aes: use cpubits crate

The `cpubits` crate contains a macro for selecting between 32-bit and
64-bit backends, incorporating 64-bit overrides for certain natively
32-bit targets with native ISA support for operations "register pairs",
namely ARMv7 and WASM. It also supports overriding the selection with
`cfg(cpubits = "...")`

This switches the selection of the 32-bit vs 64-bit soft backend, always
compiled into the crate as it's used as a fallback implementation if
intrinsics aren't available, to using `cpubits`.

The crate contains a vendored copy of `cfg-if`, so this also replaces
`cfg-if` as a dependency with `cpubits`.

Author: tarcieri

Cargo.lock

@@ -13,8 +13,8 @@ keywords = ["crypto", "aes", "rijndael", "block-cipher"]
 categories = ["cryptography", "no-std"]
 
 [dependencies]
-cfg-if = "1"
 cipher = "0.5.0-rc.7"
+cpubits = "0.1.0-rc.3"
 zeroize = { version = "1.5.6", optional = true, default-features = false, features = ["aarch64"] }
 
 [target.'cfg(any(target_arch = "aarch64", target_arch = "x86_64", target_arch = "x86"))'.dependencies]
@@ -29,7 +29,13 @@ hazmat = [] # Expose cryptographically hazardous APIs
 
 [lints.rust.unexpected_cfgs]
 level = "warn"
-check-cfg = ["cfg(aes_compact)", "cfg(aes_force_soft)", "cfg(aes_avx256)", "cfg(aes_avx512)"]
+check-cfg = [
+    'cfg(aes_compact)',
+    'cfg(aes_force_soft)',
+    'cfg(aes_avx256)',
+    'cfg(aes_avx512)',
+    'cfg(cpubits, values("16", "32", "64"))'
+]
 
 [package.metadata.docs.rs]
 all-features = true

aes/Cargo.toml

@@ -129,9 +129,7 @@ pub mod hazmat;
 mod macros;
 mod soft;
 
-use cfg_if::cfg_if;
-
-cfg_if! {
+cpubits::cfg_if! {
     if #[cfg(all(target_arch = "aarch64", not(aes_force_soft)))] {
         mod armv8;
         mod autodetect;

aes/src/lib.rs

@@ -8,9 +8,16 @@
 
 #![deny(unsafe_code)]
 
-#[cfg_attr(not(target_pointer_width = "64"), path = "soft/fixslice32.rs")]
-#[cfg_attr(target_pointer_width = "64", path = "soft/fixslice64.rs")]
-pub(crate) mod fixslice;
+cpubits::cpubits! {
+    16 | 32 => {
+        #[path = "soft/fixslice32.rs"]
+        pub(crate) mod fixslice;
+    }
+    64 => {
+        #[path = "soft/fixslice32.rs"]
+        pub(crate) mod fixslice;
+    }
+}
 
 use crate::Block;
 use cipher::{