[PATCH] Fix compiler warnings with newer GCC 7.2.1

[Indu Bhagat]

This patch fixes a few of the compiler warnings when using gcc 7.2.1.

In general there are many other warnings (a majority of which are implicit fallthrough warnings in jdk.crypto.ec package's ecp_*.c files and java.desktop package's splashscreen_gfx_impl.h file).
I can post them if there is interest.
Most warnings, however, look benign (modulo some warnings about unintialized access that I cannot comment on; they need that the code be thoroughly understood).
libfdlibm misleading indentation warnings are already knownhttp://mail.openjdk.java.net/pipermail/jdk9-dev/2016-June/004494.html

Thanks

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The patch fixes the following warnings :

1. <jdk-dev>/jdk/src/jdk.jdwp.agent/share/native/libdt_socket/socketTransport.c: In function ‘mask_s2u’:
    <jdk-dev>/jdk/src/jdk.jdwp.agent/share/native/libdt_socket/socketTransport.c:389:22: warning: left shift of negative value [-Wshift-negative-value]
      *mask = htonl(-1 << (32 - m));
                       ^
2. <jdk-dev>/jdk/src/java.desktop/share/native/libjavajpeg/jdphuff.c: In function ‘decode_mcu_AC_refine’:
    <jdk-dev>/jdk/src/java.desktop/share/native/libjavajpeg/jdphuff.c:505:17: warning: left shift of negative value [-Wshift-negative-value]
    int m1 = (-1) << cinfo->Al;   /* -1 in the bit position being coded */
                  ^~
3. <jdk-dev>/jdk/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-ot-shape-fallback.cc: In function ‘void position_mark(const hb_ot_shape_plan_t*, hb_font_t*, hb_buffer_t*, hb_glyph_extents_t&, unsigned int, unsigned int)’:
    <jdk-dev>/jdk/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-ot-shape-fallback.cc:223:14: warning: this statement may fall through [-Wimplicit-fallthrough=]
        } else if (buffer->props.direction == HB_DIRECTION_RTL) {
               ^~
    <jdk-dev>/jdk/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-ot-shape-fallback.cc:229:5: note: here
      default:
      ^~~~~~~
    <jdk-dev>/jdk/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-ot-shape-fallback.cc:261:27: warning: this statement may fall through [-Wimplicit-fallthrough=]
        base_extents.height -= y_gap;
        ~~~~~~~~~~~~~~~~~~~~^~~~~~~~
    <jdk-dev>/jdk/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-ot-shape-fallback.cc:264:5: note: here
      case HB_UNICODE_COMBINING_CLASS_ATTACHED_BELOW_LEFT:
      ^~~~
    <jdk-dev>/jdk/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-ot-shape-fallback.cc:282:27: warning: this statement may fall through [-Wimplicit-fallthrough=]
        base_extents.height -= y_gap;
        ~~~~~~~~~~~~~~~~~~~~^~~~~~~~
    <jdk-dev>/jdk/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-ot-shape-fallback.cc:285:5: note: here
      case HB_UNICODE_COMBINING_CLASS_ATTACHED_ABOVE:
      ^~~~
-----------

diff --git a/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-private.hh b/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-private.hh
--- a/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-private.hh
+++ b/src/java.desktop/share/native/libfontmanager/harfbuzz/hb-private.hh
@@ -158,6 +158,9 @@
      */
  #  include <sal.h>
  #  define HB_FALLTHROUGH __fallthrough
+#elif __GNUC__ >= 7
+   /* GNU fallthrough attribute is available from GCC7 */
+#  define HB_FALLTHROUGH __attribute__((fallthrough))
  #else
  #  define HB_FALLTHROUGH /* FALLTHROUGH */
  #endif
diff --git a/src/java.desktop/share/native/libjavajpeg/jdphuff.c b/src/java.desktop/share/native/libjavajpeg/jdphuff.c
--- a/src/java.desktop/share/native/libjavajpeg/jdphuff.c
+++ b/src/java.desktop/share/native/libjavajpeg/jdphuff.c
@@ -501,8 +501,10 @@
  {
    phuff_entropy_ptr entropy = (phuff_entropy_ptr) cinfo->entropy;
    int Se = cinfo->Se;
-  int p1 = 1 << cinfo->Al;      /* 1 in the bit position being coded */
-  int m1 = (-1) << cinfo->Al;   /* -1 in the bit position being coded */
+  /* 1 in the bit position being coded */
+  int p1 = 1 << cinfo->Al;
+  /* -1 in the bit position being coded */
+  int m1 = (int)((unsigned)(~0) << cinfo->Al);
    register int s, k, r;
    unsigned int EOBRUN;
    JBLOCKROW block;
diff --git a/src/jdk.jdwp.agent/share/native/libdt_socket/socketTransport.c b/src/jdk.jdwp.agent/share/native/libdt_socket/socketTransport.c
--- a/src/jdk.jdwp.agent/share/native/libdt_socket/socketTransport.c
+++ b/src/jdk.jdwp.agent/share/native/libdt_socket/socketTransport.c
@@ -386,7 +386,7 @@
         return instr;
      }
  
-    *mask = htonl(-1 << (32 - m));
+    *mask = htonl((uint32_t)(~0) << (32 - m));
      return s;
  }