Re: [Xen-devel] [XEN PATCH v3 16/23] xen/build: introduce if_changed and if_changed_rule

[Jan Beulich <jbeulich@suse.com>]

On 18.03.2020 11:44, Anthony PERARD wrote:
> On Wed, Mar 04, 2020 at 04:45:36PM +0100, Jan Beulich wrote:
>> On 26.02.2020 12:33, Anthony PERARD wrote:
>>> The if_changed macro from Linux can record the command used to build a
>>> target then compare it on rebuild. Thus if a command has changed, for
>>> example due to introducing new flags in CFLAGS or due to using a
>>> different compiler, the target will be rebuilt.
>>
>> As to using a different compiler - I suppose this means "a compiler
>> with a different executable name" here? What about me having, say
>> gcc-5 in use, and then updating my system such that a 5.2 based
>> compiler of this name would be upgraded to a 5.4 based one of this
>> same name. If this newer compiler has better capabilities (that we
>> would want to use if available), would this or anything else trigger
>> a rebuild then too?
> 
> I think I should have written "command line" instead of just "command".
> When writing about "different compiler" I was mostly thinking about GCC
> vs clang, not really about versions. I think Linux has something that
> detects when the compiler version changes, but that maybe to only
> trigger kconfig, to regenerate the .config file.
> 
> But as you say, if the newer compiler has better capabilities, and the
> *FLAGS are changed, then that would trigger a rebuild if other
> dependency hasn't changed.

"Would" as in "would", or merely "it would be nice if it did"? I'm
simply not seeing where such a detection would be happening.

Jan

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