Re: [PATCH] build: fix dependency tracking for preprocessed files

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/06/2020 11:22, Jan Beulich wrote:
> While the issue is more general, I noticed that asm-macros.i not getting
> re-generated as needed. This was due to its .*.d file mentioning
> asm-macros.o instead of asm-macros.i. Use -MQ here as well, and while at
> it also use -MQ to avoid the somewhat fragile sed-ary on the *.lds
> dependency tracking files. While there, further avoid open-coding $(CPP)
> and drop the bogus -Ui386.

Its not bogus.  It really is needed to prevent OUTPUT_ARCH(i386:x86-64)
being preprocessed to OUTPUT_ARCH(1:x86-64)

This explodes properly with 32bit builds, but we might get away with it
now on a 64bit build (preprocessing without -m32 does appear to skip
this transformation).

However, the robust way to deal with it is:

/* Don't clobber the ld directive */
#undef i386

unconditionally in xen.lds.S

> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/Rules.mk
> +++ b/xen/Rules.mk
> @@ -201,13 +201,13 @@ $(filter %.init.o,$(obj-y) $(obj-bin-y)
>       $(call if_changed,obj_init_o)
>  
>  quiet_cmd_cpp_i_c = CPP     $@
> -cmd_cpp_i_c = $(CPP) $(filter-out -Wa$(comma)%,$(c_flags)) $< -o $@
> +cmd_cpp_i_c = $(CPP) $(filter-out -Wa$(comma)%,$(c_flags)) $< -o $@ -MQ $@

Please can -MQ come before $<, so the input and output files are still
at the end of the command.  It is a very useful property of the current
setup, when playing build system surgery.

If you're happy with both of these suggestions, Reviewed-by: Andrew
Cooper <andrew.cooper3@xxxxxxxxxx> to save another round trip.

Alternatively, I'm happy to submit the i386 as a prereq patch, seeing as
it isn't now such a trivial change any more.

~Andrew