[m-rev.] for post-commit review: document status of clang support

[Julien Fischer]

Branches: main, 11.07

Document the status of clang support.

README.clang:
 	Document how to use clang as a C compiler with Mercury.

NEWS:
 	Announce the suport for clang.

Julien.

Index: NEWS
===================================================================
RCS file: /home/mercury/mercury1/repository/mercury/NEWS,v
retrieving revision 1.591
diff -u -r1.591 NEWS
--- NEWS	19 Aug 2011 03:26:33 -0000	1.591
+++ NEWS	2 Oct 2011 13:03:11 -0000
@@ -24,6 +24,11 @@
    bag, bimap, eqvclass, map, multi_map, queue and set modules in order
    to make them more conducive to the use of state variable notation.

+Changes to the Mercury compiler:
+
+* Support for using clang (<http://clang.llvm.org/>) as a C compiler with
+  Mercury has been added.  See README.clang for further details
+
  Changes to the Mercury profiler:

  * We have added a new form of profiling, memory retention profiling, to mprof
Index: README.clang
===================================================================
RCS file: README.clang
diff -N README.clang
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ README.clang	2 Oct 2011 13:04:50 -0000
@@ -0,0 +1,51 @@
+This file documents how to use clang as the C compiler with Mercury.  Support
+for Mercury with clang has been tested on Linux and Mac OS X.  To use clang as
+the C compiler for Mercury invoke Mercury's configure script with the --with-cc
+option set as follows:
+
+    $ ./configure --with-cc=clang
+
+Note that the asm_fast* and reg* grades are not usable with clang as they rely
+on GNU C extensions that clang does not support.
+
+Versions of LLVM (the underlying compiler infrastructure used by clang) before
+about 2.9 contain bugs that require most C compiler optimizations to be
+disabled when compiling Mercury generated C code with clang (i.e. the generated
+C files have to be compiled at -O0).  If you are using a more recent version of
+LLVM then you should be able to safely increase the optimization level used by
+clang by applying the following patch:
+
+Index: configure.in
+===================================================================
+RCS file: /home/mercury/mercury1/repository/mercury/configure.in,v
+retrieving revision 1.593
+diff -u -r1.593 configure.in
+--- configure.in	21 Sep 2011 06:11:30 -0000	1.593
++++ configure.in	2 Oct 2011 12:57:59 -0000
+@@ -4483,7 +4483,7 @@
+         # XXX we need go through the warning and optimization options for clang
+         # more carefully.
+         CFLAGS_FOR_WARNINGS="-w"
+-        CFLAGS_FOR_OPT="-fomit-frame-pointer"
++        CFLAGS_FOR_OPT="-O2 -fomit-frame-pointer"
+         CFLAGS_FOR_DEBUG="-g"
+         CFLAGS_FOR_NO_STRICT_ALIASING="-fno-strict-aliasing"
+         MCFLAGS_FOR_CC=
+Index: scripts/mgnuc.in
+===================================================================
+RCS file: /home/mercury/mercury1/repository/mercury/scripts/mgnuc.in,v
+retrieving revision 1.137
+diff -u -r1.137 mgnuc.in
+--- scripts/mgnuc.in	25 Aug 2011 14:16:53 -0000	1.137
++++ scripts/mgnuc.in	2 Oct 2011 12:57:37 -0000
+@@ -72,7 +72,7 @@
+     *clang*)
+         ANSI_OPTS="-ansi"
+         CHECK_OPTS="-w"
+-        OPT_OPTS="-O0 $CFLAGS_FOR_NO_STRICT_ALIASING -fomit-frame-pointer"
++        OPT_OPTS="-O2 $CFLAGS_FOR_NO_STRICT_ALIASING -fomit-frame-pointer"
+         DEBUG_OPT="-g"
+         COMPILER=clang
+         ;;
+
+

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