[LLVMdev] Getting command line options to affect subtarget features

[Bill Schmidt]

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
wsch...@us.ibm.com
wsch...@linux.vnet.ibm.com






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[Hal Finkel]

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

[Bill Schmidt]

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

[Bill Schmidt]

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?

Thanks,
Bill

[Bill Schmidt]

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

[Bill Schmidt]

Nope, that's no good either (as I probably should have expected):

wschmidt@bns:~/src/llvm> $LLVM_INSTALL/bin/clang vectest.c -S -O3 -o vectest.s -fno-altivec
error: unknown argument: '-Xclang -target-feature'
error: unknown argument: '-Xclang -altivec'
wschmidt@bns:~/src/llvm>


I'm officially out of ideas...

[Hal Finkel]

This is indeed a bit convoluted. mattr is declared in llvm/CodeGen/CommandFlags.h. So is code-model, so looking at how that works...

Frontend/CompilerInvocation.cpp pulls out -mcode-model into an options structure:
Opts.CodeModel = Args.getLastArgValue(OPT_mcode_model)

Then in CodeGen/BackendUtil.cpp, this is transformed into an argument to createTargetMachine

// FIXME: Parse this earlier.
llvm::CodeModel::Model CM;
if (CodeGenOpts.CodeModel == "small") {
CM = llvm::CodeModel::Small;
} else if (CodeGenOpts.CodeModel == "kernel") {
CM = llvm::CodeModel::Kernel;
...
TargetMachine *TM = TheTarget->createTargetMachine(Triple, TargetOpts.CPU,
FeaturesStr, Options,
RM, CM, OptLevel);

I think that what we want for altivec is actually to put things into FeaturesStr. This is populated by:

std::string FeaturesStr;
if (TargetOpts.Features.size()) {
SubtargetFeatures Features;
for (std::vector<std::string>::const_iterator
it = TargetOpts.Features.begin(),
ie = TargetOpts.Features.end(); it != ie; ++it)
Features.AddFeature(*it);
FeaturesStr = Features.getString();
}

the code that puts things into Features is in Basic/Targets.cpp (to get defaults) and Driver/Tools.cpp (to get command-line options). Unfortunately, the code in Driver/Tools.cpp is in Clang::AddX86TargetArgs (and, thus, is X86 specific). It seems like some refactoring might be in order.

-Hal