starter procs for blending with pm4f

src/core/SkXfer4f.cpp

+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "SkXfer4f.h"
+#include "SkPM4fPriv.h"
+#include "SkUtils.h"
+
+///////////////////////////////////////////////////////////////////////////////////////////////////
+
+void CLEAR_pm41p(uint32_t dst[], const SkPM4f& src, int count) {

[mtklein, 2016/01/29 15:23:00]

pm41p / pm4np is a bit beyond my readability decryption limit.

I don't think we need the pm4 on any of these... they're all going to have that
for the foreseeable future.
I'm not sure what the last 'p' is for.

How about one of:
    static void CLEAR_const_src(uint32_t dst[], const SkPM4f& src, int count)
    static void CLEAR_array_src(uint32_t dst[], const SkPM4f* src, int count)
or
    static void CLEAR_1(uint32_t dst[], const SkPM4f& src, int n)
    static void CLEAR_n(uint32_t dst[], const SkPM4f* src, int n)

(I think all these functions in here can/should be static.)

+    sk_bzero(dst, count * sizeof(uint32_t));
+}
+
+void CLEAR_pm4np(uint32_t dst[], const SkPM4f src[], int count) {
+    sk_bzero(dst, count * sizeof(uint32_t));
+}
+
+//////////
+
+template <bool isSRGB> void SRC_pm41p(uint32_t dst[], const SkPM4f& src, int count) {
+    uint32_t res;
+    if (isSRGB) {
+        res = Sk4f_toS32(Sk4f::Load(src.fVec));
+    } else {
+        res = Sk4f_toL32(Sk4f::Load(src.fVec));
+    }
+    sk_memset32(dst, res, count);
+}
+
+template <bool isSRGB> void SRC_pm4np(uint32_t dst[], const SkPM4f src[], int count) {
+    if (isSRGB) {
+        SkPM4f_s32_src_mode(dst, src, count);
+    } else {
+        SkPM4f_l32_src_mode(dst, src, count);
+    }
+}
+
+//////////
+
+void DST_pm41p(uint32_t dst[], const SkPM4f& src, int count) {}
+void DST_pm4np(uint32_t dst[], const SkPM4f src[], int count) {}
+
+//////////
+
+template <bool isSRGB> void SRCOVER_pm41p(uint32_t dst[], const SkPM4f& src, int count) {
+    SkASSERT(src.isUnit());
+    Sk4f s4 = Sk4f::Load(src.fVec);
+    Sk4f scale(1 - s4.kth<SkPM4f::A>());
+
+    if (!isSRGB) {
+        s4 = s4 * Sk4f(255);

[mtklein, 2016/01/29 15:41:31]

I think if we break these problems apart into independent factors (sRGB vs.
linear dst, single vs many src, blend mode) we can piece them all together with
templates and end up with just as fast code.

This'd let us write each mode once, figure out sRGB and back once, and let the
compiler handle the single-src work hoisting.  (The compiler can eliminate all
the unnecessary work for special cases like Dst, Src and Clear on its own.)

I'd be happy to try to sketch this up.

+    }
+
+    for (int i = 0; i < count; ++i) {
+        if (isSRGB) {
+            Sk4f d4 = Sk4f_fromS32(dst[i]);
+            dst[i] = Sk4f_toS32(s4 + d4 * scale);
+        } else {
+            Sk4f d4 = to_4f(dst[i]);
+            dst[i] = to_4b(s4 + d4 * scale + Sk4f(0.5f));

[mtklein, 2016/01/29 15:23:00]

Add the half to s4 outside the loop too?

+        }
+    }
+}
+
+template <bool isSRGB> void SRCOVER_pm4np(uint32_t dst[], const SkPM4f src[], int count) {
+    if (isSRGB) {
+        SkPM4f_s32_srcover_mode(dst, src, count);
+    } else {
+        SkPM4f_l32_srcover_mode(dst, src, count);
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////////////////////////////
+
+struct Pair {

[mtklein, 2016/01/29 15:23:00]

This struct seems a little dangerous without an anonymous namespace around it. 
It's very easy to violate ODR with a name like Pair.

