spin off some safe parts from AVX2 CL

src/opts/SkOpts_sse41.cpp

Index: src/opts/SkOpts_sse41.cpp
diff --git a/src/opts/SkOpts_sse41.cpp b/src/opts/SkOpts_sse41.cpp
index 16ba87ad87c04fa07327bd7719b87e7675e0cbb1..f097e56c5e34a8eab9ebb6ecbff715ec3baa0010 100644
--- a/src/opts/SkOpts_sse41.cpp
+++ b/src/opts/SkOpts_sse41.cpp
@@ -12,88 +12,67 @@
 
 #ifndef SK_SUPPORT_LEGACY_X86_BLITS
 
-// This file deals mostly with unpacked 8-bit values,
-// i.e. values between 0 and 255, but in 16-bit lanes with 0 at the top.
-
-// So __m128i typically represents 1 or 2 pixels, and m128ix2 represents 4.
-struct m128ix2 { __m128i lo, hi; };
-
-// unpack{lo,hi}() get our raw pixels unpacked, from half of 4 packed pixels to 2 unpacked pixels.
-static inline __m128i unpacklo(__m128i x) { return _mm_cvtepu8_epi16(x); }
-static inline __m128i unpackhi(__m128i x) { return _mm_unpackhi_epi8(x, _mm_setzero_si128()); }
-
-// pack() converts back, from 4 unpacked pixels to 4 packed pixels.
-static inline __m128i pack(__m128i lo, __m128i hi) { return _mm_packus_epi16(lo, hi); }
-
-// These nextN() functions abstract over the difference between iterating over
-// an array of values and returning a constant value, for uint8_t and uint32_t.
-// The nextN() taking pointers increment that pointer past where they read.
-//
-// nextN() returns N unpacked pixels or 4N unpacked coverage values.
-
-static inline __m128i next1(uint8_t val) { return _mm_set1_epi16(val); }
-static inline __m128i next2(uint8_t val) { return _mm_set1_epi16(val); }
-static inline m128ix2 next4(uint8_t val) { return { next2(val), next2(val) }; }
-
-static inline __m128i next1(uint32_t val) { return unpacklo(_mm_cvtsi32_si128(val)); }
-static inline __m128i next2(uint32_t val) { return unpacklo(_mm_set1_epi32(val)); }
-static inline m128ix2 next4(uint32_t val) { return { next2(val), next2(val) }; }
+namespace sk_sse41 {
 
-static inline __m128i next1(const uint8_t*& ptr) { return _mm_set1_epi16(*ptr++); }
-static inline __m128i next2(const uint8_t*& ptr) {
-    auto r = _mm_cvtsi32_si128(*(const uint16_t*)ptr);
-    ptr += 2;
-    const int _ = ~0;
-    return _mm_shuffle_epi8(r, _mm_setr_epi8(0,_,0,_,0,_,0,_, 1,_,1,_,1,_,1,_));
-}
-static inline m128ix2 next4(const uint8_t*& ptr) {
-    auto r = _mm_cvtsi32_si128(*(const uint32_t*)ptr);
+// An SSE register holding at most 64 bits of useful data in the low lanes.
+struct m64i {
+    __m128i v;
+    /*implicit*/ m64i(__m128i v) : v(v) {}
+    operator __m128i() const { return v; }
+};
+
+// Load 4, 2, or 1 constant pixels or coverages (4x replicated).
+static __m128i next4(uint32_t val) { return _mm_set1_epi32(val); }
+static m64i    next2(uint32_t val) { return _mm_set1_epi32(val); }
+static m64i    next1(uint32_t val) { return _mm_set1_epi32(val); }
+
+static __m128i next4(uint8_t val) { return _mm_set1_epi8(val); }
+static m64i    next2(uint8_t val) { return _mm_set1_epi8(val); }
+static m64i    next1(uint8_t val) { return _mm_set1_epi8(val); }
+
+// Load 4, 2, or 1 variable pixels or coverages (4x replicated),
+// incrementing the pointer past what we read.
+static __m128i next4(const uint32_t*& ptr) {
+    auto r = _mm_loadu_si128((const __m128i*)ptr);
     ptr += 4;
-    const int _ = ~0;
-    auto lo = _mm_shuffle_epi8(r, _mm_setr_epi8(0,_,0,_,0,_,0,_, 1,_,1,_,1,_,1,_)),
-         hi = _mm_shuffle_epi8(r, _mm_setr_epi8(2,_,2,_,2,_,2,_, 3,_,3,_,3,_,3,_));
-    return { lo, hi };
+    return r;
 }
-
-static inline __m128i next1(const uint32_t*& ptr) { return unpacklo(_mm_cvtsi32_si128(*ptr++)); }
-static inline __m128i next2(const uint32_t*& ptr) {
-    auto r = unpacklo(_mm_loadl_epi64((const __m128i*)ptr));
+static m64i next2(const uint32_t*& ptr) {
+    auto r = _mm_loadl_epi64((const __m128i*)ptr);
     ptr += 2;
     return r;
 }
-static inline m128ix2 next4(const uint32_t*& ptr) {
-    auto packed = _mm_loadu_si128((const __m128i*)ptr);
-    ptr += 4;
-    return { unpacklo(packed), unpackhi(packed) };
+static m64i next1(const uint32_t*& ptr) {
+    auto r = _mm_cvtsi32_si128(*ptr);
+    ptr += 1;
+    return r;
 }
 
