Re: [Xen-devel] [PATCH] ARM: refine compiler target architecture

[Andre Przywara <andre.przywara@xxxxxxxxxx>]

On 12/18/2013 09:29 AM, Ian Campbell wrote:
> On Wed, 2013-12-18 at 08:48 +0100, Andre Przywara wrote:
> > But actually the ABI is orthogonal to the -march setting, right? AFAIK
> > -march just controls which instruction the compiler emits and which
> > instructions the assembler allows.
> I thought they were subtly intertwined, even if technically they are
> orthogonal.
eek.

> > $ arm-linux-gnueabi-gcc -v
> > Target: arm-linux-gnueabi
> > ... --with-arch=armv4t ...
> >
> > This is pretty conservative, but the compiler is fine and with
> > -march=armv7-a -mfloat-abi=hard it even generates modern code.
> I think we can't force float-abi=hard unless the rest of the libraries
> on the system use it? Which won't be the case on armel.
Right. Well, we can force it, but it will not work ;-) Not sure if it 
even links.
> > Are there really people out there still using OABI?
> I thought the Debian armel port was OABI, it seems I was mistaken,
> sorry. From the native Debian armel compiler:
>
> $ gcc -v
> [...]
> Target: arm-linux-gnueabi
> Configured with: [...]
>   --with-arch=armv4t --with-float=soft --enable-checking=release
> --build=arm-linux-gnueabi --host=arm-linux-gnueabi
> --target=arm-linux-gnueabi
> [...]
> gcc version 4.8.2 (Debian 4.8.2-7)
>
> https://wiki.debian.org/ArmEabiPort confirms that I was wrong.
>
> The armhf port is EABI and v7:
> $ gcc -v
> [...]
> Target: arm-linux-gnueabihf
> Configured with: [...]
>   --with-arch=armv7-a --with-fpu=vfpv3-d16 --with-float=hard
> --with-mode=thumb --enable-checking=release --build=arm-linux-gnueabihf
> --host=arm-linux-gnueabihf --target=arm-linux-gnueabihf
> [...]
> gcc version 4.6.3 (Debian 4.6.3-14)
>
> > Do we care?
> I care about people not getting confused into think that things will
> work on such systems when it will not. The bugs resulting from an OABI
> vs EABI mismatch will likely be quite subtle.
>
> I guess this means the original Debian arm (not -el nor -hf) port which
> I suppose people are less likely to have these days.
Right, this matches my quick research. armel should work, arm not.

> > Both compilers I tested (Slackware and Debian cross) are purely EABI,
> > but have less than armv7 as their default.
> I wonder if we can either add some option which will force EABI and
> therefore fail to build on an OABI system (maybe march v7 is that test
> given the age of OABI?) or add a configure check for struct alignment?
Right, that sounds like a good thing. If I can find some time still this 
week, I can try to come up with something.
> In any case looking back to the original patch I am now convinced that
> it would be OK. I'm not sure about for 4.4 though, I'm a bit wary of
> changing compiler defaults at this stage.
I agree. Lets push it out for 4.5.

I can write an email with the error message I got, the explanation and 
the fix to the list to help people with this problem meanwhile. Google 
should pick it up from the archives then ;-)
Regards,
Andre.


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