Added warning with supported OSes list for VSX check

CMakeLists.txt

@@ -132,6 +132,8 @@ if(${CMAKE_SYSTEM_PROCESSOR} MATCHES "^powerpc*" OR
       add_definitions(-DPNG_POWERPC_VSX_OPT=2)
     elseif(${PNG_POWERPC_VSX} STREQUAL "check")
       add_definitions(-DPNG_POWERPC_VSX_CHECK_SUPPORTED)
+      message(WARNING
+        " Please check contrib/powerpc/README file for the list of supported OSes.")
     endif()
   else()
     add_definitions(-DPNG_POWERPC_VSX_OPT=0)

configure.ac

@@ -448,7 +448,9 @@ AS_HELP_STRING([[[--enable-powerpc-vsx]]],
          enable_powerpc_vsx=no;;
       check)
          AC_DEFINE([PNG_POWERPC_VSX_CHECK_SUPPORTED], [],
-                   [Check for POWERPC VSX support at run-time]);;
+                   [Check for POWERPC VSX support at run-time])
+         AC_MSG_WARN([--enable-powerpc-vsx Please check contrib/powerpc/README file]
+            [for the list of supported OSes.]);;
       api)
          AC_DEFINE([PNG_POWERPC_VSX_API_SUPPORTED], [],
                    [Turn on POWERPC VSX optimizations at run-time]);;

contrib/powerpc-vsx/README

@@ -0,0 +1,81 @@
+OPERATING SYSTEM SPECIFIC POWERPC DETECTION
+--------------------------------------------
+
+Detection of the ability to execute POWERPC on processor requires
+operating system support.  (The information is not available in user mode.)
+
+Currently only this feature is supported only for linux platform.
+
+HOW TO USE THIS
+---------------
+
+This directory contains C code fragments that can be included in powerpc/powerpc_init.c
+by setting the macro PNG_POWERPC_VSX_FILE to the file name in "" or <> at build
+time.  This setting is not recorded in pnglibconf.h and can be changed simply by
+rebuilding arm/arm_init.o with the required macro definition.
+
+For any of this code to be used the POWERPC code must be enabled and run time
+checks must be supported.  I.e.:
+
+#if PNG_POWERPC_VSX_OPT > 0
+#ifdef PNG_POWERPC_VSX_CHECK_SUPPORTED
+
+This is done in a 'configure' build by passing configure the argument:
+
+   --enable-powerpc-vsx=check
+
+FILE FORMAT
+-----------
+
+Each file documents its testing status as of the last time it was tested (which
+may have been a long time ago):
+
+STATUS: one of:
+   SUPPORTED: This indicates that the file is included in the regularly
+         performed test builds and bugs are fixed when discovered.
+   COMPILED: This indicates that the code did compile at least once.  See the
+         more detailed description for the extent to which the result was
+         successful.
+   TESTED: This means the code was fully compiled into the libpng test programs
+         and these were run at least once.
+
+BUG REPORTS: an email address to which to send reports of problems
+
+The file is a fragment of C code. It should not define any 'extern' symbols;
+everything should be static.  It must define the function:
+
+static int png_have_vsx(png_structp png_ptr);
+
+That function must return 1 if ARM NEON instructions are supported, 0 if not.
+It must not execute png_error unless it detects a bug.  A png_error will prevent
+the reading of the PNG and in the future, writing too.
+
+BUG REPORTS
+-----------
+
+If you mail a bug report for any file that is not SUPPORTED there may only be
+limited response.  Consider fixing it and sending a patch to fix the problem -
+this is more likely to result in action.
+
+CONTRIBUTIONS
+-------------
+
+You may send contributions of new implementations to
+png-mng-implement@sourceforge.net.  Please write code in strict C90 C where
+possible.  Obviously OS dependencies are to be expected.  If you submit code you
+must have the authors permission and it must have a license that is acceptable
+to the current maintainer; in particular that license must permit modification
+and redistribution.
+
+Please try to make the contribution a single file and give the file a clear and
+unambiguous name that identifies the target OS.  If multiple files really are
+required put them all in a sub-directory.
+
+You must also be prepared to handle bug reports from users of the code, either
+by joining the png-mng-implement mailing list or by providing an email for the
+"BUG REPORTS" entry or both.  Please make sure that the header of the file
+contains the STATUS and BUG REPORTS fields as above.
+
+Please list the OS requirements as precisely as possible.  Ideally you should
+also list the environment in which the code has been tested and certainly list
+any environments where you suspect it might not work.

contrib/powerpc-vsx/linux.c

@@ -7,7 +7,7 @@
  * For conditions of distribution and use, see the disclaimer
  * and license in png.h
  *
- * STATUS: COMPILED
+ * STATUS: TESTED
  * BUG REPORTS: png-mng-implement@sourceforge.net
  *
  * png_have_vsx implemented for Linux by reading the widely available

contrib/powerpc-vsx/linux_aux.c

@@ -7,7 +7,7 @@
  * For conditions of distribution and use, see the disclaimer
  * and license in png.h
  *
- * STATUS: COMPILED
+ * STATUS: TESTED
  * BUG REPORTS: png-mng-implement@sourceforge.net
  *
  * png_have_vsx implemented for Linux by using the auxiliary vector mechanism.