Debugging, optimization and profiling - which happens at which stage(s) (compilation and link)

[Tim Prince]

Tim wrote:
> Hi,
> 
> In most Makefiles I have seen, CXXFLAGS is used in the compile rule, either implicitly or explicitly. A few days ago, however I saw a Makefile that uses CXXFLAGS in link stage. Its compile rule is implicit which I suppose to use CXXFLAGS as well. The Makefile is as follows:
> 
> ifeq ($(STATIC),yes)
>   LDFLAGS=-static -lm -ljpeg -lpng -lz
> else
>   LDFLAGS=-lm -ljpeg -lpng
> endif
> 
> ifeq ($(DEBUG),yes)
>   OPTIMIZE_FLAG = -ggdb3 -DDEBUG -fno-omit-frame-pointer
> else
>   OPTIMIZE_FLAG = -ggdb3 -O3
> endif
> 
> ifeq ($(PROFILE),yes)
>   PROFILE_FLAG = -pg
> endif
> 
> CXXFLAGS = -Wall $(OPTIMIZE_FLAG) $(PROFILE_FLAG) $(CXXGLPK)
> 
> all: test
> 
> test: test.o rgb_image.o 
>     $(CXX) $(CXXFLAGS) -o $@ $^ $(LDFLAGS)
> 
> Makefile.depend: *.h *.cc Makefile

> 
> Does it mean that the optimization, debug options or/and profiling options are necessary in both compile and link stages? So is it better to use CXXFLAGS in link stage too?
> 

When $(CXX) is used to drive the link step, it's entirely appropriate to 
pass it the same flags used at compile time, even though many of them 
will be ignored.  In this case, the flag -pg must be consistent between 
compile and link steps.
I believe most C++ compilers implicitly include -lm, so that is 
redundant yere.