llvm dev - Mar 2014 - [LLVMdev] How to query for active subtarget features?

Hi all!

In my compiler I like to define some identifiers based on subtarget 
features. E.g. for the MIPS platform: if the user wants to compile for 
N32 (-mattr=+n32), then I want to define the identifier MIPS_N32.
This is the simple case because I can parse the command line. But if the 
user specifies nothing, then the MIPS target defaults to N64. How can I 
determine this default value?

I think the solution would be if there is a way to enumerate the 
subtarget features which are active right now. Can this be done 
currently? I found no way but I may miss something here.

Regards,
Kai

I'm not aware of a way to obtain defaults from the backend. In the case of
your example Clang currently overrides the backend default in all cases and has
its own default value. It then uses its own opinion of the ABI to define
__mips_n32, etc.

The relevant code is in tools/clang/lib/Basic/Targets.cpp:
  void getDefaultFeatures(llvm::StringMap<bool> &Features) const
override {
    // The backend enables certain ABI's by default according to the
    // architecture.
    // Disable both possible defaults so that we don't end up with multiple
    // ABI's selected and trigger an assertion.
    Features["o32"] = false;
    Features["n64"] = false;

    Features[ABI] = true;
    Features[CPU] = true;
  }
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu]
> On Behalf Of Kai Nacke
> Sent: 26 March 2014 16:45
> To: llvmdev at cs.uiuc.edu
> Subject: [LLVMdev] How to query for active subtarget features?
> 
> Hi all!
> 
> In my compiler I like to define some identifiers based on subtarget
features.
> E.g. for the MIPS platform: if the user wants to compile for
> N32 (-mattr=+n32), then I want to define the identifier MIPS_N32.
> This is the simple case because I can parse the command line. But if the
user
> specifies nothing, then the MIPS target defaults to N64. How can I
determine
> this default value?
> 
> I think the solution would be if there is a way to enumerate the subtarget
> features which are active right now. Can this be done currently? I found no
> way but I may miss something here.
> 
> Regards,
> Kai
> 
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