Provide way to build using existing C compiler on Windows.

Summary:
There are various distros that build GHC using their own C compilers
such as MSYS2. Currently they have to patch the build scripts everytime.

This patch provides the configure argument `--enable-distro-toolchain`
which allows one to build using any C compiler on the path.

This is also useful for testing new versions of GCC.

Test Plan:
./configure --enable-distro-toolchain && make - && make THREADS=9 test
./validate

Reviewers: austin, hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, erikd, #ghc_windows_task_force

GHC Trac Issues: #13792

Differential Revision: https://phabricator.haskell.org/D3637

3 files changed, 65 insertions, 10 deletions

diff --git a/aclocal.m4 b/aclocal.m4
index d566f83e8c..db394f38d6 100644
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -460,7 +460,7 @@ AC_DEFUN([GET_ARM_ISA],
 # Set the variables used in the settings file
 AC_DEFUN([FP_SETTINGS],
 [
-    if test "$windows" = YES
+    if test "$windows" = YES -a "$EnableDistroToolchain" = "NO"
     then
         mingw_bin_prefix=mingw/bin/
         SettingsCCompilerCommand="\$topdir/../${mingw_bin_prefix}gcc.exe"
@@ -472,6 +472,18 @@ AC_DEFUN([FP_SETTINGS],
         SettingsDllWrapCommand="\$topdir/../${mingw_bin_prefix}dllwrap.exe"
         SettingsWindresCommand="\$topdir/../${mingw_bin_prefix}windres.exe"
         SettingsTouchCommand='$topdir/bin/touchy.exe'
+    elif test "$EnableDistroToolchain" = "YES"
+    then
+        SettingsCCompilerCommand="$(basename $CC)"
+        SettingsCCompilerFlags="$CONF_CC_OPTS_STAGE2"
+        SettingsHaskellCPPCommand="$(basename $HaskellCPPCmd)"
+        SettingsHaskellCPPFlags="$HaskellCPPArgs"
+        SettingsLdCommand="$(basename $LdCmd)"
+        SettingsArCommand="$(basename $ArCmd)"
+        SettingsPerlCommand="$(basename $PerlCmd)"
+        SettingsDllWrapCommand="$(basename $DllWrapCmd)"
+        SettingsWindresCommand="$(basename $WindresCmd)"
+        SettingsTouchCommand='$topdir/bin/touchy.exe'
     else
         SettingsCCompilerCommand="$CC"
         SettingsHaskellCPPCommand="$HaskellCPPCmd"
@@ -479,17 +491,17 @@ AC_DEFUN([FP_SETTINGS],
         SettingsLdCommand="$LdCmd"
         SettingsArCommand="$ArCmd"
         SettingsPerlCommand="$PerlCmd"
-        if test -z "$DllWrap"
+        if test -z "$DllWrapCmd"
         then
             SettingsDllWrapCommand="/bin/false"
         else
-            SettingsDllWrapCommand="$DllWrap"
+            SettingsDllWrapCommand="$DllWrapCmd"
         fi
-        if test -z "$Windres"
+        if test -z "$WindresCmd"
         then
             SettingsWindresCommand="/bin/false"
         else
-            SettingsWindresCommand="$Windres"
+            SettingsWindresCommand="$WindresCmd"
         fi
        SettingsTouchCommand='touch'
     fi
diff --git a/configure.ac b/configure.ac
index a32e6b408c..721f0e7b8b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -113,6 +113,17 @@ AC_ARG_ENABLE(tarballs-autodownload,
         TarballsAutodownload=NO
 )
 
+AC_ARG_ENABLE(distro-toolchain,
+[AC_HELP_STRING([--enable-distro-toolchain],
+                [Do not use bundled Windows toolchain binaries.])],
+        EnableDistroToolchain=YES,
+        EnableDistroToolchain=NO
+)
+
+if test "$EnableDistroToolchain" = "YES"; then
+  TarballsAutodownload=NO
+fi
+
 dnl CC_STAGE0 is like the "previous" variable CC (inherited by CC_STAGE[123])
 dnl but instead used by stage0 for bootstrapping stage1
 AC_ARG_VAR(CC_STAGE0, [C compiler command (bootstrap)])
@@ -365,7 +376,7 @@ set_up_tarballs() {
     fi
 }
 
