalar)
-        }
+        scalar_mul::variable_base::mul(self, scalar)
     }
 }
 
@@ -634,14 +634,7 @@ impl MultiscalarMul for EdwardsPoint {
         // size-dependent algorithm dispatch, use this as the hint.
         let _size = s_lo;
 
-        // If we built with AVX2, use the AVX2 backend.
-        #[cfg(all(feature="avx2_backend", target_feature="avx2"))]
-        use backend::vector::scalar_mul::straus::Straus;
-        // Otherwise, proceed as normal:
-        #[cfg(not(all(feature="avx2_backend", target_feature="avx2")))]
-        use backend::serial::scalar_mul::straus::Straus;
-
-        Straus::multiscalar_mul(scalars, points)
+        scalar_mul::straus::Straus::multiscalar_mul(scalars, points)
     }
 }
 
@@ -672,29 +665,19 @@ impl VartimeMultiscalarMul for EdwardsPoint {
         // size-dependent algorithm dispatch, use this as the hint.
         let _size = s_lo;
 
-        // If we built with AVX2, use the AVX2 backend.
-        #[cfg(all(feature="avx2_backend", target_feature="avx2"))]
-        use backend::vector::scalar_mul::straus::Straus;
-        // Otherwise, proceed as normal:
-        #[cfg(not(all(feature="avx2_backend", target_feature="avx2")))]
-        use backend::serial::scalar_mul::straus::Straus;
-
-        Straus::optional_multiscalar_mul(scalars, points)
+        scalar_mul::straus::Straus::optional_multiscalar_mul(scalars, points)
     }
 }
 
 impl EdwardsPoint {
     /// Compute \\(aA + bB\\) in variable time, where \\(B\\) is the Ed25519 basepoint.
     #[cfg(feature = "stage2_build")]
-    pub fn vartime_double_scalar_mul_basepoint(a: &Scalar, A: &EdwardsPoint, b: &Scalar) -> EdwardsPoint {
-        // If we built with AVX2, use the AVX2 backend.
-        #[cfg(all(feature="avx2_backend", target_feature="avx2"))]
-        use backend::vector::scalar_mul::vartime_double_base;
-        // Otherwise, use the serial backend:
-        #[cfg(not(all(feature="avx2_backend", target_feature="avx2")))]
-        use backend::serial::scalar_mul::vartime_double_base;
-
-        vartime_double_base::mul(a, A, b)
+    pub fn vartime_double_scalar_mul_basepoint(
+        a: &Scalar,
+        A: &EdwardsPoint,
+        b: &Scalar,
+    ) -> EdwardsPoint {
+        scalar_mul::vartime_double_base::mul(a, A, b)
     }
 }
 

src/lib.rs

@@ -9,14 +9,18 @@
 // - Henry de Valence <hdevalence@hdevalence.ca>
 
 #![no_std]
-
-#![cfg_attr(feature = "ifma_backend", feature(simd_ffi))]
-#![cfg_attr(feature = "ifma_backend", feature(link_llvm_intrinsics))]
-
+#![cfg_attr(
+    all(feature = "simd_backend", target_feature = "avx512ifma"),
+    feature(simd_ffi)
+)]
+#![cfg_attr(
+    all(feature = "simd_backend", target_feature = "avx512ifma"),
+    feature(link_llvm_intrinsics)
+)]
+#![cfg_attr(feature = "nightly", feature(test))]
 #![cfg_attr(all(feature = "alloc", not(feature = "std")), feature(alloc))]
 #![cfg_attr(feature = "nightly", feature(cfg_target_feature))]
 #![cfg_attr(feature = "nightly", feature(external_doc))]
-
 // Refuse to compile if documentation is missing, but only on nightly.
 //
 // This means that missing docs will still fail CI, but means we can use
@@ -41,21 +45,21 @@ extern crate alloc;
 #[macro_use]
 extern crate std;
 
-#[cfg(all(feature = "nightly", any(feature = "avx2_backend", feature = "ifma_backend")))]
+#[cfg(all(feature = "nightly", feature = "packed_simd"))]
 extern crate packed_simd;
 
-extern crate rand;
-extern crate clear_on_drop;
 extern crate byteorder;
+extern crate clear_on_drop;
 pub extern crate digest;
+extern crate rand;
 
 // Used for traits related to constant-time code.
 extern crate subtle;
 
-#[cfg(feature = "serde")]
-extern crate serde;
 #[cfg(all(test, feature = "serde"))]
 extern crate bincode;
+#[cfg(feature = "serde")]
+extern crate serde;
 
 // Internal macros. Must come first!
 #[macro_use]