Add SK_BEGIN_REQUIRE_DENSE / SK_END_REQUIRE_DENSE.

Use in on a few examples of things we hash.

BUG=skia:

Review URL: https://codereview.chromium.org/1468483002

include/core/SkMatrix.h

@@ -22,6 +22,7 @@ class SkString;
     using either reset() - to construct an identity matrix, or one of the set
     functions (e.g. setTranslate, setRotate, etc.).
 */
+SK_BEGIN_REQUIRE_DENSE
 class SK_API SkMatrix {
 public:
     static SkMatrix SK_WARN_UNUSED_RESULT MakeScale(SkScalar sx, SkScalar sy) {
@@ -826,5 +827,6 @@ private:
 
     friend class SkPerspIter;
 };
+SK_END_REQUIRE_DENSE
 
 #endif

include/core/SkStrokeRec.h

@@ -12,6 +12,7 @@
 
 class SkPath;
 
+SK_BEGIN_REQUIRE_DENSE
 class SkStrokeRec {
 public:
     enum InitStyle {
@@ -125,5 +126,6 @@ private:
     uint32_t        fJoin : 15;            // SkPaint::Join
     uint32_t        fStrokeAndFill : 1;    // bool
 };
+SK_END_REQUIRE_DENSE
 
 #endif

include/core/SkTypes.h

@@ -287,6 +287,16 @@ static inline bool SkIsU16(long x) {
 template <typename T, size_t N> char (&SkArrayCountHelper(T (&array)[N]))[N];
 #define SK_ARRAY_COUNT(array) (sizeof(SkArrayCountHelper(array)))
 
+// Can be used to bracket data types that must be dense, e.g. hash keys.
+#if defined(__clang__)  // This should work on GCC too, but GCC diagnostic pop didn't seem to work!
+    #define SK_BEGIN_REQUIRE_DENSE _Pragma("GCC diagnostic push") \
+                                   _Pragma("GCC diagnostic error \"-Wpadded\"")
+    #define SK_END_REQUIRE_DENSE   _Pragma("GCC diagnostic pop")
+#else
+    #define SK_BEGIN_REQUIRE_DENSE
+    #define SK_END_REQUIRE_DENSE
+#endif
+
 #define SkAlign2(x)     (((x) + 1) >> 1 << 1)
 #define SkIsAlign2(x)   (0 == ((x) & 1))
 

src/core/SkGlyph.h

@@ -22,6 +22,7 @@ class SkGlyphCache;
 
 #define kMaxGlyphWidth (1<<13)
 
+SK_BEGIN_REQUIRE_DENSE
 class SkGlyph {
     enum {
         kSubBits = 2,
@@ -182,5 +183,6 @@ class SkGlyph {
 #endif
     uint32_t    fID;
 };
+SK_END_REQUIRE_DENSE
 
 #endif