x86/doc/Makefile: DBG=1 to preprocess external asm

The macro hell sometimes make it difficult to trace the source of
an error, so it is easier to analyze the preprocessed output.

This patch makes this automatical by specifying DBG=1 on the
command line: a file ffmpeg/dir/file.asm gets preprocessed to
builddir/dir/file.dbg.asm, which is then compiled.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

common.mak

@@ -5,6 +5,14 @@
 # first so "all" becomes default target
 all: all-yes
 
+DEFAULT_YASMD=.dbg
+
+ifndef DBG
+YASMD=
+else
+YASMD=$(DEFAULT_YASMD)
+endif
+
 ifndef SUBDIR
 
 ifndef V
@@ -138,7 +146,7 @@ $(TOOLOBJS): | tools
 
 OBJDIRS := $(OBJDIRS) $(dir $(OBJS) $(HOBJS) $(HOSTOBJS) $(SLIBOBJS) $(TESTOBJS))
 
-CLEANSUFFIXES     = *.d *.o *~ *.h.c *.map *.ver *.ho *.gcno *.gcda
+CLEANSUFFIXES     = *.d *.o *~ *.h.c *.map *.ver *.ho *.gcno *.gcda *$(DEFAULT_YASMD).asm
 DISTCLEANSUFFIXES = *.pc
 LIBSUFFIXES       = *.a *.lib *.so *.so.* *.dylib *.dll *.def *.dll.a
 

doc/build_system.txt

@@ -7,6 +7,11 @@ V
     Disable the default terse mode, the full command issued by make and its
     output will be shown on the screen.
 
+DBG
+    Preprocess x86 external assembler files to a .dbg.asm file in the object
+    directory, which then gets compiled. Helps developping those assembler
+    files.
+
 DESTDIR
     Destination directory for the install targets, useful to prepare packages
     or install FFmpeg in cross-environments.

doc/optimization.txt

@@ -191,6 +191,11 @@ __asm__() block.
 Use external asm (nasm/yasm) or inline asm (__asm__()), do not use intrinsics.
 The latter requires a good optimizing compiler which gcc is not.
 
+When debugging a x86 external asm compilation issue, if lost in the macro
+expansions, add DBG=1 to your make command-line: the input file will be
+preprocessed, stripped of the debug/empty lines, then compiled, showing the
+actual lines causing issues.
+
 Inline asm vs. external asm
 ---------------------------
 Both inline asm (__asm__("..") in a .c file, handled by a compiler such as gcc)

library.mak

@@ -22,7 +22,11 @@ $(SUBDIR)%-test.i: $(SUBDIR)%-test.c
 $(SUBDIR)%-test.i: $(SUBDIR)%.c
 	$(CC) $(CCFLAGS) $(CC_E) $<
 
-$(SUBDIR)x86/%.o: $(SUBDIR)x86/%.asm
+$(SUBDIR)x86/%$(DEFAULT_YASMD).asm: $(SUBDIR)x86/%.asm
+	$(DEPYASM) $(YASMFLAGS) -I $(<D)/ -M -o $@ $< > $(@:.asm=.d)
+	$(YASM) $(YASMFLAGS) -I $(<D)/ -e $< | sed '/^%/d;/^$$/d;' > $@
+
+$(SUBDIR)x86/%.o: $(SUBDIR)x86/%$(YASMD).asm
 	$(DEPYASM) $(YASMFLAGS) -I $(<D)/ -M -o $@ $< > $(@:.o=.d)
 	$(YASM) $(YASMFLAGS) -I $(<D)/ -o $@ $<
 	-$(if $(ASMSTRIPFLAGS), $(STRIP) $(ASMSTRIPFLAGS) $@)