Makefile now automatically detects modifications of compilation flags,
and produce object files in directories dedicated to this compilation flags.
This makes it possible, for example, to compile libzstd with different DEBUGLEVEL.
Object files sharing the same configration will be generated into their dedicated directories.
Also : new compilation variables
- DEBUGLEVEL : select the debug level (assert & traces) inserted during compilation (default == 0 == release)
- HASH : select a hash function to differentiate configuration (default == md5sum)
- BUILD_DIR : skip the hash stage, store object files into manually specified directory
%.o object files generated for dynamic library
must be different from those generated for static library.
Due to this difference, %.o were so far only generated for the static library.
The dynamic library was rebuilt from %.c source.
This meant that, for every minor change, the entire dynamic library had to be rebuilt.
This is fixed in this PR :
only the modified %.c source get rebuilt.
That was a subtle one :
VPATH is affecting search for both %.c source and %.o object files.
This meant that, when an object file already exists in lib/,
it's used in programs/,
even though programs/ is supposed to generate its own %.o object files.
With the new vpath directive, this is no longer the case :
the search is only activated for %.c source files.
Now, local programs/%.o are always generated
even if equivalent ones are already created in lib/.
It more clearly guarantees that lib/ and programs/ can use different compilation directives
without mixing resulting %.o object files.
Building the zstd CLI costs time.
Some part of it is incompressible, leading to substantial iteration delay when testing code modifications.
That's mainly because all source files from the library must be rebuilt from source every time.
The main reason we don't build the CLI from library object files
is that we can't just build the object directly in the lib/ directory
(which they would by default)
since they use different compilation flags.
Specifically, the CLI enables multithreading, while the library doesn't (by default).
This is solved in this commit, by generating the object files locally.
Now, the CLI and the library can employ different sets of flags, without tripping over each other.
All library object files are generated directly into programs/ dir.
This works because no 2 source files have the same name.
Now, modifying a file doesn't require to recompile the entire lib, just the modified files.
The recipe is also compatible with `-j` parallel build, leading to large build time reductions on multi-core systems.
previous recipe would build object files directly within programs/
which could be in competition with other local builds happening in programs/ at the same time.
fixed by generating the relevant object file locally.
