use attribute to alias for punning float to int

Bug: libyuv:791
Test: g++ -Iinclude -I../libvpx/third_party/libwebm -I../libvpx/vp8 -I../libvpx/vp8 -I../libvpx/vp9 -I../libvpx/vp9 -Iinclude -m64 -DNDEBUG -O3 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -Wall -Wdisabled-optimization -Wfloat-conversion -Wpointer-arith -Wtype-limits -Wcast-qual -Wvla -Wuninitialized -Wunused -Wextra -I. -I"../libvpx" -D__STDC_CONSTANT_MACROS -D__STDC_LIMIT_MACROS -Wno-unused-parameter -c -o third_party/libyuv/source/row_common.cc.o source/row_common.cc
Change-Id: Ia006cb9212b671ae668cab5ec0b29759024a2c8a
Reviewed-on: https://chromium-review.googlesource.com/1012462Reviewed-by: Johann Koenig <johannkoenig@google.com>
Commit-Queue: Frank Barchard <fbarchard@chromium.org>

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 1708
+Version: 1709
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 1708
+#define LIBYUV_VERSION 1709
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_

source/row_common.cc

@@ -2762,6 +2762,13 @@ void ARGBPolynomialRow_C(const uint8_t* src_argb,
 // simply extract the low bits of the exponent and the high
 // bits of the mantissa from our float and we're done.
 
+// Work around GCC 7 punning warning -Wstrict-aliasing
+#if defined(__GNUC__)
+typedef uint32_t __attribute__((__may_alias__)) uint32_alias_t;
+#else
+typedef uint32_t uint32_alias_t;
+#endif
+
 void HalfFloatRow_C(const uint16_t* src,
                     uint16_t* dst,
                     float scale,
@@ -2770,7 +2777,7 @@ void HalfFloatRow_C(const uint16_t* src,
   float mult = 1.9259299444e-34f * scale;
   for (i = 0; i < width; ++i) {
     float value = src[i] * mult;
-    dst[i] = (uint16_t)((*(uint32_t*)&value) >> 13);
+    dst[i] = (uint16_t)((*(const uint32_alias_t*)&value) >> 13);
   }
 }