llvm dev - Jan 2013 - [LLVMdev] Getting command line options to affect subtarget features

The problem I'm trying to solve:  Invoking clang on PowerPC with
-fno-altivec has no effect.
>From what I've been able to piece together, PPC.td specifies various
CPUs and the processor features available on each.  So for example we
have:

def FeatureAltivec   :
SubtargetFeature<"altivec","HasAltivec",
"true",
                                        "Enable Altivec
instructions">;

def : Processor<"pwr7", G5Itineraries,
                  [DirectivePwr7, FeatureAltivec,
                   FeatureMFOCRF, FeatureFSqrt, FeatureSTFIWX,
                   FeatureISEL, Feature64Bit /*, Feature64BitRegs */]>;

During compilation the subtarget class (such as PPCSubtarget) is
instantiated with the strings obtained from -mcpu and -mattr.
ParseSubtargetFeatures then uses the -mcpu and -mattr values to
determine which features to use for code generation.

What seems problematic is that -mattr is the only way that the static
features of the target processor can be overridden.

It seems there must be a way for command-line parameters such as
-fno-altivec to be used to override which processor features are
exploited.  But I can't see immediately how this should work.  I don't
know whether or how the command line flags from Clang are made available
to LLVM for inspection.

I'm sure there must be an obvious way to do this, but peeking at code
from other targets hasn't enlightened me.  Any advice for how I can
accomplish this?

Thanks!  I appreciate any help.

Bill

-- 
Bill Schmidt, Ph.D.
IBM Advance Toolchain for PowerLinux
IBM Linux Technology Center
wschmidt at us.ibm.com
wschmidt at linux.vnet.ibm.com

----- Original Message -----
> From: "Bill Schmidt" <wschmidt at linux.vnet.ibm.com>
> To: llvmdev at cs.uiuc.edu
> Sent: Thursday, January 31, 2013 9:26:15 AM
> Subject: [LLVMdev] Getting command line options to affect subtarget
features
> 
> The problem I'm trying to solve:  Invoking clang on PowerPC with
> -fno-altivec has no effect.
> 
> From what I've been able to piece together, PPC.td specifies various
> CPUs and the processor features available on each.  So for example we
> have:
> 
> def FeatureAltivec   :
SubtargetFeature<"altivec","HasAltivec",
> "true",
>                                         "Enable Altivec
>                                         instructions">;
> 
> def : Processor<"pwr7", G5Itineraries,
>                   [DirectivePwr7, FeatureAltivec,
>                    FeatureMFOCRF, FeatureFSqrt, FeatureSTFIWX,
>                    FeatureISEL, Feature64Bit /*, Feature64BitRegs
>                    */]>;
> 
> During compilation the subtarget class (such as PPCSubtarget) is
> instantiated with the strings obtained from -mcpu and -mattr.
> ParseSubtargetFeatures then uses the -mcpu and -mattr values to
> determine which features to use for code generation.
> 
> What seems problematic is that -mattr is the only way that the static
> features of the target processor can be overridden.
> 
> It seems there must be a way for command-line parameters such as
> -fno-altivec to be used to override which processor features are
> exploited.  But I can't see immediately how this should work.  I
> don't
> know whether or how the command line flags from Clang are made
> available
> to LLVM for inspection.
> 
> I'm sure there must be an obvious way to do this, but peeking at code
> from other targets hasn't enlightened me.  Any advice for how I can
> accomplish this?
Off of the top of my head...

Within the current infrastructure, I think that we need to alter the Clang
PowerPC toolchain code (Clang::AddPPCTargetArgs in Tools.cpp) to do something
like this:

if (Args.hasFlag(options::OPT_fno_altivec, options::OPT_faltivec, true)) {
  CmdArgs.push_back("-backend-option");
  CmdArgs.push_back("-mattr=-altivec");
}

Why this not done currently I don't know.

 -Hal
> 
> Thanks!  I appreciate any help.
> 
> Bill
> 
> --
> Bill Schmidt, Ph.D.
> IBM Advance Toolchain for PowerLinux
> IBM Linux Technology Center
> wschmidt at us.ibm.com
> wschmidt at linux.vnet.ibm.com
> 
> 
> 
> 
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

On Thu, 2013-01-31 at 09:42 -0600, Hal Finkel wrote:
> ----- Original Message -----
> > From: "Bill Schmidt" <wschmidt at linux.vnet.ibm.com>
> > To: llvmdev at cs.uiuc.edu
> > Sent: Thursday, January 31, 2013 9:26:15 AM
> > Subject: [LLVMdev] Getting command line options to affect subtarget
features
> > 
> > The problem I'm trying to solve:  Invoking clang on PowerPC with
> > -fno-altivec has no effect.
> > 
> > From what I've been able to piece together, PPC.td specifies
various
> > CPUs and the processor features available on each.  So for example we
> > have:
> > 
> > def FeatureAltivec   :
SubtargetFeature<"altivec","HasAltivec",
> > "true",
> >                                         "Enable Altivec
> >                                         instructions">;
> > 
> > def : Processor<"pwr7", G5Itineraries,
> >                   [DirectivePwr7, FeatureAltivec,
> >                    FeatureMFOCRF, FeatureFSqrt, FeatureSTFIWX,
> >                    FeatureISEL, Feature64Bit /*, Feature64BitRegs
> >                    */]>;
> > 
> > During compilation the subtarget class (such as PPCSubtarget) is
> > instantiated with the strings obtained from -mcpu and -mattr.
> > ParseSubtargetFeatures then uses the -mcpu and -mattr values to
> > determine which features to use for code generation.
> > 
> > What seems problematic is that -mattr is the only way that the static
> > features of the target processor can be overridden.
> > 
> > It seems there must be a way for command-line parameters such as
> > -fno-altivec to be used to override which processor features are
> > exploited.  But I can't see immediately how this should work.  I
> > don't
> > know whether or how the command line flags from Clang are made
> > available
> > to LLVM for inspection.
> > 
> > I'm sure there must be an obvious way to do this, but peeking at
code
> > from other targets hasn't enlightened me.  Any advice for how I
can
> > accomplish this?
> 
> Off of the top of my head...
> 
> Within the current infrastructure, I think that we need to alter the Clang
PowerPC toolchain code (Clang::AddPPCTargetArgs in Tools.cpp) to do something
like this:
> 
> if (Args.hasFlag(options::OPT_fno_altivec, options::OPT_faltivec, true)) {
>   CmdArgs.push_back("-backend-option");
>   CmdArgs.push_back("-mattr=-altivec");
> }
Perfect, Hal, thanks!  That makes good sense.  I'll fix this and also
bring some of the CPU handling more up-to-date (several missing
processors that ought to be in there).

Thanks,
Bill
> 
> Why this not done currently I don't know.
> 
>  -Hal
> 
> > 
> > Thanks!  I appreciate any help.
> > 
> > Bill
> > 
> > --
> > Bill Schmidt, Ph.D.
> > IBM Advance Toolchain for PowerLinux
> > IBM Linux Technology Center
> > wschmidt at us.ibm.com
> > wschmidt at linux.vnet.ibm.com
> > 
> > 
> > 
> > 
> > 
> > 
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > 
>