docs: document the AVX2 instructions (#203)

Author: nikneym

src/simd/avx2.rs

@@ -21,25 +21,62 @@ pub unsafe fn match_uri_vectored(bytes: &mut Bytes) {
 #[allow(non_snake_case, overflowing_literals)]
 #[allow(unused)]
 unsafe fn match_url_char_32_avx(buf: &[u8]) -> usize {
+    // NOTE: This check might be not necessary since this function is only used in
+    // `match_uri_vectored` where buffer overflow is taken care of.
     debug_assert!(buf.len() >= 32);
 
     #[cfg(target_arch = "x86")]
     use core::arch::x86::*;
     #[cfg(target_arch = "x86_64")]
     use core::arch::x86_64::*;
 
+    // pointer to buffer
     let ptr = buf.as_ptr();
 
     // %x21-%x7e %x80-%xff
+    //
+    // Character ranges allowed by this function, can also be interpreted as:
+    // 33 =< (x != 127) =< 255
+    //
+    // Create a vector full of DEL (0x7f) characters.
     let DEL: __m256i = _mm256_set1_epi8(0x7f);
+    // Create a vector full of exclamation mark (!) (0x21) characters.
+    // Used as lower threshold, characters in URLs cannot be smaller than this.
     let LOW: __m256i = _mm256_set1_epi8(0x21);
 
+    // Load a chunk of 32 bytes from `ptr` as a vector.
+    // We can check 32 bytes in parallel at most with AVX2 since
+    // YMM registers can only have 256 bits most.
     let dat = _mm256_lddqu_si256(ptr as *const _);
+
     // unsigned comparison dat >= LOW
+    //
+    // We create a new via `_mm256_max_epu8` which compares vectors `dat` and `LOW`
+    // and picks the max. values from each for all indices.
+    // So if a byte in `dat` is <= 32, it'll be represented as 33
+    // which is the smallest valid character.
+    //
+    // Then, we compare the new vector with `dat` for equality.
+    //
+    // `_mm256_cmpeq_epi8` returns a new vector where;
+    // * matching bytes are set to 0xFF (all bits set),
+    // * nonmatching bytes are set to 0 (no bits set).
     let low = _mm256_cmpeq_epi8(_mm256_max_epu8(dat, LOW), dat);
+    // Similar to what we did before, but now invalid characters are set to 0xFF.
     let del = _mm256_cmpeq_epi8(dat, DEL);
+
+    // We glue the both comparisons via `_mm256_andnot_si256`.
+    //
+    // Since the representation of truthy/falsy differ in these comparisons,
+    // we cannot use 
+    // we are in need of bitwise NOT to convert valid characters of `del`.
     let bit = _mm256_andnot_si256(del, low);
+    // This creates a bitmask from the most significant bit of each byte.
+    // Simply, we're converting a vector value to scalar value here.
     let res = _mm256_movemask_epi8(bit) as u32;
+
+    // Count trailing zeros to find the first encountered invalid character.
+    // Bitwise NOT is required once again to flip truthiness.
     // TODO: use .trailing_ones() once MSRV >= 1.46
     (!res).trailing_zeros() as usize
 }
@@ -72,17 +109,38 @@ unsafe fn match_header_value_char_32_avx(buf: &[u8]) -> usize {
     let ptr = buf.as_ptr();
 
     // %x09 %x20-%x7e %x80-%xff
+    // Create a vector full of horizontal tab (\t) (0x09) characters.
     let TAB: __m256i = _mm256_set1_epi8(0x09);
+    // Create a vector full of DEL (0x7f) characters.
     let DEL: __m256i = _mm256_set1_epi8(0x7f);
+    // Create a vector full of space (0x20) characters.
     let LOW: __m256i = _mm256_set1_epi8(0x20);
 
+    // Load a chunk of 32 bytes from `ptr` as a vector.
     let dat = _mm256_lddqu_si256(ptr as *const _);
+
     // unsigned comparison dat >= LOW
+    //
+    // Same as what we do in `match_url_char_32_avx`.
+    // This time the lower threshold is set to space character though.
     let low = _mm256_cmpeq_epi8(_mm256_max_epu8(dat, LOW), dat);
+    // Check if `dat` includes `TAB` characters.
     let tab = _mm256_cmpeq_epi8(dat, TAB);
+    // Check if `dat` includes `DEL` characters.
     let del = _mm256_cmpeq_epi8(dat, DEL);
+
+    // Combine all comparisons together, notice that we're also using OR
+    // to connect `low` and `tab` but flip bits of `del`.
+    //
+    // In the end, this is simply:
+    // ~del & (low | tab)
     let bit = _mm256_andnot_si256(del, _mm256_or_si256(low, tab));
+    // This creates a bitmask from the most significant bit of each byte.
+    // Creates a scalar value from vector value.
     let res = _mm256_movemask_epi8(bit) as u32;
+
+    // Count trailing zeros to find the first encountered invalid character.
+    // Bitwise NOT is required once again to flip truthiness.
     // TODO: use .trailing_ones() once MSRV >= 1.46
     (!res).trailing_zeros() as usize
 }