LLVM codegen loop ranges

Troubleshooting
#1

Using tvm.tir.ir_builder.create(), I’ve been playing around with for ranges. However, I’ve found that it doesn’t allow for loop ranges that don’t start at zero.

E.g., what is a perfectly valid for loop range in Python:

for i in range(5, 10):
  do_something()

Is invalid in the IR:

with irb.for_range(5, 10, name='i') as i:
  do_something()

Failing with the error:

  File "../src/target/llvm/codegen_cpu.cc", line 960
  TVMError: Check failed: is_zero(op->min):

If we visit void CodeGenCPU::VisitStmt_ we see this check is indeed performed.

Of course, we can always sidestep this issue by having a for loop like:

with irb.for_range(0, 5, name='i') as i:
  # add 5 to all usages of i
  do_something()

However this opens us up to errors, and potentially more arithmetic depending on how the compilation works.

Is this a limitation of LLVM codegen, or an oversight on the TVM side?

If the former, this should be explained in the code via a comment.