[libpng16] Fixed GCC unsigned int->float warnings. Various versions of GCC

seem to generate warnings when an unsigned value is implicitly
converted to double. This is probably a GCC bug but this change
avoids the issue by explicitly converting to (int) where safe.

CHANGES

@@ -5053,6 +5053,10 @@ Version 1.6.15beta05 [November 5, 2014]
     example.c, pngtest.c, and applications in the contrib directory.
   Avoid out-of-bounds memory access in png_user_version_check().
   Simplified and future-proofed png_user_version_check().
+  Fixed GCC unsigned int->float warnings. Various versions of GCC
+    seem to generate warnings when an unsigned value is implicitly
+    converted to double. This is probably a GCC bug but this change
+    avoids the issue by explicitly converting to (int) where safe.
 
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png.c

@@ -3719,7 +3719,31 @@ png_gamma_8bit_correct(unsigned int value, png_fixed_point gamma_val)
    if (value > 0 && value < 255)
    {
 #     ifdef PNG_FLOATING_ARITHMETIC_SUPPORTED
-         double r = floor(255*pow(value/255.,gamma_val*.00001)+.5);
+         /* 'value' is unsigned, ANSI-C90 requires the compiler to correctly
+          * convert this to a floating point value.  This includes values that
+          * would overflow if 'value' were to be converted to 'int'.
+          *
+          * Apparently GCC, however, does an intermediate convertion to (int)
+          * on some (ARM) but not all (x86) platforms, possibly because of
+          * hardware FP limitations.  (E.g. if the hardware convertion always
+          * assumes the integer register contains a signed value.)  This results
+          * in ANSI-C undefined behavior for large values.
+          *
+          * Other implementations on the same machine might actually be ANSI-C90
+          * conformant and therefore compile spurious extra code for the large
+          * values.
+          *
+          * We can be reasonably sure that an unsigned to float convertion
+          * won't be faster than an int to float one.  Therefore this code
+          * assumes responsibility for the undefined behavior, which it knows
+          * can't happen because of the check above.
+          *
+          * Note the argument to this routine is an (unsigned int) because, on
+          * 16-bit platforms, it is assigned a value which might be out of
+          * range for an (int); that would result in undefined behavior in the
+          * caller if the *argument* ('value') were to be declared (int).
+          */
+         double r = floor(255*pow((int)/*SAFE*/value/255.,gamma_val*.00001)+.5);
          return (png_byte)r;
 #     else
          png_int_32 lg2 = png_log8bit(value);
@@ -3743,7 +3767,13 @@ png_gamma_16bit_correct(unsigned int value, png_fixed_point gamma_val)
    if (value > 0 && value < 65535)
    {
 #     ifdef PNG_FLOATING_ARITHMETIC_SUPPORTED
-         double r = floor(65535*pow(value/65535.,gamma_val*.00001)+.5);
+         /* The same (unsigned int)->(double) constraints apply here as above,
+          * however in this case the (unsigned int) to (int) convertion can
+          * overflow on an ANSI-C90 compliant system so the cast needs to ensure
+          * that this is not possible.
+          */
+         double r = floor(65535*pow((png_int_32)value/65535.,
+                     gamma_val*.00001)+.5);
          return (png_uint_16)r;
 #     else
          png_int_32 lg2 = png_log16bit(value);
@@ -3797,6 +3827,12 @@ png_build_16bit_table(png_structrp png_ptr, png_uint_16pp *ptable,
 {
    /* Various values derived from 'shift': */
    PNG_CONST unsigned int num = 1U << (8U - shift);
+#ifdef PNG_FLOATING_ARITHMETIC_SUPPORTED
+   /* CSE the division and work round wacky GCC warnings (see the comments
+    * in png_gamma_8bit_correct for where these come from.)
+    */
+   PNG_CONST double fmax = 1./(((png_int_32)1 << (16U - shift))-1);
+#endif
    PNG_CONST unsigned int max = (1U << (16U - shift))-1U;
    PNG_CONST unsigned int max_by_2 = 1U << (15U-shift);
    unsigned int i;
@@ -3828,7 +3864,10 @@ png_build_16bit_table(png_structrp png_ptr, png_uint_16pp *ptable,
             png_uint_32 ig = (j << (8-shift)) + i;
 #           ifdef PNG_FLOATING_ARITHMETIC_SUPPORTED
                /* Inline the 'max' scaling operation: */
-               double d = floor(65535*pow(ig/(double)max, gamma_val*.00001)+.5);
+               /* See png_gamma_8bit_correct for why the cast to (int) is
+                * required here.
+                */
+               double d = floor(65535.*pow(ig*fmax, gamma_val*.00001)+.5);
                sub_table[j] = (png_uint_16)d;
 #           else
                if (shift != 0)