[LLVMdev] Getting command line options to affect subtarget features

[Bill Schmidt]

On Thu, 2013-01-31 at 11:23 -0600, Bill Schmidt wrote:
> On Thu, 2013-01-31 at 10:17 -0600, Bill Schmidt wrote:
> > 
> > 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).
> 
> Well, I spoke too soon.  I tried this and received the following
> objection:
> 
> ~/src/llvm> $LLVM_INSTALL/bin/clang vectest.c -S -O3 -o vectest.s -fno-altivec
> clang: Unknown command line argument '-mattr=-altivec'.  Try: 'clang -help'
> clang: Did you mean '-mxgot=-altivec'?
> ~/src/llvm> 
> 
> Yet I can do the following:
> 
> ~/src/llvm> $LLVM_INSTALL/bin/llc -O3 -mattr=-altivec vectest.ll
> ~/src/llvm> 
> 
> The general mechanism seems valid since code like this exists:
> 
>   if (Args.hasFlag(options::OPT_mstackrealign, options::OPT_mno_stackrealign,
>                    false)) {
>     CmdArgs.push_back("-backend-option");
>     CmdArgs.push_back("-force-align-stack");
>   }
> 
> But it doesn't like -mattr.  Anyone have thoughts why?

I found some ideas to try in
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2011-September/017087.html.
Perhaps I can try pushing these tokens onto CmdArgs and then enable
altivec as a feature in PPCTargetInfo::setFeatureEnabled():

-Xclang -target-feature -Xclang -altivec

I'll give that a whirl and see what happens.

Bill

> 
> Thanks,
> Bill
> 
> > 
> > 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
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > _______________________________________________
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> > > > 
> > >