On Mon, Jun 06, 2016 at 04:32:50PM +0200, Paolo Bonzini wrote: > On 06/06/2016 16:15, Eduardo Habkost wrote: > >> > No, the second time you'll get the expansion of > >> > > >> > DOCKER_SRC_COPY := docker-src.$(CUR_TIME) > > And why it isn't enough to simply write the above line in the > > Makefile without any eval trick? > > Because 1) initially the idea was to delay the $(mkdir) too 2) Too many > $(shell) end up slowing down make.
Now I see: you wanted to delay the shell command, but evaluate it only once. But I don't see why it makes sense here, because the variable is being expanded immediately after it is defined (in the "$(DOCKER_SRC_COPY):" line), and CUR_TIME is already defined using ":=". > > >>> > > Why did you use this trick instead of just relying on ":=" to > >>> > > expand the shell command only once? > >> > > >> > I think initially the $(mkdir) was in DOCKER_SRC_COPY too. Also, too > >> > many $(shell) end up slowing down make. > > And how exactly does the eval trick helps avoiding slowing down > > make, that wouldn't happen if using just: > > DOCKER_SRC_COPY = docker-src.$(CUR_TIME) > > ? Oops, I mean: DOCKER_SRC_COPY := docker-src.$(CUR_TIME) > > If you define CUR_TIME with "=", the DOCKER_SRC_COPY value changes every > second. If you define CUR_TIME with ":=", again you have a useless > $(shell). Why a useless shell? > > In this particular case I agree that it's probably premature optimization. In this particular case it doesn't seem to be an optimization at all. -- Eduardo
