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Unified Diff: src/codec/SkPngFilters.cpp

Issue 1657503002: Look beyond SSE2 for Paeth (Closed) Base URL: https://skia.googlesource.com/skia.git@master
Patch Set: kill sse4.1 Created 4 years, 11 months ago
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Index: src/codec/SkPngFilters.cpp
diff --git a/src/codec/SkPngFilters.cpp b/src/codec/SkPngFilters.cpp
index 2185936c44b6b0b0dd506947905c99554fbc31a0..472123fa15e6c6010eda52f1b37efd49ab06068b 100644
--- a/src/codec/SkPngFilters.cpp
+++ b/src/codec/SkPngFilters.cpp
@@ -80,16 +80,27 @@
}
}
- // Returns bytewise |x-y|.
- static __m128i absdiff_u8(__m128i x, __m128i y) {
- // One of these two saturated subtractions will be the answer, the other zero.
- return _mm_or_si128(_mm_subs_epu8(x,y), _mm_subs_epu8(y,x));
+ // Returns |x| for 16-bit lanes.
+ static __m128i abs_i16(__m128i x) {
+ #if defined(__SSSE3__)
+ return _mm_abs_epi16(x);
+ #else
+ // Read this all as, return x<0 ? -x : x.
+ // To negate two's complement, you flip all the bits then add 1.
+ __m128i is_negative = _mm_cmplt_epi16(x, _mm_setzero_si128());
+ x = _mm_xor_si128(x, is_negative); // Flip negative lanes.
+ x = _mm_add_epi16(x, _mm_srli_epi16(is_negative, 15)); // +1 to negative lanes, else +0.
+ return x;
+ #endif
}
// Bytewise c ? t : e.
static __m128i if_then_else(__m128i c, __m128i t, __m128i e) {
- // SSE 4.1+ would be: return _mm_blendv_epi8(e,t,c);
+ #if 0 && defined(__SSE4_1__) // Make sure we have a bot testing this before enabling.
+ return _mm_blendv_epi8(e,t,c);
+ #else
return _mm_or_si128(_mm_and_si128(c, t), _mm_andnot_si128(c, e));
+ #endif
}
template <int bpp>
@@ -103,39 +114,33 @@
// The first pixel has no left context, and so uses an Up filter, p = b.
// This works naturally with our main loop's p = a+b-c if we force a and c to zero.
// Here we zero b and d, which become c and a respectively at the start of the loop.
- __m128i c, b = _mm_setzero_si128(),
- a, d = _mm_setzero_si128();
+ const __m128i zero = _mm_setzero_si128();
+ __m128i c, b = zero,
+ a, d = zero;
int rb = row_info->rowbytes;
while (rb > 0) {
- c = b; b = load<bpp>(prev);
- a = d; d = load<bpp>(row );
+ // It's easiest to do this math (particularly, deal with pc) with 16-bit intermediates.
+ c = b; b = _mm_unpacklo_epi8(load<bpp>(prev), zero);
+ a = d; d = _mm_unpacklo_epi8(load<bpp>(row ), zero);
- // We can't express p in 8 bits, but luckily we can use this faux p instead.
- // (I have no deep insight here... I just proved this with brute force.)
- __m128i min = _mm_min_epu8(a,b),
- max = _mm_max_epu8(a,b),
- faux_p = _mm_adds_epu8(min, _mm_subs_epu8(max, c));
-
- // We could use faux_p for calculating all three of pa, pb, and pc,
- // but it's a little quicker to calculate the correct pa and pb directly,
- // and the predictor remains the same. (Again, brute force.)
- __m128i pa = absdiff_u8(b,c), // |a+b-c - a| == |b-c|
- pb = absdiff_u8(a,c), // |a+b-c - b| == |a-c|
- faux_pc = absdiff_u8(faux_p, c);
-
- // From here, things are straightforward. Find the smallest distance to p...
- __m128i smallest = _mm_min_epu8(_mm_min_epu8(pa, pb), faux_pc);
-
- // ... then the predictor is the input corresponding to that smallest distance,
- // breaking ties in favor of a over b over c.
- __m128i nearest = if_then_else(_mm_cmpeq_epi8(smallest, pa), a,
- if_then_else(_mm_cmpeq_epi8(smallest, pb), b,
- c));
-
- // We've reconstructed d! Leave it for next round to become a, and write it out.
- d = _mm_add_epi8(d, nearest);
- store<bpp>(row, d);
+ __m128i pa = _mm_sub_epi16(b,c), // (p-a) == (a+b-c - a) == (b-c)
+ pb = _mm_sub_epi16(a,c), // (p-b) == (a+b-c - b) == (a-c)
+ pc = _mm_add_epi16(pa,pb); // (p-c) == (a+b-c - c) == (a+b-c-c) == (b-c)+(a-c)
+
+ pa = abs_i16(pa); // |p-a|
+ pb = abs_i16(pb); // |p-b|
+ pc = abs_i16(pc); // |p-c|
+
+ __m128i smallest = _mm_min_epi16(pc, _mm_min_epi16(pa, pb));
+
+ // Paeth breaks ties favoring a over b over c.
+ __m128i nearest = if_then_else(_mm_cmpeq_epi16(smallest, pa), a,
+ if_then_else(_mm_cmpeq_epi16(smallest, pb), b,
+ c));
+
+ d = _mm_add_epi8(d, nearest); // Note `_epi8`: we need addition to wrap modulo 255.
+ store<bpp>(row, _mm_packus_epi16(d,d));
prev += bpp;
row += bpp;
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