The compiler can generate smulbb perfectly well nowadays.

src/opts/SkBlitRow_opts_mips_dsp.cpp

 /*
  * Copyright 2014 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBlitRow.h"
 #include "SkBlitMask.h"
 #include "SkColorPriv.h"
@@ skipping 735 matching lines @@
         : [alpha]"r"(alpha)
         : "memory", "hi", "lo"
     );
 
     if (count == 1) {
         SkPMColor sc = *src++;
         SkPMColorAssert(sc);
         if (sc) {
             uint16_t dc = *dst;
             unsigned dst_scale = 255 - SkMulDiv255Round(SkGetPackedA32(sc), alpha);
-            unsigned dr = SkMulS16(SkPacked32ToR16(sc), alpha) +
-                          SkMulS16(SkGetPackedR16(dc), dst_scale);
-            unsigned dg = SkMulS16(SkPacked32ToG16(sc), alpha) +
-                          SkMulS16(SkGetPackedG16(dc), dst_scale);
-            unsigned db = SkMulS16(SkPacked32ToB16(sc), alpha) +
-                          SkMulS16(SkGetPackedB16(dc), dst_scale);
+            unsigned dr = (SkPacked32ToR16(sc) * alpha) + (SkGetPackedR16(dc) * dst_scale);
+            unsigned dg = (SkPacked32ToG16(sc) * alpha) + (SkGetPackedG16(dc) * dst_scale);
+            unsigned db = (SkPacked32ToB16(sc) * alpha) + (SkGetPackedB16(dc) * dst_scale);
             *dst = SkPackRGB16(SkDiv255Round(dr), SkDiv255Round(dg), SkDiv255Round(db));
         }
         dst += 1;
     }
 }
 
 static void S32_Blend_BlitRow32_mips_dsp(SkPMColor* SK_RESTRICT dst,
                                          const SkPMColor* SK_RESTRICT src,
                                          int count, U8CPU alpha) {
     register int32_t t0, t1, t2, t3, t4, t5, t6, t7;
@@ skipping 174 matching lines @@
     return platform_565_procs_mips_dsp[flags];
 }
 
 SkBlitRow::ColorProc16 SkBlitRow::PlatformColorFactory565(unsigned flags) {
     return NULL;
 }
 
 SkBlitRow::Proc32 SkBlitRow::PlatformProcs32(unsigned flags) {
     return platform_32_procs_mips_dsp[flags];
 }