build: per-file automatic dependencies (.deps), retire the Depend/sed machinery

Replace the monolithic per-directory `Depend` file -- generated by a separate,
serial `make depend` pass (one `gcc -MM` over all sources) and post-processed by
sed -- with automake-style per-file dependency fragments produced as a side
effect of each compile.

rules.mak's %.o rule now adds, when the compiler supports it,
  -MMD -MP -MF .deps/<stem>.d
so every object records the (non-system) headers it used.  Those fragments are
`-include`d best-effort: absent on a first build, present and correct after.
Benefits:
  * self-maintaining -- no explicit depend step; deps refresh on every compile;
  * parallel -- generated during the (already parallel) compile, no barrier;
  * -MMD already drops system headers (what the sed did) and -MP tolerates a
    removed/renamed header, so the sed post-processing is gone;
  * fixes out-of-tree incremental header tracking: with an absolute srcdir the
    old sed stripped the now-absolute *local* header paths too, silently losing
    them; the .d files keep them.

configure probes `-MMD -MP` (AUTODEP_FLAGS; empty and best-effort-skipped if the
compiler cannot).  clean/distclean remove .deps (rm -r); .gitignore replaces the
obsolete */Depend with .deps/.  `make depend` is kept as a no-op for callers that
still invoke it (e.g. the WASM build script).

Verified out-of-tree (--disable-ccache): 324 .d files, 0 Depend files, a header
touch recompiles its dependents, no-op rebuild is idle, clean clears .deps.  (The
top-level depend phase still runs here as a no-op; removed next.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -70,8 +70,8 @@ doc/latexfiles/Makefile
 *.o
 *.a
 *.so
-*/Depend
-*/Depend*.tmp
+# Per-file dependency fragments (in-tree they land in each source module dir;
+# out-of-tree they are under the build dir).  Replaces the old */Depend files.
 .deps/
 database/database.h
 # autoconf config header, generated by ./configure (in-tree location; out-of-tree

rules.mak

@@ -2,31 +2,23 @@
 
 module: lib${MODULE}.o
 
-depend: ${DEPEND_FILE}
-
-# New Depend file generating line (Tim Edwards, 1/25/06).  This gets around
-# problems with gcc.  The purpose of "make depend" is to generate a list of
-# all local dependencies, but gcc insists that anything that is in, for
-# example, the /usr/X11R6 path should also be included.  The sed scripts
-# (respectively) do:  1) remove comment lines generated by gcc, 2) remove
-# any header (.h) files with an absolute path (beginning with "/"), and
-# 3) remove isolated backslash-returns just to clean things up a bit.
-
-# The $$PPID allows this to be run on the same dir in parallel without
-# corrupting the output, the final MV is transactional. If that is needed
-# it indicates a missing dependency somewhere in a upstream/parent Makefile.
-${DEPEND_FILE}: ${DEPSRCS}
-	${CC} ${CFLAGS} ${CPPFLAGS} ${DFLAGS} ${DEPEND_FLAG} $(addprefix $(srcdir)/,${DEPSRCS}) > ${DEPEND_FILE}$$PPID.tmp
-	${SED} -e "/#/D" -e "/ \//s/ \/.*\.h//" -e "/  \\\/D" -i ${DEPEND_FILE}$$PPID.tmp
-	${MV} -f ${DEPEND_FILE}$$PPID.tmp ${DEPEND_FILE}
-
-# Original Depend file generating line:
-#	${CC} ${CFLAGS} ${CPPFLAGS} ${DFLAGS} ${DEPEND_FLAG} ${SRCS} > ${DEPEND_FILE}
+# Dependencies are now generated automatically as a side effect of each compile
+# (see the %.o rule below); "make depend" is a no-op kept only so scripts/habits
+# that still invoke it do not error.
+.PHONY: depend
+depend:
 
+# Compile a source into an object.  When the compiler supports it
+# (AUTODEP_FLAGS = "-MMD -MP"), also write .deps/<stem>.d listing the non-system
+# headers this object used -- generated as a side effect of the compile, so there
+# is no separate serial "make depend" pass and it parallelises for free.  -MMD
+# already drops system headers (the job the old sed did), and -MP adds a phony
+# target per header so a later removed/renamed header does not break the build.
 %.o: %.c
 	@echo --- compiling ${MODULE}/$*.o
+	@test -z "${AUTODEP_FLAGS}" || mkdir -p .deps
 	${RM} $*.o
-	${CC} ${CFLAGS} ${CPPFLAGS} ${DFLAGS}  -c $< -o $@
+	${CC} ${CFLAGS} ${CPPFLAGS} ${DFLAGS} ${AUTODEP_FLAGS} $(if ${AUTODEP_FLAGS},-MF .deps/$*.d) -c $< -o $@
 
 lib${MODULE}.o: ${OBJS}
 	@echo --- linking lib${MODULE}.o
@@ -53,12 +45,13 @@ ${DESTDIR}${BINDIR}/${MODULE}${EXEEXT}: ${MODULE}${EXEEXT}
 .PHONY: clean
 clean:
 	${RM} ${CLEANS}
+	${RM} -r .deps
 
 tags: ${SRCS} ${LIB_SRCS}
 	ctags $(addprefix $(srcdir)/,${SRCS} ${LIB_SRCS})
 
