llvm dev - May 2016 - Automake Assembler Assumptions with LLVM-MC

On Wed, May 18, 2016 at 01:10:50PM +0000, Daniel Sanders via llvm-dev
wrote:
> It's my understanding that llvm-mc is intended to be a testing tool
> for LLVM developers rather than an assembler for end users. Users
> should be assembling with clang.
Not all LLVM users are clang users.  For example, we're using LLVM to build
OpenVMS cross-compilers to x86 for our porting effort.  We have our own set
of frontends.  We're using llvm-mc as our assembler for the lower level OS
code. 

We'll eventually port clang as well to bootstrap going forward, but OpenVMS
doesn't have the same kind of driver-style so even there, I can't just
the
'clang' command to control assembly.

As for bug-for-bug compatibility in any sort of software, that's a tough
choice.
Traditionally, I've resisted such pressure but sooner or later, I've had
to give
in now and then.  As for the command line option matching, that is an area I
wouldn't attempt to match since such options vary greatly across platforms.

John

We are in a similar situation.  We technically have a pseudo-functional clang
frontend, but many of our initial developers work directly in assembly.

I was able to get our user off the ground using our version of clang.  However,
I also agree that we do a significant amount of testing/work with early
architectures using llvm-mc (especially when we’re only bouncing code off
simulators).

John D. Leidel


On May 19, 2016, at 8:33 AM, John Reagan via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> On Wed, May 18, 2016 at 01:10:50PM +0000, Daniel Sanders via llvm-dev
wrote:
>> It's my understanding that llvm-mc is intended to be a testing tool
>> for LLVM developers rather than an assembler for end users. Users
>> should be assembling with clang.
> 
> Not all LLVM users are clang users.  For example, we're using LLVM to
build
> OpenVMS cross-compilers to x86 for our porting effort.  We have our own set
> of frontends.  We're using llvm-mc as our assembler for the lower level
OS
> code. 
> 
> We'll eventually port clang as well to bootstrap going forward, but
OpenVMS
> doesn't have the same kind of driver-style so even there, I can't
just the
> 'clang' command to control assembly.
> 
> As for bug-for-bug compatibility in any sort of software, that's a
tough choice.
> Traditionally, I've resisted such pressure but sooner or later,
I've had to give
> in now and then.  As for the command line option matching, that is an area
I
> wouldn't attempt to match since such options vary greatly across
platforms.
> 
> John
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

On Thu, May 19, 2016 at 09:33:39AM -0400, John Reagan via llvm-dev
wrote:
> 
> On Wed, May 18, 2016 at 01:10:50PM +0000, Daniel Sanders via llvm-dev
wrote:
> > It's my understanding that llvm-mc is intended to be a testing
tool
> > for LLVM developers rather than an assembler for end users. Users
> > should be assembling with clang.
> 
> Not all LLVM users are clang users.  For example, we're using LLVM to
build
> OpenVMS cross-compilers to x86 for our porting effort.  We have our own set
> of frontends.  We're using llvm-mc as our assembler for the lower level
OS
> code. 
> 
Maybe I'm missing something here, but it seems like if you can run llvm-mc
you
should also be able to run clang with the -x assembler flag.  Is the problem
that you are unable to install clang on your dev system for some reason?

-Tom
> We'll eventually port clang as well to bootstrap going forward, but
OpenVMS
> doesn't have the same kind of driver-style so even there, I can't
just the
> 'clang' command to control assembly.
> 
> As for bug-for-bug compatibility in any sort of software, that's a
tough choice.
> Traditionally, I've resisted such pressure but sooner or later,
I've had to give
> in now and then.  As for the command line option matching, that is an area
I
> wouldn't attempt to match since such options vary greatly across
platforms.
> 
> John
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

> From: Tom Stellard [mailto:tom at stellard.net]
> Sent: Thursday, May 19, 2016 1:56 PM
> 
> On Thu, May 19, 2016 at 09:33:39AM -0400, John Reagan via llvm-dev
> wrote:
> >
> > On Wed, May 18, 2016 at 01:10:50PM +0000, Daniel Sanders via llvm-dev
> wrote:
> > > It's my understanding that llvm-mc is intended to be a
testing tool
> > > for LLVM developers rather than an assembler for end users. Users
> > > should be assembling with clang.
> >
> > Not all LLVM users are clang users.  For example, we're using LLVM
to
> > build OpenVMS cross-compilers to x86 for our porting effort.  We have
> > our own set of frontends.  We're using llvm-mc as our assembler
for
> > the lower level OS code.
> >
> 
> Maybe I'm missing something here, but it seems like if you can run
> llvm-mc you should also be able to run clang with the -x assembler
> flag.  Is the problem that you are unable to install clang on your dev
> system for some reason?
> 
> -Tom
> 
We haven't attempted to build clang yet as it isn't needed for our OS
bootstrap.  In addition, given the somewhat stale C++ compiler we're using
on
OpenVMS Itanium (EDG/Intel-based), we're using LLVM 3.4.2 since it
doesn't
use C++11 syntax in source code.

Even then, I had to make some changes to the LLVM source code to
get it past my C++ compiler (it has bugs with no hope of fixes).  
I suspect I'll have to do the same in the clang 3.4.2 tree.  
I just didn't bother with that yet.

I won't need clang until we have a working version of OpenVMS x86 
so I can then bootstrap my way to something newer.  Once running
natively, we'll use clang to build things going forward.

The OS team needs a cross-assembler.  So I could spend my time to get
binutils
and gas working (it has an OpenVMS target so it isn't that complicated) or I
can just use the llvm-mc that builds with the rest of the LLVM tools.  It
may not be the long-term solution once we get clang on the native systems.

Yes, the downside to the model is that there is no pre-processor that is 
automatically run over the source (either using llvm-mc or gas) so that is
often run manually by the build system.  That's what the OS team prefers
at the moment.

Sorry for missing any context. It appears the thread is forked somewhere.
> On May 19, 2016, at 6:33 AM, John Reagan via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> 
> On Wed, May 18, 2016 at 01:10:50PM +0000, Daniel Sanders via llvm-dev
wrote:
>> It's my understanding that llvm-mc is intended to be a testing tool
>> for LLVM developers rather than an assembler for end users.
This is true.
>> Users
>> should be assembling with clang.
This is mostly true, as it’s atypical to be using LLVM for an assembler and not
using clang as a C/C++ front end. It’s not, however, supposed to be a
requirement to use clang.

Creating an as(1) style user-facing driver to package up the assembler
independently of clang is a totally reasonable thing to do. That should,
however, be a separate tool from llvm-mc.

> 
> Not all LLVM users are clang users.  For example, we're using LLVM to
build
> OpenVMS cross-compilers to x86 for our porting effort.  We have our own set
> of frontends.  We're using llvm-mc as our assembler for the lower level
OS
> code. 
> 
> We'll eventually port clang as well to bootstrap going forward, but
OpenVMS
> doesn't have the same kind of driver-style so even there, I can't
just the
> 'clang' command to control assembly.
> 
> As for bug-for-bug compatibility in any sort of software, that's a
tough choice.
> Traditionally, I've resisted such pressure but sooner or later,
I've had to give
> in now and then.  As for the command line option matching, that is an area
I
> wouldn't attempt to match since such options vary greatly across
platforms.
> 
> John
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev