add musl-clang, a wrapper for system clang installs

musl-clang allows the user to compile musl-powered programs using their
already existent clang install, without the need of a special cross compiler.
it achieves this by wrapping around both the system clang install and the
linker and passing them special flags to re-target musl at runtime.
it does only affect invocations done through the special musl-clang wrapper
script, so that the user setup remains fully intact otherwise.

the clang wrapper consists of the compiler frontend wrapper script,
musl-clang, and the linker wrapper script, ld.musl-clang.
musl-clang makes sure clang invokes ld.musl-clang to link objects; neither
script needs to be in PATH for the wrapper to work.

.gitignore

@@ -7,5 +7,7 @@ arch/*/bits/alltypes.h
 config.mak
 include/bits
 tools/musl-gcc
+tools/musl-clang
+tools/ld.musl-clang
 lib/musl-gcc.specs
 src/internal/version.h

Makefile

@@ -52,6 +52,7 @@ ALL_LIBS = $(CRT_LIBS) $(STATIC_LIBS) $(SHARED_LIBS) $(EMPTY_LIBS) $(TOOL_LIBS)
 ALL_TOOLS = tools/musl-gcc
 
 WRAPCC_GCC = gcc
+WRAPCC_CLANG = clang
 
 LDSO_PATHNAME = $(syslibdir)/ld-musl-$(ARCH)$(SUBARCH).so.1
 
@@ -160,6 +161,10 @@ tools/musl-gcc: config.mak
 	printf '#!/bin/sh\nexec "$${REALGCC:-$(WRAPCC_GCC)}" "$$@" -specs "%s/musl-gcc.specs"\n' "$(libdir)" > $@
 	chmod +x $@
 
+tools/%-clang: tools/%-clang.in config.mak
+	sed -e 's!@CC@!$(WRAPCC_CLANG)!g' -e 's!@PREFIX@!$(prefix)!g' -e 's!@INCDIR@!$(includedir)!g' -e 's!@LIBDIR@!$(libdir)!g' -e 's!@LDSO@!$(LDSO_PATHNAME)!g' $< > $@
+	chmod +x $@
+
 $(DESTDIR)$(bindir)/%: tools/%
 	$(INSTALL) -D $< $@
 

configure

@@ -134,6 +134,7 @@ shared=auto
 static=yes
 wrapper=auto
 gcc_wrapper=no
+clang_wrapper=no
 
 for arg ; do
 case "$arg" in
@@ -158,8 +159,9 @@ case "$arg" in
 --enable-visibility|--enable-visibility=yes) visibility=yes ;;
 --disable-visibility|--enable-visibility=no) visibility=no ;;
 --enable-wrapper|--enable-wrapper=yes) wrapper=detect ;;
---enable-wrapper=all) wrapper=yes ; gcc_wrapper=yes ;;
+--enable-wrapper=all) wrapper=yes ; gcc_wrapper=yes ; clang_wrapper=yes ;;
 --enable-wrapper=gcc) wrapper=yes ; gcc_wrapper=yes ;;
+--enable-wrapper=clang) wrapper=yes ; clang_wrapper=yes ;;
 --disable-wrapper|--enable-wrapper=no) wrapper=no ;;
 --enable-gcc-wrapper|--enable-gcc-wrapper=yes) wrapper=yes ; gcc_wrapper=yes ;;
 --disable-gcc-wrapper|--enable-gcc-wrapper=no) wrapper=no ;;
@@ -230,6 +232,8 @@ cc_ver="$(LC_ALL=C $CC -v 2>&1)"
 cc_family=unknown
 if fnmatch '*gcc\ version*' "$cc_ver" ; then
 cc_family=gcc
+elif fnmatch '*clang\ version*' "$cc_ver" ; then
+cc_family=clang
 fi
 echo "$cc_family"
 
@@ -247,6 +251,9 @@ echo "none"
 elif test "$cc_family" = gcc ; then
 gcc_wrapper=yes
 echo "gcc"
+elif test "$cc_family" = clang ; then
+clang_wrapper=yes
+echo "clang"
 else
 echo "none"
 if test "$wrapper" = detect ; then
@@ -259,6 +266,9 @@ if test "$gcc_wrapper" = yes ; then
 tools="$tools tools/musl-gcc"
 tool_libs="$tool_libs lib/musl-gcc.specs"
 fi
+if test "$clang_wrapper" = yes ; then
+tools="$tools tools/musl-clang tools/ld.musl-clang"
+fi
 
 #
 # Find the target architecture
@@ -600,6 +610,7 @@ EOF
 test "x$static" = xno && echo "STATIC_LIBS ="
 test "x$shared" = xno && echo "SHARED_LIBS ="
 test "x$cc_family" = xgcc && echo 'WRAPCC_GCC = $(CC)'
+test "x$cc_family" = xclang && echo 'WRAPCC_CLANG = $(CC)'
 exec 1>&3 3>&-
 
 printf "done\n"

tools/ld.musl-clang.in

+#!/bin/sh
+cc="@CC@"
+libc_lib="@LIBDIR@"
+ldso="@LDSO@"
+cleared=
+shared=
+userlinkdir=
+userlink=
+
+for x ; do
+    test "$cleared" || set -- ; cleared=1
+
+    case "$x" in
+        -L-user-start)
+            userlinkdir=1
+            ;;
+        -L-user-end)
+            userlinkdir=
+            ;;
+        -L*)
+            test "$userlinkdir" && set -- "$@" "$x"
+            ;;
+        -l-user-start)
+            userlink=1
+            ;;
+        -l-user-end)
+            userlink=
+            ;;
+        crtbegin*.o|crtend*.o)
+            set -- "$@" $($cc -print-file-name=$x)
+            ;;
+        -lgcc|-lgcc_eh)
+            file=lib${x#-l}.a
+            set -- "$@" $($cc -print-file-name=$file)
+            ;;
+        -l*)
+            test "$userlink" && set -- "$@" "$x"
+            ;;
+        -shared)
+            shared=1
+            set -- "$@" -shared
+            ;;
+        -sysroot=*|--sysroot=*)
+            ;;
+        *)
+            set -- "$@" "$x"
+            ;;
+    esac
+done
+
+exec $($cc -print-prog-name=ld) -nostdlib "$@" -lc -dynamic-linker "$ldso"

tools/musl-clang.in

+#!/bin/sh
+cc="@CC@"
+libc="@PREFIX@"
+libc_inc="@INCDIR@"
+libc_lib="@LIBDIR@"
+thisdir="`cd "$(dirname "$0")"; pwd`"
+
+# prevent clang from running the linker (and erroring) on no input.
+sflags=
+eflags=
+for x ; do
+    case "$x" in
+        -l*) input=1 ;;
+        *) input= ;;
+    esac
+    if test "$input" ; then
+        sflags="-l-user-start"
+        eflags="-l-user-end"
+        break
+    fi
+done
+
+exec $cc \
+    -B"$thisdir" \
+    -fuse-ld=musl-clang \
+    -static-libgcc \
+    -nostdinc \
+    --sysroot "$libc" \
+    -isystem "$libc_inc" \
+    -L-user-start \
+    $sflags \
+    "$@" \
+    $eflags \
+    -L"$libc_lib" \
+    -L-user-end