I am going to keep this short and to the point, but if further clarifications are needed please let me know.
I have an i64 Value that I want to check the top bits of if they are zeros or not. If they are zeros, I would do something, if they are not, I would do something else. How do I instrument the IR to allow this to happen at runtime?
One thing I found is that LLVM has an intrinsic "llvm.ctlz" that counts the leading zeros and puts them in an i64 Value, but how do I use its return value to do the checking? Or how do I instrument so the checking happens at runtime?
Any help or suggestions would be appreciated. Thanks!
You didn't say how many top bits, so I'll do an example with the top 32 bits. Given i64 %x, I'd check it with %result = icmp uge i64 %x, i64 4294967296 because 4294967296 is 2^32 and that is the first value which has a 1 bit in the top 32-bits. If you want to check the top two bits to be zero, use 2^62 (4611686018427387904) instead.
In order to do two different things based on the value of %result in general you'll want to branch on it. BasicBlock has a method splitBasicBlock that takes an instruction to split at. Use that to split your block into a before and after. Create new blocks for the true side an false side, add a branch on your result to your new blocks, br i1 %result, label %cond_true, label %cond_false. Make sure those two new blocks branch back to the after block.
Depending on what you want to do, you may not need an entire block, for instance if you're only calculating a value and not doing any side-effecting operations you might be able to use a select instruction instead of a branch and separate blocks.