Enable / Disable a processor feature

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1

I’m trying to enable/disable a target feature through clang.

Here is how my target looks like

// Esencia subtarget features
//===----------------------------------------------------------------------===//
def FeatureMul : SubtargetFeature<“mul”, “HasMul”, “true”,
“Enable hardware multiplier”>;
def FeatureDiv : SubtargetFeature<“div”, “HasDiv”, “true”,
“Enable hardware divider”>;
def FeatureCmov : SubtargetFeature<“cmov”, “HasCmov”, “true”,
“Enable conditional move instruction”>;
def FeatureAddc : SubtargetFeature<“addc”, “HasAddc”, “true”,
“Enable add with carry instruction”>;

//===----------------------------------------------------------------------===//
// Esencia processors supported.
//===----------------------------------------------------------------------===//
class Proc<string Name, SchedMachineModel Model,
list Features>
: ProcessorModel<Name, Model, Features>;

def : Proc<“esencia”, EsenciaModel, [FeatureMul,
FeatureDiv,
FeatureCmov,
FeatureAddc
]>;

This is what I have in EsenciaTargetInfo::setFeatureEnabled(…) in clang/lib/Basic/Targets.cpp

virtual void setFeatureEnabled(llvm::StringMap &Features,
StringRef Name,
bool Enabled) const {
if (Name == “mul” ||
Name == “div” ||
Name == “cmov” ||
Name == “addc”) {
Features[Name] = Enabled;
}
}

Is there a way to enable/disable a given feature through clang? Any flags I need to pass?

Any help is appreciated.

2

The easiest way here is to take a look at an existing target, say x86, and do what it does :slight_smile:

For specific guidance within x86, take a look at how -msse3 works.

-eric

3

Thanks for the reply. The issue was that I did look at an existing target, but wasn’t able to figure it out, at least initially. I figured it right now (with llc use -mattr=+feature1,-feature2,+feature3 to enable feature1, disable feature2 and enable feature3 respectively), but still have questions. For example, clang has this -target-feature=+feature1,-feature2,+feature3, but for some reason it didn’t work for my target. I’m wondering why it didn’t. I would really like to know why it didn’t work. I can post more code if you are interested in helping out.

4

I’m not suggesting at the command line, but rather how this is handled in Targets.cpp for your target. The code that handles this is a little hairy (unfortunately), but if you take a look at how x86 implements that then you can look at your target.

-eric