Fix 59828 - Broken assembly on ppc* with two -mcpu= options

[Alan Modra]

On Tue, Jan 20, 2015 at 09:26:12AM -0500, David Edelsohn wrote:
> On Tue, Jan 20, 2015 at 12:41 AM, Alan Modra <amodra@gmail.com> wrote:
> > On Mon, Jan 19, 2015 at 10:43:29PM -0500, David Edelsohn wrote:
> >> On Fri, Jan 17, 2014 at 10:58 PM, Alan Modra <amodra@gmail.com> wrote:
> >> > This patch cures PR59828 by translating all the -mcpu options at once,
> >> > in order, to their equivalent assembler -m options by using a new spec
> >> > function.  In the process this removes some duplication.
> >>
> >> ASM_CPU_SPEC is too fragile a mechanism.  I would much prefer to
> >> expand on the ".machine" directive that I added to
> >> rs6000_file_start().  The initial implementation explicitly avoids
> >> .machine when -mcpu= or --with-cpu= is present as a conservative
> >> start.
> >>
> >> It seems much better to select a .machine directive based on the
> >> actual target ISA flag bits enabled than translating CPU command line
> >> options to ASM options.  Patches to replace ASM_CPU_SPEC with .machine
> >> and expand functionality for AIX are welcome.
> >
> > This might make sense when looking only at gcc, but when considering
> > the whole toolchain I think you'll run into difficulty.  gas and other
> > powerpc assemblers have always been invoked with -m options to select
> > the cpu, so if you do away with ASM_CPU_SPEC and rely on .machine then
> > you will be exercising the assembler in a new way.  I am sure that
> > this will not work for all powerpc assemblers currently in use.
> 
> It is stressing .machine more than that feature has been in the past,
> but it is functionality that is suppose to work.  .machine already has
> been stressed more with IFUNC pushing and popping ISAs.
> 
> Are you concerned about the fundamental functionality of the pseudo-op
> or a particular GAS release missing support for a particular ISA?  AIX
> supports .machine, but I think that it expects slightly different
> processor names.  I am not certain about LLVM-AS, but it normally is
> not fed external assembly language files.

I'm concerned about initialisation that might happen based on a -m
option that doesn't happen with .machine.  A quick look over the
gas source showed we currently have at least one case like this:
ppc_dwarf2_line_min_insn_length is only set from command line -m
options.  This means that if you want debug with VLE insns, you cannot
currently invoke gas without -mvle.  ".machine vle" by itself won't
work.  I think there may be other similar problems with setting the
bfd arch/mach pair.

-- 
Alan Modra
Australia Development Lab, IBM