cmov: extract backends module (#1358)

Now that we have some additional modules like `array` and `slice` which
aren't backends, it makes sense to group all the backends as submodules
under a `backends` module for the purposes of code hygeine.

Author: tarcieri

cmov/src/backends.rs

@@ -0,0 +1,17 @@
+//! Backends for `cmov`, only one of which will be selected at compile-time.
+
+// Architecture-specific backends for target architectures with native predication instructions
+#[cfg(not(miri))]
+#[cfg(target_arch = "aarch64")]
+mod aarch64;
+#[cfg(not(miri))]
+#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+mod x86;
+
+// Fallback portable implementation for targets which don't have native predication instructions
+// (or if they do, aren't currently supported)
+#[cfg(any(
+    not(any(target_arch = "aarch64", target_arch = "x86", target_arch = "x86_64")),
+    miri
+))]
+mod soft;

cmov/src/aarch64.rs cmov/src/backends/aarch64.rscmov/src/aarch64.rs renamed to cmov/src/backends/aarch64.rs

@@ -1,6 +1,6 @@
 //! Portable "best effort" implementation of `Cmov`/`CmovEq`.
 //!
-//! This implementation is based on portable bitwise arithmetic augmented with tactical usage of
+//! This implementation is based on portable arithmetic operations augmented with tactical usage of
 //! `core::hint::black_box` based on past observations of where the optimizer has inserted branches
 //! (see CVE-2026-23519), but the fully portable implementation cannot guarantee that the resulting
 //! generated assembly is free of branch instructions.

cmov/src/portable.rs cmov/src/backends/soft.rscmov/src/portable.rs renamed to cmov/src/backends/soft.rs

@@ -30,19 +30,9 @@
 #[macro_use]
 mod macros;
 
-#[cfg(not(miri))]
-#[cfg(target_arch = "aarch64")]
-mod aarch64;
 mod array;
-#[cfg(any(
-    not(any(target_arch = "aarch64", target_arch = "x86", target_arch = "x86_64")),
-    miri
-))]
-mod portable;
+mod backends;
 mod slice;
-#[cfg(not(miri))]
-#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
-mod x86;
 
 /// Condition
 pub type Condition = u8;