Re: [Xen-devel] [PATCH for-4.7] docs/build: Work around apparent bug with multi-target rules

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/04/16 14:50, Wei Liu wrote:
> On Mon, Apr 25, 2016 at 02:35:56PM +0100, Ian Jackson wrote:
>> Andrew Cooper writes ("[PATCH for-4.7] docs/build: Work around apparent bug 
>> with multi-target rules"):
>>> The `make` manual documents that a rule of the form
>>>
>>>   target1 target2: prereq
>>>     recipe
>>>
>>> is equivilent to
>>>
>>>   target1: prereq
>>>     recipe
>>>   target2: prereq
>>>     recipe
>>>
>>> This is correct if only target1 or target2 is wanted, but is not the
>>> case if both target1 and target2 are wanted to be rebuilt in the
>>> same pass.  In such a case, executing the recipe to generate target1
>>> causes the entire rule to be considered complete, short circuiting
>>> the search to regenerate target2.
>> This is not a bug in make.  From the manual I have here (wheezy):
>>
>>      Suppose you would like to vary the prerequisites according to the
>>    target, much as the variable `$@' allows you to vary the commands.
>>    You cannot do this with multiple targets in an ordinary rule, but
>>    you can do it with a "static pattern rule".  *Note Static Pattern
>>    Rules: Static Pattern.
>>
>> and (from `Pattern Intro'):
>>
>>      Pattern rules may have more than one target.  Unlike normal
>>    rules, this does not act as many different rules with the same
>>    prerequisites and commands.  If a pattern rule has multiple
>>    targets, `make' knows that the rule's commands are responsible for
>>    making all of the targets.  The commands are executed only once to
>>    make all the targets.
>>
>> So this is a bug in the Makefile.  Your patch looks like a right
>> approach to me.  A static pattern rule would be the other option.
>>
>> Acked-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
>>
> Release-acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
>
> Thank you both for handling this.
>
> Andrew, I think you need to resubmit with updated subject line and
> commit message.

I will adjust and roll it into a short series with the cpuid feature
doc.  (Guess what I was doing to find this issue...)

~Andrew

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