-if test "$HostOS" = "mingw32"
+if test "$HostOS" = "mingw32" -a "$EnableDistroToolchain" = "NO"
 then
     test -d inplace || mkdir inplace
 
@@ -395,6 +406,29 @@ then
     fi
 fi
 
+# We don't want to bundle a MinGW-w64 toolchain
+# So we have to find these individual tools.
+if test "$EnableDistroToolchain" = "YES"
+then
+    # Ideally should use AC_CHECK_TARGET_TOOL but our triples
+    # are screwed up. Configure doesn't think they're ever equal and
+    # so never tried without the prefix.
+    AC_PATH_PROG([CC],[gcc], [clang])
+    AC_PATH_PROG([LD],[ld], [lld])
+    AC_PATH_PROG([NM],[nm])
+    AC_PATH_PROG([AR],[ar])
+    AC_PATH_PROG([RANLIB],[ranlib])
+    AC_PATH_PROG([OBJDUMP],[objdump])
+    AC_PATH_PROG([DllWrap],[dllwrap])
+    AC_PATH_PROG([Windres],[windres])
+
+    DllWrapCmd="$DllWrap"
+    WindresCmd="$Windres"
+
+    AC_SUBST([DllWrapCmd])
+    AC_SUBST([WindresCmd])
+fi
+
 FP_ICONV
 FP_GMP
 FP_CURSES
@@ -461,6 +495,7 @@ fi
 AC_SUBST(CrossCompiling)
 AC_SUBST(CrossCompilePrefix)
 AC_SUBST(TargetPlatformFull)
+AC_SUBST(EnableDistroToolchain)
 
 dnl ** Which gcc to use?
 dnl --------------------------------------------------------------
@@ -621,7 +656,11 @@ SplitObjsBroken=NO
 dnl ** look for `perl'
 case $HostOS_CPP in
 cygwin32|mingw32)
-  PerlCmd=$hardtop/inplace/perl/perl
+  if test "$EnableDistroToolchain" = "NO"; then
+    PerlCmd=$hardtop/inplace/perl/perl
+  else
+    AC_PATH_PROG([PerlCmd],[perl])
+  fi
   ;;
 *)
     AC_PATH_PROG([PerlCmd],[perl])
@@ -1258,8 +1297,8 @@ echo "\
    libtool      : $LibtoolCmd
    objdump      : $ObjdumpCmd
    ranlib       : $RanlibCmd
-   windres      : $Windres
-   dllwrap      : $DllWrap
+   windres      : $WindresCmd
+   dllwrap      : $DllWrapCmd
    Happy        : $HappyCmd ($HappyVersion)
    Alex         : $AlexCmd ($AlexVersion)
    Perl         : $PerlCmd
diff --git a/docs/users_guide/8.4.1-notes.rst b/docs/users_guide/8.4.1-notes.rst
index 5929998e95..f23cb360c9 100644
--- a/docs/users_guide/8.4.1-notes.rst
+++ b/docs/users_guide/8.4.1-notes.rst
@@ -76,6 +76,10 @@ Now we generate ::
 
       _ == _ = error ...
 
+- Configure on Windows now supports ``--enable-distro-toolchain`` which can be
+  used to build a GHC using compilers on your ``PATH`` instead of using the
+  bundled bindist. See :ghc-ticket:`13792`
+
 - Lots of other bugs. See `Trac <https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.4.1&col=id&col=summary&col=status&col=type&col=priority&col=milestone&col=component&order=priority>`_
   for a complete list.
 
@@ -84,7 +88,7 @@ Runtime system
 
 - Function ``hs_add_root()`` was removed. It was a no-op since GHC-7.2.1
   where module initialisation stopped requiring a call to ``hs_add_root()``.
-  
+
 - Proper import library support added to GHC which can handle all of the libraries produced
   by dlltool. The limitation of them needing to be named with the suffix .dll.a is also removed.
   See :ghc-ticket:`13606`, :ghc-ticket:`12499`, :ghc-ticket:`12498`