Fix UB in SkDivBits

This used to:
  DIVBITS_ITER was shifting bits up into the sign bit, which is a no-no.
  This turns numer into a uint32_t to make those defined, and adds a few notes.

  x >= 0 is always true for unsigned x, so we needed a few small logic refactors.

Instead it now:
  Only call SkDivBits if the old behavior is required.
  Usually, just do the divide with /.

BUG=skia:3562

Committed: https://skia.googlesource.com/skia/+/988adddd48322bfa3e3cb0c017cfce71fbbf1123

Committed: https://skia.googlesource.com/skia/+/6c7b104b4c08ae2332a6ce3c8c906da4e8c51e5f

TBR=reed@google.com
No API change.

Review URL: https://codereview.chromium.org/1455163004

BUILD.public

@@ -439,7 +439,7 @@ cc_test(
         "--nogpu",
         "--verbose",
         # TODO(mtklein): maybe investigate why these fail?
-        "--match ~FontMgr ~Scalar ~Canvas ~Codec_stripes ~Codec_Dimensions ~Codec ~Stream ~skps ~Math ~RecordDraw_TextBounds",
+        "--match ~FontMgr ~Scalar ~Canvas ~Codec_stripes ~Codec_Dimensions ~Codec ~Stream ~skps ~RecordDraw_TextBounds",
         "--resourcePath %s/resources" % BASE_DIR,
         "--images %s/resources" % BASE_DIR,
     ],

include/core/SkFixed.h

@@ -82,7 +82,14 @@ typedef int32_t             SkFixed;
 #define SkFixedAbs(x)       SkAbs32(x)
 #define SkFixedAve(a, b)    (((a) + (b)) >> 1)
 
-#define SkFixedDiv(numer, denom)    SkDivBits(numer, denom, 16)
+// Blink layout tests are baselined to Clang optimizing through undefined behavior in SkDivBits.
+#if defined(SK_SUPPORT_LEGACY_DIVBITS_UB)
+    #define SkFixedDiv(numer, denom) SkDivBits(numer, denom, 16)
+#else
+    // TODO(reed): this clamp shouldn't be needed.  Use SkToS32().
+    #define SkFixedDiv(numer, denom) \
+        SkTPin<int32_t>(((int64_t)numer << 16) / denom, SK_MinS32, SK_MaxS32)
+#endif
 
 //////////////////////////////////////////////////////////////////////////////////////////////////////
 // Now look for ASM overrides for our portable versions (should consider putting this in its own file)

tools/dm_flags.json

@@ -1221,9 +1221,7 @@
     "_", 
     "image", 
     "_", 
-    "interlaced3.png", 
-    "--match", 
-    "~Math"
+    "interlaced3.png"
   ], 
   "Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-Valgrind": [
     "--matrix", 

tools/dm_flags.py

@@ -169,11 +169,6 @@ def get_args(bot):
   if 'Valgrind' in bot: # skia:3021
     match.append('~Threaded')
 
-  # skia:3562
-  if ('TSAN' in bot or
-      'Test-Mac10.8-Clang-MacMini4.1-CPU-SSE4-x86_64-Release' in bot):
-    match.append('~Math')
-
   if 'GalaxyS3' in bot:  # skia:1699
     match.append('~WritePixels')