+    SkPM4fXfer1Proc fProc1;
+    SkPM4fXferNProc fProcN;
+};
+
+const Pair gClearPairs[] = {

[mtklein, 2016/01/29 15:23:00]

static for all these too?

+    { CLEAR_pm41p, CLEAR_pm4np },
+    { CLEAR_pm41p, CLEAR_pm4np },
+    { CLEAR_pm41p, CLEAR_pm4np },
+    { CLEAR_pm41p, CLEAR_pm4np },
+};
+
+const Pair gSrcPairs[] = {
+    { SRC_pm41p<false>, SRC_pm4np<false> }, // linear [alpha  ignored]
+    { SRC_pm41p<false>, SRC_pm4np<false> }, // linear [opaque ignored]
+    { SRC_pm41p<true>,  SRC_pm4np<true>  }, // srgb   [alpha  ignored]
+    { SRC_pm41p<true>,  SRC_pm4np<true>  }, // srgb   [opaque ignored]
+};
+
+const Pair gDstPairs[] = {
+    { DST_pm41p, DST_pm4np },
+    { DST_pm41p, DST_pm4np },
+    { DST_pm41p, DST_pm4np },
+    { DST_pm41p, DST_pm4np },
+};
+
+const Pair gSrcOverPairs[] = {
+    { SRCOVER_pm41p<false>,   SRCOVER_pm4np<false>  },  // linear   alpha
+    { SRC_pm41p<false>,       SRC_pm4np<false>      },  // linear   opaque
+    { SRCOVER_pm41p<true>,    SRCOVER_pm4np<true>   },  // srgb     alpha
+    { SRC_pm41p<true>,        SRC_pm4np<true>       },  // srgb     opaque
+};
+
+static const Pair* find_pair(SkXfermode::Mode mode, uint32_t flags) {
+    SkASSERT(0 == (flags & ~3));
+    const Pair* pairs = nullptr;
+
+    switch (mode) {
+        case SkXfermode::kClear_Mode:   pairs = gClearPairs;

[mtklein, 2016/01/29 15:23:00]

I think you've missed some breaks?

+        case SkXfermode::kSrc_Mode:     pairs = gSrcPairs; break;
+        case SkXfermode::kDst_Mode:     pairs = gDstPairs;
+        case SkXfermode::kSrcOver_Mode: pairs = gSrcOverPairs; break;
+        default: return nullptr;
+    }
+    return &pairs[flags & 3];

[mtklein, 2016/01/29 15:23:00]

I think this could be a lot simpler if we return Pair instead of const Pair*.

static Pair find_pair(SkXfermode::Mode mode, uint32_t flags) {
    SkASSERT(0 == (flags& ~3));
    flags &= 3;
    switch (mode) { 
       case SkXfermode::kClear_Mode: return {CLEAR_pm41p, CLEAR_pm4np};
       case SkXfermode::kDst_Mode:   return {  DST_pm41p,   DST_pm4np};
       ...
       case SkXfermode::kSrc_Mode:     return gSrcPairs    [flags];
       case SkXfermode::kSrcOver_Mode: return gSrcOverPairs[flags];
       ...
       default: return {nullptr, nullptr}
    }
}

and the callers:

   return find_pair(mode, flags).fProc1;
or
   return find_pair(mode, flags).fProcN;

+}
+
+SkPM4fXfer1Proc SkPM4fXfer1ProcFactory(SkXfermode::Mode mode, uint32_t flags) {
+    const Pair* pair = find_pair(mode, flags);
+    return pair ? pair->fProc1 : nullptr;
+}
+
+SkPM4fXferNProc SkPM4fXferNProcFactory(SkXfermode::Mode mode, uint32_t flags) {
+    const Pair* pair = find_pair(mode, flags);
+    return pair ? pair->fProcN : nullptr;
+}