makefiles - compile all c files at once

I want to experiment with GCC whole program optimizations. To do so I have to pass all C-files at once to the compiler frontend. However, I use makefiles to automate my build process, and I'm not an expert when it comes to makefile magic.

How should I modify the makefile if I want to compile (maybe even link) using just one call to GCC?

For reference - my makefile looks like this:

LIBS  = -lkernel32 -luser32 -lgdi32 -lopengl32
CFLAGS = -Wall

OBJ = 64bitmath.o    \
      monotone.o     \
      node_sort.o    \
      planesweep.o   \
      triangulate.o  \
      prim_combine.o \
      welding.o      \
      test.o         \
      main.o
      
%.o : %.c
    gcc -c $(CFLAGS) $< -o $@

test: $(OBJ)
    gcc -o $@ $^ $(CFLAGS) $(LIBS)

LIBS  = -lkernel32 -luser32 -lgdi32 -lopengl32
CFLAGS = -Wall

# Should be equivalent to your list of C files, if you don't build selectively
SRC=$(wildcard *.c)

test: $(SRC)
    gcc -o $@ $^ $(CFLAGS) $(LIBS)

SRCS=$(wildcard *.c)

OBJS=$(SRCS:.c=.o)

all: $(OBJS)

You need to take out your suffix rule (%.o: %.c) in favour of a big-bang rule. Something like this:

LIBS  = -lkernel32 -luser32 -lgdi32 -lopengl32
CFLAGS = -Wall

OBJ = 64bitmath.o    \
      monotone.o     \
      node_sort.o    \
      planesweep.o   \
      triangulate.o  \
      prim_combine.o \
      welding.o      \
      test.o         \
      main.o

SRCS = $(OBJ:%.o=%.c)

test: $(SRCS)
    gcc -o $@  $(CFLAGS) $(LIBS) $(SRCS)

If you're going to experiment with GCC's whole-program optimization, make sure that you add the appropriate flag to CFLAGS, above.

On reading through the docs for those flags, I see notes about link-time optimization as well; you should investigate those too.