Bug 670659 - Detect GNU ld bug with debugging symbols when using --gc-sections and don't use it then. r=khuey

configure.in

@@ -1549,10 +1549,8 @@ if test "$GNU_CC"; then
     MKCSHLIB='$(CC) $(CFLAGS) $(DSO_PIC_CFLAGS) $(DSO_LDOPTS) -Wl,-h,$@ -o $@'
     DSO_LDOPTS='-shared'
     if test "$GCC_USE_GNU_LD"; then
-        # Don't allow undefined symbols in libraries, and remove dead symbols
-        DSO_LDOPTS="$DSO_LDOPTS -Wl,-z,defs -Wl,--gc-sections"
-        CFLAGS="$CFLAGS -ffunction-sections -fdata-sections"
-        CXXFLAGS="$CXXFLAGS -ffunction-sections -fdata-sections"
+        # Don't allow undefined symbols in libraries
+        DSO_LDOPTS="$DSO_LDOPTS -Wl,-z,defs"
     fi
     WARNINGS_AS_ERRORS='-Werror'
     DSO_CFLAGS=''
@@ -7298,6 +7296,47 @@ if test -n "$MOZ_DEBUG" -o -n "$MOZ_DEBUG_SYMBOLS"; then
     export MOZ_DEBUG_SYMBOLS
 fi
 
+dnl ========================================================
+dnl = Automatically remove dead symbols
+dnl ========================================================
+
+if test "$GNU_CC" -a "$GCC_USE_GNU_LD" -a -n "$MOZ_DEBUG_FLAGS"; then
+   dnl See bug 670659
+   AC_CACHE_CHECK([whether removing dead symbols breaks debugging],
+       GC_SECTIONS_BREAKS_DEBUG_RANGES,
+       [echo 'int foo() {return 42;}' \
+             'int bar() {return 1;}' \
+             'int main() {return foo();}' > conftest.${ac_ext}
+        if AC_TRY_COMMAND([${CC-cc} -o conftest.${ac_objext} $CFLAGS $MOZ_DEBUG_FLAGS -ffunction-sections conftest.${ac_ext} 1>&2]) &&
+           AC_TRY_COMMAND([${CC-cc} -o conftest${ac_exeext} $LDFLAGS $MOZ_DEBUG_FLAGS -ffunction-sections -Wl,--gc-sections conftest.${ac_ext} $LIBS 1>&2]) &&
+           test -s conftest${ac_exeext} -a -s conftest.${ac_objext}; then
+            dnl objdump -WR would be much simpler, but ancient objdump
+            dnl doesn't support it.
+            debug_ranges_lines() {
+                awk '/^Contents/ { content=0 } /^Contents of the \.debug_ranges/ { content=1 } content { n += 1 } END { print n }'
+            }
+            dnl We expect the number of lines after "Contents of the
+            dnl .debug_ranges" to be the same in the object file and the
+            dnl linked program. If it's not, then we are hitting the linker
+            dnl bug.
+            if test `objdump -W conftest.${ac_objext} 2> /dev/null | debug_ranges_lines` = \
+                    `objdump -W conftest${ac_exeext} 2> /dev/null | debug_ranges_lines`; then
+                GC_SECTIONS_BREAKS_DEBUG_RANGES=no
+            else
+                GC_SECTIONS_BREAKS_DEBUG_RANGES=yes
+            fi
+        else
+             dnl We really don't expect to get here, but just in case
+             AC_ERROR([couldn't compile a simple C file])
+        fi
+        rm -rf conftest*])
+    if test "$GC_SECTIONS_BREAKS_DEBUG_RANGES" = no; then
+        DSO_LDOPTS="$DSO_LDOPTS -Wl,--gc-sections"
+        CFLAGS="$CFLAGS -ffunction-sections -fdata-sections"
+        CXXFLAGS="$CXXFLAGS -ffunction-sections -fdata-sections"
+    fi
+fi
+
 dnl ========================================================
 dnl = Disable any treating of compile warnings as errors
 dnl ========================================================