Correct CMAKE_INSTALL_PREFIX value for Win64 apps (#9992)

The default value for CMAKE_INSTALL_PREFIX should be
based on what architecture the built targets are, not
what architecture CMake itself is.

This fix merely guesses better what the built targets
architecture is. It still may guess incorrectly in some
cases. For those cases, it will have to be up to build
scripts and developers on projects to pass in a correct
value for CMAKE_INSTALL_PREFIX with -D on the command line
or via 'force cache value' logic in CMakeLists.txt files.

Modules/CMakeGenericSystem.cmake

@@ -52,6 +52,94 @@ IF(CMAKE_GENERATOR MATCHES "Makefiles")
   ENDIF(DEFINED CMAKE_RULE_MESSAGES)
 ENDIF(CMAKE_GENERATOR MATCHES "Makefiles")
 
+
+# GetDefaultWindowsPrefixBase
+#
+# Compute the base directory for CMAKE_INSTALL_PREFIX based on:
+#  - is this 32-bit or 64-bit Windows
+#  - is this 32-bit or 64-bit CMake running
+#  - what architecture targets will be built
+#
+function(GetDefaultWindowsPrefixBase var)
+
+  # Try to guess what architecture targets will end up being built as,
+  # even if CMAKE_SIZEOF_VOID_P is not computed yet... We need to know
+  # the architecture of the targets being built to choose the right
+  # default value for CMAKE_INSTALL_PREFIX.
+  #
+  if("${CMAKE_GENERATOR}" MATCHES "Win64")
+    set(arch_hint "x64")
+  elseif("${CMAKE_SIZEOF_VOID_P}" STREQUAL "8")
+    set(arch_hint "x64")
+  elseif("$ENV{LIB}" MATCHES "(amd64|ia64)")
+    set(arch_hint "x64")
+  endif()
+
+  if(NOT arch_hint)
+    set(arch_hint "x86")
+  endif()
+
+  # default env in a 64-bit app on Win64:
+  # ProgramFiles=C:\Program Files
+  # ProgramFiles(x86)=C:\Program Files (x86)
+  # ProgramW6432=C:\Program Files
+  #
+  # default env in a 32-bit app on Win64:
+  # ProgramFiles=C:\Program Files (x86)
+  # ProgramFiles(x86)=C:\Program Files (x86)
+  # ProgramW6432=C:\Program Files
+  #
+  # default env in a 32-bit app on Win32:
+  # ProgramFiles=C:\Program Files
+  # ProgramFiles(x86) NOT DEFINED
+  # ProgramW6432 NOT DEFINED
+
+  # By default, use the ProgramFiles env var as the base value of
+  # CMAKE_INSTALL_PREFIX:
+  #
+  set(_PREFIX_ENV_VAR "ProgramFiles")
+
+  if ("$ENV{ProgramW6432}" STREQUAL "")
+    # running on 32-bit Windows
+    # must be a 32-bit CMake, too...
+    #message("guess: this is a 32-bit CMake running on 32-bit Windows")
+  else()
+    # running on 64-bit Windows
+    if ("$ENV{ProgramW6432}" STREQUAL "$ENV{ProgramFiles}")
+      # 64-bit CMake
+      #message("guess: this is a 64-bit CMake running on 64-bit Windows")
+      if(NOT "${arch_hint}" STREQUAL "x64")
+      # building 32-bit targets
+        set(_PREFIX_ENV_VAR "ProgramFiles(x86)")
+      endif()
+    else()
+      # 32-bit CMake
+      #message("guess: this is a 32-bit CMake running on 64-bit Windows")
+      if("${arch_hint}" STREQUAL "x64")
+      # building 64-bit targets
+        set(_PREFIX_ENV_VAR "ProgramW6432")
+      endif()
+    endif()
+  endif()
+
+  #if("${arch_hint}" STREQUAL "x64")
+  #  message("guess: you are building a 64-bit app")
+  #else()
+  #  message("guess: you are building a 32-bit app")
+  #endif()
+
+  if(NOT "$ENV{${_PREFIX_ENV_VAR}}" STREQUAL "")
+    file(TO_CMAKE_PATH "$ENV{${_PREFIX_ENV_VAR}}" _base)
+  elseif(NOT "$ENV{SystemDrive}" STREQUAL "")
+    set(_base "$ENV{SystemDrive}/Program Files")
+  else()
+    set(_base "C:/Program Files")
+  endif()
+
+  set(${var} "${_base}" PARENT_SCOPE)
+endfunction()
+
+
 # Set a variable to indicate whether the value of CMAKE_INSTALL_PREFIX
 # was initialized by the block below.  This is useful for user
 # projects to change the default prefix while still allowing the
@@ -65,23 +153,11 @@ IF(CMAKE_HOST_UNIX)
   SET(CMAKE_INSTALL_PREFIX "/usr/local"
     CACHE PATH "Install path prefix, prepended onto install directories.")
 ELSE(CMAKE_HOST_UNIX)
-  IF("$ENV{ProgramFiles}" MATCHES "^$")
-    IF("$ENV{SystemDrive}" MATCHES "^$")
-      SET(CMAKE_GENERIC_PROGRAM_FILES "C:/Program Files")
-    ELSE("$ENV{SystemDrive}" MATCHES "^$")
-      SET(CMAKE_GENERIC_PROGRAM_FILES "$ENV{SystemDrive}/Program Files")
-    ENDIF("$ENV{SystemDrive}" MATCHES "^$")
-  ELSE("$ENV{ProgramFiles}" MATCHES "^$")
-    SET(CMAKE_GENERIC_PROGRAM_FILES "$ENV{ProgramFiles}")
-  ENDIF("$ENV{ProgramFiles}" MATCHES "^$")
+  GetDefaultWindowsPrefixBase(CMAKE_GENERIC_PROGRAM_FILES)
   SET(CMAKE_INSTALL_PREFIX
     "${CMAKE_GENERIC_PROGRAM_FILES}/${PROJECT_NAME}"
     CACHE PATH "Install path prefix, prepended onto install directories.")
   SET(CMAKE_GENERIC_PROGRAM_FILES)
-
-  # Make sure the prefix uses forward slashes.
-  STRING(REGEX REPLACE "\\\\" "/"
-    CMAKE_INSTALL_PREFIX "${CMAKE_INSTALL_PREFIX}")
 ENDIF(CMAKE_HOST_UNIX)
 
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