flags and hacks to get MSAN bot going

src/opts/SkXfermode_opts.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef Sk4pxXfermode_DEFINED
 #define Sk4pxXfermode_DEFINED
 
 #include "Sk4px.h"
+#include "SkMSAN.h"
 #include "SkNx.h"
 #include "SkXfermode_proccoeff.h"
 
 namespace {
 
 // Most xfermodes can be done most efficiently 4 pixels at a time in 8 or 16-bit fixed point.
 #define XFERMODE(Xfermode) \
     struct Xfermode { Sk4px operator()(const Sk4px&, const Sk4px&) const; }; \
     inline Sk4px Xfermode::operator()(const Sk4px& d, const Sk4px& s) const
 
@@ skipping 173 matching lines @@
 #define XFERMODE_AA(Xfermode) \
     template <> Sk4px xfer_aa<Xfermode>(const Sk4px& d, const Sk4px& s, const Sk4px& aa)
 
 // Plus' clamp needs to happen after AA.  skia:3852
 XFERMODE_AA(Plus) {  // [ clamp( (1-AA)D + (AA)(S+D) ) == clamp(D + AA*S) ]
     return d.saturatedAdd(s.approxMulDiv255(aa));
 }
 
 #undef XFERMODE_AA
 
+// Src and Clear modes are safe to use with unitialized dst buffers,
+// even if the implementation branches based on bytes from dst (e.g. asserts in Debug mode).
+// For those modes, just lie to MSAN that dst is always intialized.
+template <typename Xfermode> static void mark_dst_initialized_if_safe(void*, void*) {}
+template <> void mark_dst_initialized_if_safe<Src>(void* dst, void* end) {
+    sk_msan_mark_initialized(dst, end, "Src doesn't read dst.");
+}
+template <> void mark_dst_initialized_if_safe<Clear>(void* dst, void* end) {
+    sk_msan_mark_initialized(dst, end, "Clear doesn't read dst.");
+}
+
 template <typename Xfermode>
 class Sk4pxXfermode : public SkProcCoeffXfermode {
 public:
     Sk4pxXfermode(const ProcCoeff& rec, SkXfermode::Mode mode)
         : INHERITED(rec, mode) {}
 
     void xfer32(SkPMColor dst[], const SkPMColor src[], int n, const SkAlpha aa[]) const override {
+        mark_dst_initialized_if_safe<Xfermode>(dst, dst+n);
         if (nullptr == aa) {
             Sk4px::MapDstSrc(n, dst, src, Xfermode());
         } else {
             Sk4px::MapDstSrcAlpha(n, dst, src, aa, xfer_aa<Xfermode>);
         }
     }
 
     void xfer16(uint16_t dst[], const SkPMColor src[], int n, const SkAlpha aa[]) const override {
+        mark_dst_initialized_if_safe<Xfermode>(dst, dst+n);
         SkPMColor dst32[4];
         while (n >= 4) {
             dst32[0] = SkPixel16ToPixel32(dst[0]);
             dst32[1] = SkPixel16ToPixel32(dst[1]);
             dst32[2] = SkPixel16ToPixel32(dst[2]);
             dst32[3] = SkPixel16ToPixel32(dst[3]);
 
             this->xfer32(dst32, src, 4, aa);
 
             dst[0] = SkPixel32ToPixel16(dst32[0]);
@@ skipping 107 matching lines @@
     #undef CASE
 
         default: break;
     }
     return nullptr;
 }
 
 } // namespace SK_OPTS_NS
 
 #endif//Sk4pxXfermode_DEFINED