fn fguv 32x32xn_rust: Elide val bounds checks, fixing a perf regr…

…ession (#560)

This elides bounds checks from indexing with `val` by clipping the index
to a valid one. Initially, I copied #558's approach, which uses
`cmp::min(val, scaling.as_ref().len() - 1)`, and this removes 3 bounds
checks. But since the scaling size is a `const` and is `1 <<
BD::SCALING_BITS`, we can just truncate the other bits with `&`, which
avoids a `csel` and 2 more bounds checks.

include/common/bitdepth.rs

@@ -119,7 +119,8 @@ pub trait BitDepth: Clone + Copy {
         + ToPrimitive<c_int>;
 
     type Scaling: AsRef<[u8]> + AsMut<[u8]> + ArrayDefault + Copy;
-    const SCALING_SIZE: usize;
+    const SCALING_BITS: usize;
+    const SCALING_SIZE: usize = 1 << Self::SCALING_BITS;
 
     type BitDepthMax;
 
@@ -206,7 +207,7 @@ impl BitDepth for BitDepth8 {
     type Entry = i8;
 
     type Scaling = [u8; Self::SCALING_SIZE];
-    const SCALING_SIZE: usize = 1 << 8;
+    const SCALING_BITS: usize = 8;
 
     type BitDepthMax = ();
 
@@ -287,7 +288,7 @@ impl BitDepth for BitDepth16 {
     type Entry = i16;
 
     type Scaling = [u8; Self::SCALING_SIZE];
-    const SCALING_SIZE: usize = 1 << 12;
+    const SCALING_BITS: usize = 12;
 
     type BitDepthMax = Self::Pixel;
 

src/filmgrain.rs

@@ -792,8 +792,10 @@ unsafe fn fguv_32x32xn_rust<BD: BitDepth>(
                     bd.bitdepth_max().as_::<c_int>(),
                 );
             }
+            // `val` isn't out of bounds, so we can
+            // eliminate extra panicking code by bit-truncating `val`.
             let noise = round2(
-                scaling.as_ref()[val as usize] as c_int * grain,
+                scaling.as_ref()[val as usize % scaling.as_ref().len()] as c_int * grain,
                 data.scaling_shift,
             );
             *dst = iclip((*src).as_::<c_int>() + noise, min_value, max_value).as_::<BD::Pixel>();