-// Divide by 255 with rounding.
-// (x+127)/255 == ((x+128)*257)>>16.
-// Sometimes we can be more efficient by breaking this into two parts.
-static inline __m128i div255_part1(__m128i x) { return _mm_add_epi16(x, _mm_set1_epi16(128)); }
-static inline __m128i div255_part2(__m128i x) { return _mm_mulhi_epu16(x, _mm_set1_epi16(257)); }
-static inline __m128i div255(__m128i x) { return div255_part2(div255_part1(x)); }
-
-// (x*y+127)/255, a byte multiply.
-static inline __m128i scale(__m128i x, __m128i y) {
-    return div255(_mm_mullo_epi16(x, y));
+// xyzw -> xxxx yyyy zzzz wwww
+static __m128i replicate_coverage(__m128i xyzw) {
+    const uint8_t mask[] = { 0,0,0,0, 1,1,1,1, 2,2,2,2, 3,3,3,3 };
+    return _mm_shuffle_epi8(xyzw, _mm_load_si128((const __m128i*)mask));
 }
 
-// (255 - x).
-static inline __m128i inv(__m128i x) {
-    return _mm_xor_si128(_mm_set1_epi16(0x00ff), x);  // This seems a bit faster than _mm_sub_epi16.
+static __m128i next4(const uint8_t*& ptr) {
+    auto r = replicate_coverage(_mm_cvtsi32_si128(*(const uint32_t*)ptr));
+    ptr += 4;
+    return r;
 }
-
-// ARGB argb -> AAAA aaaa
-static inline __m128i alphas(__m128i px) {
-    const int a = 2 * (SK_A32_SHIFT/8);  // SK_A32_SHIFT is typically 24, so this is typically 6.
-    const int _ = ~0;
-    return _mm_shuffle_epi8(px, _mm_setr_epi8(a+0,_,a+0,_,a+0,_,a+0,_, a+8,_,a+8,_,a+8,_,a+8,_));
+static m64i next2(const uint8_t*& ptr) {
+    auto r = replicate_coverage(_mm_cvtsi32_si128(*(const uint16_t*)ptr));
+    ptr += 2;
+    return r;
+}
+static m64i next1(const uint8_t*& ptr) {
+    auto r = replicate_coverage(_mm_cvtsi32_si128(*ptr));
+    ptr += 1;
+    return r;
 }
 
