Re: GCC Optimization Levels - Seeking Insights

[Aran Nokan <nokanaran@gmail.com>]

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Hi,

Thank you. I noticed that it is not working with the following error:

No rule to make target 'test.0',


So I changed it to this:

$(PROGS): test.% : $(SRC) $(DEPS)
>         $(CC) -o $@ $(CFLAGS) $(LDLAGS) -O$* $(SRC)


But now another problem:

make: *** No rule to make target 'levmarg.c', needed by 'test.0'.  Stop.
>

Do you have any idea how to fix it?

Sincerely,
Aran

On Fri, Dec 22, 2023 at 10:26 AM David Brown via Gcc-help <
gcc-help@gcc.gnu.org> wrote:

> On 21/12/2023 20:57, Jonathan Wakely via Gcc-help wrote:
> > On Thu, 21 Dec 2023, 19:33 Aran Nokan via Gcc-help, <
> gcc-help@gcc.gnu.org>
> > wrote:
> >
> >> Hello GCC Community,
> >>
> >> I recently conducted an experiment where I tested the impact of
> different
> >> GCC optimization levels on the performance of a code
> >> <https://github.com/leechwort/levenberg-maquardt-example>. I observed
> that
> >> higher optimization levels didn't necessarily result in faster code
> >> execution.
> >>
> >> Is it correct or I have made a mistake? Do we have any other parameters
> for
> >> optimization?
> >>
> >
> > There is no guarantee that optimization make code faster, except that -O0
> > will invariably be slower.
> >
> >
> > However, I suspect the problem is that your makefile only builds the .o
> > objects once, for the first target that needs them. That means they will
> be
> > optimized (or not optimized) according to the CFLAGS set when building
> that
> > first target.
> >
> > Unless you do 'make clean' before building each target, you weren't
> > actually testing what you intend to test.
> >
> >
>
> For this test, since there are not many source files, I'd put them all
> on the same gcc line :
>
>
> default: all
> .PHONY: all
>
>
> OPTS = 0 1 2 3 g fast
>
> PROGS = $(foreach opt, $(OPTS), test.$(opt))
> all: $(PROGS)
>
> CC = gcc
> CFLAGS_COMMON = -I.
> LDLAGS = -lm
> DEPS = levmarq.h makefile
> SRC = main.c levmarg.c
>
> test.% : $(SRC) $(DEPS)
>         gcc -o $@ $(CFLAGS) $(LDLAGS) -O$* $(SRC)
>
> clean:
>         rm -f $(PROGS)
>
>
>
> (I haven't tried the makefile at all, but it might be a starting point
> for the OP.)
>
>
> >>
> >> My make file was as follows:
> >>
> >>
> >> CC=gcc
> >>> CFLAGS_COMMON=-I.
> >>> LDLAGS=-lm
> >>> DEPS = levmarq.h
> >>> OBJ = main.o levmarq.o
> >>>
> >>> %.o: %.c $(DEPS)
> >>>          $(CC) -c -o $@ $< $(CFLAGS) $(LDLAGS)
> >>>
> >>> main: $(OBJ)
> >>>          gcc -o $@ $^ $(CFLAGS) $(LDLAGS)
> >>>
> >>> # No optimization
> >>> main_no_opt: CFLAGS += -O0
> >>> main_no_opt: $(OBJ)
> >>>          gcc -o $@ $^ $(CFLAGS) $(LDLAGS)
> >>>
> >>> # Basic optimization
> >>> main_opt1: CFLAGS += -O1
> >>> main_opt1: $(OBJ)
> >>>          gcc -o $@ $^ $(CFLAGS) $(LDLAGS)
> >>>
> >>> # Moderate optimization
> >>> main_opt2: CFLAGS += -O2
> >>> main_opt2: $(OBJ)
> >>>          gcc -o $@ $^ $(CFLAGS) $(LDLAGS)
> >>>
> >>> # High optimization
> >>> main_opt3: CFLAGS += -O3
> >>> main_opt3: $(OBJ)
> >>>          gcc -o $@ $^ $(CFLAGS) $(LDLAGS)
> >>>
> >>> # Clean rule
> >>> clean:
> >>>          rm -f *.o main main_no_opt main_opt2 main_opt3
> >>>
> >>
> >> Best regards,
> >> Aran
> >>
> >
>
>
>