-# Depends are a somewhat optional part of the build process that are only useful when incremental
-# building.  If the file is here it's here, if not continue with build optimistically
-ifneq (,$(wildcard ${DEPEND_FILE}))
-include ${DEPEND_FILE}
-endif
+# Per-file dependency fragments written by the %.o rule above.  Best effort: none
+# exist on a first build (everything compiles anyway); on later builds they cause
+# a recompile when a used header changes.  `-include` tolerates their absence and
+# the `-MP` phony header targets tolerate a removed/renamed header.
+-include $(wildcard .deps/*.d)

scripts/configure

@@ -636,7 +636,7 @@ MSED
 MCPP
 LD_RUN_PATH
 SHLIB_CFLAGS
-DEPEND_FLAG
+AUTODEP_FLAGS
 gr_hprog
 gr_hsrcs
 gr_srcs
@@ -6349,7 +6349,7 @@ extra_defs="$extra_defs -DCAD_DIR=\\\"\${LIBDIR}\\\" -DBIN_DIR=\\\"\${BINDIR}\\\
 X_LIBS=
 X_CFLAGS=
 INC_SPECS=
-DEPEND_FLAG=
+AUTODEP_FLAGS=
 SHLIB_CFLAGS=""
 LD_RUN_PATH=""
 WISH_EXE=""
@@ -9435,9 +9435,36 @@ fi
 
 fi
 
-if test "$GCC" = "yes" ; then
-   DEPEND_FLAG="-MM"
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether $CC generates dependencies with -MMD -MP" >&5
+$as_echo_n "checking whether $CC generates dependencies with -MMD -MP... " >&6; }
+magic_save_cflags="$CFLAGS"
+CFLAGS="$CFLAGS -MMD -MP -MF conftest.autodep.d"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  if test -f conftest.autodep.d ; then
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }; AUTODEP_FLAGS="-MMD -MP"
+     else
+        { $as_echo "$as_me:${as_lineno-$LINENO}: result: no (produced no .d)" >&5
+$as_echo "no (produced no .d)" >&6; }
+     fi
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
 fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+rm -f conftest.autodep.d
+CFLAGS="$magic_save_cflags"
 
 
 

scripts/configure.in

@@ -370,7 +370,7 @@ extra_defs="$extra_defs -DCAD_DIR=\\\"\${LIBDIR}\\\" -DBIN_DIR=\\\"\${BINDIR}\\\
 X_LIBS=
 X_CFLAGS=
 INC_SPECS=
-DEPEND_FLAG=
+AUTODEP_FLAGS=
 SHLIB_CFLAGS=""
 LD_RUN_PATH=""
 WISH_EXE=""
@@ -1987,9 +1987,25 @@ if test $usingTcl ; then
 
 fi
 
-if test "$GCC" = "yes" ; then
-   DEPEND_FLAG="-MM"
-fi
+dnl Automatic per-file dependency generation.  Compilers that support -MMD -MP
+dnl write a .deps/<stem>.d listing the (non-system) headers a source used, as a
+dnl side effect of compiling it -- replacing the old serial "make depend" pass
+dnl and its sed post-processing, and parallelising with the compile.  Best
+dnl effort: empty if unsupported, and the build proceeds without incremental
+dnl header tracking.  Probed (not assumed from $GCC) so clang and others are
+dnl covered only when they actually produce the file.
+AC_MSG_CHECKING([whether $CC generates dependencies with -MMD -MP])
+magic_save_cflags="$CFLAGS"
+CFLAGS="$CFLAGS -MMD -MP -MF conftest.autodep.d"
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [])],
+    [if test -f conftest.autodep.d ; then
+        AC_MSG_RESULT([yes]); AUTODEP_FLAGS="-MMD -MP"
+     else
+        AC_MSG_RESULT([no (produced no .d)])
+     fi],
+    [AC_MSG_RESULT([no])])
+rm -f conftest.autodep.d
+CFLAGS="$magic_save_cflags"
 
 
 dnl Substitute all variables
@@ -2032,7 +2048,7 @@ AC_SUBST(CFLAGS)
 AC_SUBST(CPPFLAGS)
 AC_SUBST(CXX)
 AC_SUBST(CXXFLAGS)
-AC_SUBST(DEPEND_FLAG)
+AC_SUBST(AUTODEP_FLAGS)
 AC_SUBST(SHLIB_CFLAGS)
 AC_SUBST(LDFLAGS)
 AC_SUBST(LD_RUN_PATH)

scripts/defs.mak.in

@@ -200,8 +200,10 @@ LDFLAGS                = @LDFLAGS@
 READLINE_DEFS          = @rl_defs@
 READLINE_LIBS          = @rl_libs@
 
-DEPEND_FILE	       = Depend
-DEPEND_FLAG	       = @DEPEND_FLAG@
+# Per-file dependency flags ("-MMD -MP" when the compiler supports it, else
+# empty); used by the %.o rule in rules.mak to write .deps/<stem>.d as a compile
+# side effect.  Replaces the old serial "make depend" / Depend-file machinery.
+AUTODEP_FLAGS	       = @AUTODEP_FLAGS@
 EXEEXT		       = @EXEEXT@
 
 GR_CFLAGS              = @X_CFLAGS@ @gr_cflags@
@@ -214,12 +216,11 @@ GR_HELPER_PROG         = @gr_hprog@
 OA		       = @OA@
 OA_LIBS		       = @OA_LIBS@
 
-DEPSRCS	  = ${SRCS}
 C_OBJS    = ${SRCS:.c=.o}
 CXX_OBJS  = ${CXXSRCS:.cpp=.o}
 OBJS      = ${CXX_OBJS} ${C_OBJS}
 LIB_OBJS  = ${LIB_SRCS:.c=.o}
-CLEANS    = Depend ${OBJS} ${LIB_OBJS} lib${MODULE}.a lib${MODULE}.o ${MODULE}
+CLEANS    = ${OBJS} ${LIB_OBJS} lib${MODULE}.a lib${MODULE}.o ${MODULE}
 
 # allows Makefile to be selective
 MAKE_TCL           = @MAKE_TCL@