 // For i = 0...n, tgt = fn(dst,src,cov), where Dst,Src,and Cov can be constants or arrays.
 template <typename Dst, typename Src, typename Cov, typename Fn>
-static inline void loop(int n, uint32_t* t, const Dst dst, const Src src, const Cov cov, Fn&& fn) {
+static void loop(int n, uint32_t* t, const Dst dst, const Src src, const Cov cov, Fn&& fn) {
     // We don't want to muck with the callers' pointers, so we make them const and copy here.
     Dst d = dst;
     Src s = src;
@@ -102,30 +81,85 @@ static inline void loop(int n, uint32_t* t, const Dst dst, const Src src, const
     // Writing this as a single while-loop helps hoist loop invariants from fn.
     while (n) {
         if (n >= 4) {
-            auto d4 = next4(d),
-                 s4 = next4(s),
-                 c4 = next4(c);
-            auto lo = fn(d4.lo, s4.lo, c4.lo),
-                 hi = fn(d4.hi, s4.hi, c4.hi);
-            _mm_storeu_si128((__m128i*)t, pack(lo,hi));
+            _mm_storeu_si128((__m128i*)t, fn(next4(d), next4(s), next4(c)));
             t += 4;
             n -= 4;
             continue;
         }
         if (n & 2) {
-            auto r = fn(next2(d), next2(s), next2(c));
-            _mm_storel_epi64((__m128i*)t, pack(r,r));
+            _mm_storel_epi64((__m128i*)t, fn(next2(d), next2(s), next2(c)));
             t += 2;
         }
         if (n & 1) {
-            auto r = fn(next1(d), next1(s), next1(c));
-            *t = _mm_cvtsi128_si32(pack(r,r));
+            *t = _mm_cvtsi128_si32(fn(next1(d), next1(s), next1(c)));
         }
         return;
     }
 }
 
-namespace sk_sse41 {
+//                                             packed
+// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ //
+//                                            unpacked
+
+// Everything on the packed side of the squiggly line deals with densely packed 8-bit data,
+// e.g. [BGRA bgra ... ] for pixels or [ CCCC cccc ... ] for coverage.
+//
+// Everything on the unpacked side of the squiggly line deals with unpacked 8-bit data,
+// e.g [B_G_ R_A_ b_g_ r_a_ ] for pixels or [ C_C_ C_C_ c_c_ c_c_ c_c_ ] for coverage,
+// where _ is a zero byte.
+//
+// Adapt<Fn> / adapt(fn) allow the two sides to interoperate,
+// by unpacking arguments, calling fn, then packing the results.
+//
+// This lets us write most of our code in terms of unpacked inputs (considerably simpler)
+// and all the packing and unpacking is handled automatically.
+
+template <typename Fn>
+struct Adapt {
+    Fn fn;
+
+    __m128i operator()(__m128i d, __m128i s, __m128i c) {
+        auto lo = [](__m128i x) { return _mm_unpacklo_epi8(x, _mm_setzero_si128()); };
+        auto hi = [](__m128i x) { return _mm_unpackhi_epi8(x, _mm_setzero_si128()); };
+        return _mm_packus_epi16(fn(lo(d), lo(s), lo(c)),
+                                fn(hi(d), hi(s), hi(c)));
+    }
+
+    m64i operator()(const m64i& d, const m64i& s, const m64i& c) {
+        auto lo = [](__m128i x) { return _mm_unpacklo_epi8(x, _mm_setzero_si128()); };
+        auto r = fn(lo(d), lo(s), lo(c));
+        return _mm_packus_epi16(r, r);
+    }
+};
+
+template <typename Fn>
+static Adapt<Fn> adapt(Fn&& fn) { return { fn }; }
+
+// These helpers all work exclusively with unpacked 8-bit values,
+// except div255() with is 16-bit -> unpacked 8-bit, and mul255() which is the reverse.
+
+// Divide by 255 with rounding.
+// (x+127)/255 == ((x+128)*257)>>16.
+// Sometimes we can be more efficient by breaking this into two parts.
+static __m128i div255_part1(__m128i x) { return _mm_add_epi16(x, _mm_set1_epi16(128)); }
+static __m128i div255_part2(__m128i x) { return _mm_mulhi_epu16(x, _mm_set1_epi16(257)); }
+static __m128i div255(__m128i x) { return div255_part2(div255_part1(x)); }
+
+// (x*y+127)/255, a byte multiply.
+static __m128i scale(__m128i x, __m128i y) { return div255(_mm_mullo_epi16(x, y)); }
+
+// (255 * x).
+static __m128i mul255(__m128i x) { return _mm_sub_epi16(_mm_slli_epi16(x, 8), x); }
+
+// (255 - x).
+static __m128i inv(__m128i x) { return _mm_xor_si128(_mm_set1_epi16(0x00ff), x); }
+
+// ARGB argb -> AAAA aaaa
+static __m128i alphas(__m128i px) {
+    const int a = 2 * (SK_A32_SHIFT/8);  // SK_A32_SHIFT is typically 24, so this is typically 6.
+    const int _ = ~0;
+    return _mm_shuffle_epi8(px, _mm_setr_epi8(a+0,_,a+0,_,a+0,_,a+0,_, a+8,_,a+8,_,a+8,_,a+8,_));
+}
 
