perf: AVX2 gather for PixmapRef::gather

src/pipeline/mod.rs

@@ -154,18 +154,10 @@ pub const STAGES_COUNT: usize = Stage::GammaCompressSrgb as usize + 1;
 impl PixmapRef<'_> {
     #[inline(always)]
     pub(crate) fn gather(&self, index: u32x8) -> [PremultipliedColorU8; highp::STAGE_WIDTH] {
-        let index: [u32; 8] = bytemuck::cast(index);
         let pixels = self.pixels();
-        [
-            pixels[index[0] as usize],
-            pixels[index[1] as usize],
-            pixels[index[2] as usize],
-            pixels[index[3] as usize],
-            pixels[index[4] as usize],
-            pixels[index[5] as usize],
-            pixels[index[6] as usize],
-            pixels[index[7] as usize],
-        ]
+        // safety: callers clamp indices to [0, w*h) via gather_ix.
+        let gathered = unsafe { u32x8::gather_u32(pixels.as_ptr() as *const u32, index) };
+        bytemuck::cast(gathered)
     }
 }
 

src/wide/u32x8_t.rs

@@ -81,6 +81,34 @@ impl u32x8 {
             }
         }
     }
+
+    /// Gathers 8 u32s from `base[index[i]]`.
+    ///
+    /// # Safety
+    /// Each lane in `index` must be a valid offset (in u32 units) into the
+    /// buffer at `base`. avx2 `vpgatherdd` faults on oob; the scalar fallback
+    /// indexes a raw slice and would UB on oob too.
+    #[inline(always)]
+    pub unsafe fn gather_u32(base: *const u32, index: Self) -> Self {
+        cfg_if::cfg_if! {
+            if #[cfg(all(feature = "simd", target_feature = "avx2"))] {
+                let vindex: __m256i = cast(index);
+                Self(_mm256_i32gather_epi32::<4>(base as *const i32, vindex))
+            } else {
+                let ix: [u32; 8] = bytemuck::cast(index);
+                bytemuck::cast([
+                    *base.add(ix[0] as usize),
+                    *base.add(ix[1] as usize),
+                    *base.add(ix[2] as usize),
+                    *base.add(ix[3] as usize),
+                    *base.add(ix[4] as usize),
+                    *base.add(ix[5] as usize),
+                    *base.add(ix[6] as usize),
+                    *base.add(ix[7] as usize),
+                ])
+            }
+        }
+    }
 }
 
 impl core::ops::Not for u32x8 {