[LLVMdev] "-march" trashing ARM triple

[David Meyer]

Jim,

There's a comment in llc.cpp:

  // Allocate target machine.  First, check whether the user has explicitly
  // specified an architecture to compile for. If so we have to look it up
by
  // name, because it might be a backend that has no mapping to a target
triple.
  const Target *TheTarget = 0;
  if (!MArch.empty()) {

It explicitly uses MArch over the triple for the Target lookup.

Daniel, it looks like you wrote this comment. Could you explain what you
mean by a backend with no mapping to a target triple?

Thanks,
- pdox

On Fri, Mar 2, 2012 at 9:11 AM, Jim Grosbach <grosbach at apple.com> wrote:

>
> On Mar 2, 2012, at 12:04 AM, David Meyer <pdox at google.com> wrote:
>
> > ARM subtarget features are determined by parsing the target tuple string
> TT. (ParseARMTriple(StringRef TT) in ARMMCTargetDesc.cpp)
> >
> > In llc, the -march setting overrides the architecture specified in
> -mtriple. So when you invoke:
> >
> > $ llc -march arm -mtriple armv7-none-linux ...
> >
> > ParseARMTriple() will see TT == "arm-none-linux" instead of
> "armv7-none-linux". As a result, the target features will be set
> generically. (Note that using "-march armv7" is not valid.)
> >
> > This is clearly wrong, but I'm not clear on where/how this should be
> fixed. Does the -march substitution need to happen at all? Could it be
> disabled only for ARM? Should TargetTriple or -march be made more precise?
> >
>
> When using a triple, -march doesn't add any additional information. The
> idea is that -march is a shorthand for a generic triple (e.g., -march=arm
> implies -mtriple=arm-unknown-unknown or something similar).
>
> It seems to me that using both on the llc command line should issue a
> diagnostic.
>
> -Jim
>
> > Thanks,
> > - pdox
> >
> > _______________________________________________
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>
>
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