Rework benchmarks to make it easier to get assembly.

This change:
  - Move the benchmarks from mod.rs to buffer.rs
  - Move the inner loop we benchmark into an `#[inline(never)]` function
  - Includes instructions for getting the ASM for a specific benchmark

This should hopefully reduce the variance of these benchmarks and make
it easier to figure out if we are emitting the assembly or IR we expect
for a particular implementation.

Signed-off-by: Joe Richey <joerichey@google.com>

benches/buffer.rs

@@ -0,0 +1,71 @@
+#![feature(test, maybe_uninit_uninit_array_transpose)]
+extern crate test;
+
+use std::mem::MaybeUninit;
+
+// Call getrandom on a zero-initialized stack buffer
+#[inline(always)]
+fn getrandom_ref<const N: usize>() {
+    let mut buf = [0u8; N];
+    getrandom::getrandom(&mut buf).unwrap();
+    test::black_box(&buf as &[u8]);
+}
+
+// Call getrandom_uninit on an uninitialized stack buffer
+#[inline(always)]
+fn getrandom_uninit_ref<const N: usize>() {
+    let mut uninit = [MaybeUninit::uninit(); N];
+    let buf: &[u8] = getrandom::getrandom_uninit(&mut uninit).unwrap();
+    test::black_box(buf);
+}
+
+// We benchmark using #[inline(never)] "inner" functions for two reasons:
+//  - Avoiding inlining reduces a source of variance when running benchmarks.
+//  - It is _much_ easier to get the assembly or IR for the inner loop.
+//
+// For example, using cargo-show-asm (https://github.com/pacak/cargo-show-asm),
+// we can get the assembly for a particular benchmark's inner loop by running:
+//   cargo asm --bench buffer --release buffer::p384::bench_getrandom::inner
+macro_rules! bench {
+    ( $name:ident, $size:expr ) => {
+        pub mod $name {
+            #[bench]
+            pub fn bench_getrandom(b: &mut test::Bencher) {
+                #[inline(never)]
+                fn inner() {
+                    super::getrandom_ref::<{ $size }>()
+                }
+
+                b.bytes = $size as u64;
+                b.iter(inner);
+            }
+            #[bench]
+            pub fn bench_getrandom_uninit(b: &mut test::Bencher) {
+                #[inline(never)]
+                fn inner() {
+                    super::getrandom_uninit_ref::<{ $size }>()
+                }
+
+                b.bytes = $size as u64;
+                b.iter(inner);
+            }
+        }
+    };
+}
+
+// 16 bytes (128 bits) is the size of an 128-bit AES key/nonce.
+bench!(aes128, 128 / 8);
+
+// 32 bytes (256 bits) is the seed sized used for rand::thread_rng
+// and the `random` value in a ClientHello/ServerHello for TLS.
+// This is also the size of a 256-bit AES/HMAC/P-256/Curve25519 key
+// and/or nonce.
+bench!(p256, 256 / 8);
+
+// A P-384/HMAC-384 key and/or nonce.
+bench!(p384, 384 / 8);
+
+// Initializing larger buffers is not the primary use case of this library, as
+// this should normally be done by a userspace CSPRNG. However, we have a test
+// here to see the effects of a lower (amortized) syscall overhead.
+bench!(page, 4096);