[PATCH] D42216: Use New Module Metadata String "AvoidPLT" to avoid calls via PLT

[Rafael Avila de Espindola via llvm-commits]

Reid Kleckner <rnk at google.com> writes:

>> Index: lib/Target/X86/X86ISelLowering.cpp
>> > ===================================================================
>> > --- lib/Target/X86/X86ISelLowering.cpp
>> > +++ lib/Target/X86/X86ISelLowering.cpp
>> > @@ -3732,11 +3732,23 @@
>> >      }
>> >    } else if (ExternalSymbolSDNode *S = dyn_cast<ExternalSymbolSDNode>(Callee))
>> {
>> >      const Module *Mod = DAG.getMachineFunction().
>> getFunction().getParent();
>> > -    unsigned char OpFlags =
>> > -        Subtarget.classifyGlobalFunctionReference(nullptr, *Mod);
>> > +    // If PLT must be avoided then the call should be via GOTPCREL.
>> > +    unsigned char OpFlags = X86II::MO_GOTPCREL;
>> > +
>> > +    if (!Mod->getAvoidPLT()) {
>> > +      OpFlags = Subtarget.classifyGlobalFunctionReference(nullptr,
>> *Mod);
>> > +    }
>> >
>> >      Callee = DAG.getTargetExternalSymbol(
>> >          S->getSymbol(), getPointerTy(DAG.getDataLayout()), OpFlags);
>> > +
>> > +    if (OpFlags == X86II::MO_GOTPCREL) {
>> > +      Callee = DAG.getNode(X86ISD::WrapperRIP, dl,
>> > +          getPointerTy(DAG.getDataLayout()), Callee);
>> > +      Callee = DAG.getLoad(
>> > +          getPointerTy(DAG.getDataLayout()), dl, DAG.getEntryNode(),
>> Callee,
>> > +          MachinePointerInfo::getGOT(DAG.getMachineFunction()));
>> > +    }
>>
>> The logic should be added to shouldAssumeDSOLocal.
>>
>
> We might need to extend shouldAssumeDSOLocal to know that we are calling a
> function. We only have an ExternalSymbol in this case, not a GlobalValue,
> so GV there will be null.

My memory in these areas is fuzzy. Do we never create a GV for a call to
an intrinsic or do we lose it at some point in the codegen path? If it is
lost, could we track it a bit longer?

For this patch, is there any other case when GV is null? If that only
happens when calling an intrinsic shouldAssumeDSOLocal can use that to
know that it is a call.

Maybe it can always return false if GV is null?

There are 3 ways of calling a function:

call foo
call foo at plt
call *foo at got

if shouldAssumeDSOLocal returns true, we are supposed to use the first
one. If it returns false, one of the last two.

Now, "call "foo" is almost identical to "call foo at plt" as the linker can
convert plt calls if the target is local. If the only case when GV is
null is a call to an intrinsic, it is safe to return false.

Cheers,
Rafael