 // SrcOver, with a constant source and full coverage.
 static void blit_row_color32(SkPMColor* tgt, const SkPMColor* dst, int n, SkPMColor src) {
@@ -134,14 +168,14 @@ static void blit_row_color32(SkPMColor* tgt, const SkPMColor* dst, int n, SkPMCo
 
     // But we can go one step further to ((s*255 + 128 + d*inv(alphas(s)))*257)>>16.
     // This lets us hoist (s*255+128) and inv(alphas(s)) out of the loop.
-    __m128i s = next2(src),
-            s_255_128 = div255_part1(_mm_mullo_epi16(s, _mm_set1_epi16(255))),
+    __m128i s = _mm_unpacklo_epi8(_mm_set1_epi32(src), _mm_setzero_si128()),
+            s_255_128 = div255_part1(mul255(s)),
             A = inv(alphas(s));
 
     const uint8_t cov = 0xff;
-    loop(n, tgt, dst, src, cov, [=](__m128i d, __m128i, __m128i) {
+    loop(n, tgt, dst, src, cov, adapt([=](__m128i d, __m128i, __m128i) {
         return div255_part2(_mm_add_epi16(s_255_128, _mm_mullo_epi16(d, A)));
-    });
+    }));
 }
 
 // SrcOver, with a constant source and variable coverage.
@@ -152,23 +186,26 @@ static void blit_mask_d32_a8(SkPMColor* dst,     size_t dstRB,
     if (SkColorGetA(color) == 0xFF) {
         const SkPMColor src = SkSwizzle_BGRA_to_PMColor(color);
         while (h --> 0) {
-            loop(w, dst, (const SkPMColor*)dst, src, cov, [](__m128i d, __m128i s, __m128i c) {
+            loop(w, dst, (const SkPMColor*)dst, src, cov,
+                    adapt([](__m128i d, __m128i s, __m128i c) {
                 // Src blend mode: a simple lerp from d to s by c.
                 // TODO: try a pmaddubsw version?
-                return div255(_mm_add_epi16(_mm_mullo_epi16(inv(c),d), _mm_mullo_epi16(c,s)));
-            });
+                return div255(_mm_add_epi16(_mm_mullo_epi16(inv(c),d),
+                                            _mm_mullo_epi16(    c ,s)));
+            }));
             dst += dstRB / sizeof(*dst);
             cov += covRB / sizeof(*cov);
         }
     } else {
         const SkPMColor src = SkPreMultiplyColor(color);
         while (h --> 0) {
-            loop(w, dst, (const SkPMColor*)dst, src, cov, [](__m128i d, __m128i s, __m128i c) {
+            loop(w, dst, (const SkPMColor*)dst, src, cov,
+                    adapt([](__m128i d, __m128i s, __m128i c) {
                 // SrcOver blend mode, with coverage folded into source alpha.
                 __m128i sc = scale(s,c),
                         AC = inv(alphas(sc));
                 return _mm_add_epi16(sc, scale(d,AC));
-            });
+            }));
             dst += dstRB / sizeof(*dst);
             cov += covRB / sizeof(*cov);
         }
@@ -176,6 +213,7 @@ static void blit_mask_d32_a8(SkPMColor* dst,     size_t dstRB,
 }
 
 }  // namespace sk_sse41
+
 #endif
 
 namespace SkOpts {