STABLE clang planned update MFC path (3.4.1 STABLE, 3.7.0, CURRENT) vs. powerpc64

[Mark Millard]

I had written (2015-Oct-11):

> I'm not sure about going all the way back to FreeBSD 9 but this suggests that clang was for some time --and recently has been-- insufficient on its own for reliable(?) powerpc64 builds (2015-Feb-05). It may be best to consider powerpc64 omitted from the "clang 3.4.1 and libc++" list in that last paragraph given the "upgrade easily" context intended.

But looking at stable/9 I see that -mlongcall is not in use in CFLAGS.

I would guess that stable/9 depended on code happening to be close enough together and it failed in other contexts. ("The" problems seem to move around.) I'll note that the 2012-Mar-13 stable/10 addition of -mlongcall was explained by (nwhitehorn's -r232932):

> Adding -mlongcall

> to crt1 flags causes the compiler to emit explicit TOC load instructions

> for all function calls, including _fini().


The reason for this being important was listed as:

> Work around a binutils bug on powerpc64 where the TOC would not be

> properly reloaded when calling _fini() in large binaries with multiple

> TOC sections (e.g. GCC), leading to a segmentation fault.


I'm still not sure that the stable/9 to stable/10 update would be reliably clean for powerpc64, despite -mlongcall not being used at that stage.

===
Mark Millard
markmi at dsl-only.net

On 2015-Oct-11, at 8:05 PM, Mark Millard <markmi at dsl-only.net> wrote:

> On 2014-Oct-11 Dimitry Andric wrote:
> 
>> On 11 Oct 2015, at 14:05, Piotr Kubaj <pkubaj at riseup.net
>>> wrote:
>>> 
>>> AFAIK if there had been such plans, they were dropped long ago. The
>>> reasoning it can't be done (at least for now) is that versions 3.5.0+
>>> require C++11-capable stack and that would break upgrades from 9-STABLE
>>> (if the user still uses GCC, as is by default). So, LLVM in stable/10
>>> will probably be upgraded when stable/9 goes EOL.
>> 
>> 
>> If stable/10 had clang 3.5 or higher, you could still upgrade from
>> stable/9.  It would only require you to do the upgrade in two steps:
>> 
>> * Rebuild and reinstall your stable/9 world using WITH_CLANG,
>>  WITH_CLANG_IS_CC, and WITH_LIBCPLUSPLUS.  This will install clang
>>  3.4.1 and libc++, and make clang the default compiler.
>> * Checkout stable/10 (or even head), and build/install it in the regular
>>  fashion.
>> 
>> I am personally not against merging newer llvm/clang versions into
>> stable/10.  But the "silent agreement" has always been that you could
>> upgrade easily from the latest stable/X to stable/X+1, and the above
>> two-step process breaks that, or at least makes it more complicated.
>> 
>> Last but not least, note that this would only apply to the architectures
>> that *can* actually build clang 3.4.1 and libc++ on stable/9.  This is
>> currently limited to x86, little-endian arm and powerpc (64 bit, I'm
>> unsure about 32 bit).
>> 
>> -Dimitry
> 
> lib/csu/powerpc64/Makefile in head has updates and comments (2015-Feb-05 or so) about "powerpc64 csu needs to be built by gcc, so enforce that". It is tied to clang not supporting -mlongcall and "testing shows a clang linked with a [#] clang-built csu segfaults". The forcing of gcc use in head looks like:
> 
> CC:=            gcc
> COMPILER_TYPE:= gcc
> 
> which is not in stable/10's variant.
> 
> stable/10 has a lib/csu/powerpc64/Makefile that does not force gcc but still has:
> 
> CFLAGS+=        -I${.CURDIR}/../common \
>                -I${.CURDIR}/../../libc/include \
>                -mlongcall
> 
> and so has -mlongcall in use on the command lines. Unless -mlongcall support used to be in place for clang and was later removed, a rebuilding of FreeBSD 9 or 10 that includes a lib/csu/powerpc64/ rebuild likely fails to build under WITH_CLANG_IS_CC.
> 
> I'm not sure about going all the way back to FreeBSD 9 but this suggests that clang was for some time --and recently has been-- insufficient on its own for reliable(?) powerpc64 builds (2015-Feb-05). It may be best to consider powerpc64 omitted from the "clang 3.4.1 and libc++" list in that last paragraph given the "upgrade easily" context intended.
> 
> (If there is an easy powerpc64 upgrade then I'd like to see notes about it: Other contexts might be able to use similar techniques. I started my explorations with 10.)
> 
> ===
> Mark Millard
> markmi at dsl-only.net