Re: [Xen-devel] [PATCH] xen: convert lto to Kconfig option

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Dec 05, 2016 at 03:22:30PM +0000, Wei Liu wrote:
> On Mon, Dec 05, 2016 at 08:05:35AM -0700, Jan Beulich wrote:
> > >>> On 05.12.16 at 15:39, <wei.liu2@xxxxxxxxxx> wrote:
> > > Introduce CONFIG_LTO in Kconfig. Since this is the last option to be
> > > converted to Kconfig, delete the preceding comment in Rules.mk as well.
> > > 
> > > Make it depend on BROKEN because it doesn't work at the moment.
> > 
> > Looks fine except for this one aspect: Is that brokenness with gcc,
> > clang, or both? I wouldn't be surprised if it was broken with gcc (in
> 
> It's going to be both.
> 
> Its build rune is broken.
> 
> I got:
> 
>   head.S:212:0: fatal error: opening dependency file /..d: Permission denied
> 
> That is, this rule
> 
>   CFLAGS-y += -MMD -MF $(@D)/.$(@F).d                                         
>     
> 
> is evaluated too early outside of prerequisite list, which results in
> empty string.
> 

Sorry, "evaluated too early outside ..." is non-sense. I will just copy
the manpage for automatic variables from Make:

"In particular, you cannot use them anywhere within the target list of a
rule; they have no value there and will expand to the empty string.
Also, they cannot be accessed directly within the prerequisite list of a
rule. A common mistake is attempting to use $@ within the prerequisites
list; this will not work."

The "empty string" part in my description is accurate though.

> Also I'm sure gcc can't deal with lto because at one point I tried to
> fix lto but gcc  toolchain can't cope with that.
> 
> > fact I would be surprised it has ever worked), but I would have
> > hoped that the clang variant (which iirc this was introduced for)
> > would work (also e.g. in 4.8).
> > 
> 
> I haven't tried clang, because the brokenness is somewhere else.
> 
> Wei.
> 
> > Jan
> > 

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