[LLVMdev] Giving labels to intrinsics and getting the address
eslee3 at uiuc.edu
Fri Apr 18 11:43:15 PDT 2008
call void @llvm.branchlike(label %thelabel)
Is it possible to give a label/BasicBlock* to an intrinsic? Similar to
labels for branches. No label type is currently defined in
Intrinsics.td, but would it be possible to add it there and
But I also want to take the address and put it into a physical
register and emit a desired x86 instruction.
Assuming I have the label, I should be able to do this in the Lowering
process with inline asm. Where I want the x86 instruction to be "0x0f,
0x36, 0x00" and label address put into register a.
// Get the label from instrinsic's arguments..
BasicBlock* theLabel = ...magic?...
// Create a function type that takes a label and returns void
FunctionType* asmFuncTy = FunctionType::get(Type::VoidTy, asmArgs, false);
// Create inline asm that emits the desired opcode and prepares register a
InlineAsm* asm = InlineAsm::get(asmFuncTy, ".byte 0x0f, 0x36, 0x00",
// Create call to the inline asm giving "the label" to put in register a
CallInst* callAsm = new CallInst(asm, theLabel);
// Replace the intrinsic with the call
Actually, now that I think about it, maybe I shouldn't even bother
using an intrinsic and directly use this inline asm. Assuming that the
following transformation won't cause basic blocks to be considered
%x = call i1 @llvm.myintrinsic(label %otherPath) ; @myintrinsic always
br i1 %x, label %mainPath, label %otherPath
Hopefully, even if the CFG optimizer finds that mainPath is always
taken, because there's a use of %otherPath by @myintrinsic, that path
won't be used. Additionally, to the rest of the compiler, data (e.g.,
%x) flows into *both* %mainPath and %otherPath.
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