From 03be367b8ec87aeaea1539f6f7a5ef206bf3d405 Mon Sep 17 00:00:00 2001
From: "Sergey \"Shnatsel\" Davidoff" <shnatsel@gmail.com>
Date: Sun, 29 Sep 2024 14:12:12 +0100
Subject: [PATCH] Add multiversioning for filtering and make it an optional,
 opt-in feature

---
 Cargo.toml    |  3 ++-
 src/filter.rs | 25 +++++++++++++++++--------
 2 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index e92ef4a3..4813e8af 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -26,7 +26,7 @@ crc32fast = "1.2.0"
 fdeflate = "0.3.3"
 flate2 = "1.0.11"
 miniz_oxide = { version = "0.8", features = ["simd"] }
-multiversion = "0.7.4"
+multiversion = { version = "0.7.4", optional = true }
 
 [dev-dependencies]
 byteorder = "1.5.0"
@@ -41,6 +41,7 @@ term = "0.7"
 [features]
 unstable = []
 benchmarks = []
+multiversioning = ["multiversion"]
 
 [[bench]]
 path = "benches/decoder.rs"
diff --git a/src/filter.rs b/src/filter.rs
index 41d3462d..5c310f22 100644
--- a/src/filter.rs
+++ b/src/filter.rs
@@ -1,5 +1,6 @@
 use core::convert::TryInto;
 
+#[cfg(feature = "multiversioning")]
 use multiversion::multiversion;
 
 use crate::common::BytesPerPixel;
@@ -14,6 +15,7 @@ mod simd {
     use std::simd::cmp::{SimdOrd, SimdPartialEq, SimdPartialOrd};
     use std::simd::num::{SimdInt, SimdUint};
     use std::simd::{u8x4, u8x8, LaneCount, Simd, SimdElement, SupportedLaneCount};
+    #[cfg(feature = "multiversioning")]
     use multiversion::multiversion;
 
     /// This is an equivalent of the `PaethPredictor` function from
@@ -170,10 +172,10 @@ mod simd {
         dest[0..3].copy_from_slice(&src.to_array()[0..3])
     }
 
-    #[multiversion(targets(
+    #[cfg_attr(feature = "multiversioning", multiversion(targets(
         "x86_64+sse+sse2+sse3+sse4.1+ssse3", // SSE4.1 is enough because our vectors are 128bit
         "arm+neon", // 32-bit ARM only; 64-bit always has NEON
-    ))]
+    )))]
     /// Undoes `FilterType::Paeth` for `BytesPerPixel::Three`.
     pub fn unfilter_paeth3(mut prev_row: &[u8], mut curr_row: &mut [u8]) {
         debug_assert_eq!(prev_row.len(), curr_row.len());
@@ -205,10 +207,10 @@ mod simd {
         store3(x, curr_row);
     }
 
-    #[multiversion(targets(
+    #[cfg_attr(feature = "multiversioning", multiversion(targets(
         "x86_64+sse+sse2+sse3+sse4.1+ssse3", // SSE4.1 is enough because our vectors are 128bit
         "arm+neon", // 32-bit ARM only; 64-bit always has NEON
-    ))]
+    )))]
     /// Undoes `FilterType::Paeth` for `BytesPerPixel::Four` and `BytesPerPixel::Eight`.
     ///
     /// This function calculates the Paeth predictor entirely in `Simd<u8, N>`
@@ -241,10 +243,10 @@ mod simd {
         dest[0..6].copy_from_slice(&src.to_array()[0..6])
     }
 
-    #[multiversion(targets(
+    #[cfg_attr(feature = "multiversioning", multiversion(targets(
         "x86_64+sse+sse2+sse3+sse4.1+ssse3", // x86-64-v2, higher levels provide no benefit
         "arm+neon", // 32-bit ARM only; 64-bit always has NEON
-    ))]
+    )))]
     /// Undoes `FilterType::Paeth` for `BytesPerPixel::Six`.
     pub fn unfilter_paeth6(mut prev_row: &[u8], mut curr_row: &mut [u8]) {
         debug_assert_eq!(prev_row.len(), curr_row.len());
@@ -396,10 +398,10 @@ fn filter_paeth(a: u8, b: u8, c: u8) -> u8 {
     }
 }
 
-#[multiversion(targets(
+#[cfg_attr(feature = "multiversioning", multiversion(targets(
     "x86_64+sse+sse2+sse3+sse4.1+ssse3", // SSE4.1 is enough because our vectors are 128bit
     "arm+neon", // 32-bit ARM only; 64-bit always has NEON
-))]
+)))]
 pub(crate) fn unfilter(
     mut filter: FilterType,
     tbpp: BytesPerPixel,
@@ -912,6 +914,13 @@ pub(crate) fn unfilter(
     }
 }
 
+#[cfg_attr(feature = "multiversioning", multiversion(targets(
+    // SSE4.1 only gives a +15% boost to Paeth, not worth the bloat.
+    // AVX regresses fast filters but speeds up slow ones. Another +10% to Paeth. Not worth it?
+    // AVX2 makes everything go BRRRRRRRRRR, with more than double performance for Paeth
+    "x86_64+sse+sse2+sse3+sse4.1+sse4.2+ssse3+avx+avx2+fma",
+    "arm+neon", // 32-bit ARM only; 64-bit always has NEON
+)))]
 fn filter_internal(
     method: FilterType,
     bpp: usize,