Re: [XEN PATCH v8 01/47] build: factorise generation of the linker scripts

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Tue, Dec 14, 2021 at 04:54:21PM +0000, Julien Grall wrote:
> On 25/11/2021 13:39, Anthony PERARD wrote:
> > diff --git a/xen/Rules.mk b/xen/Rules.mk
> > +# Linker scripts, .lds.S -> .lds
> > +quiet_cmd_cpp_lds_S = LDS     $@
> > +cmd_cpp_lds_S = $(CPP) -P $(call cpp_flags,$(a_flags)) -D__LINKER__ -MQ $@ 
> > -o $@ $<
> > +

> > diff --git a/xen/arch/arm/Makefile b/xen/arch/arm/Makefile
> > +xen.lds: xen.lds.S FORCE
> > +   $(call if_changed,cpp_lds_S)
> 
> Sorry, I haven't really followed the build system rework. Could you explain
> why it is necessary to add FORCE?

This new rules make use of a new macro "if_changed", and we need to
execute this new macro every time, even if the source file hasn't been
modified.

if_changed, in addition to checking if the source is newer than the
target that make does, also compare the command line used to generate
the target. If the command line have changed, the macro update the
target.

The command line to execute is stored in "cmd_$1" variables, in this case
"cmd_cpp_lds_S". The macro if_changed store the command line in the file
".${target-name}.cmd".

So, every time the macro if_changed is used, we need to tell make to
always execute the recipe, thus FORCE.

Cheers,

-- 
Anthony PERARD