XRay threshold (bug?)

Hi Dean,

I’ve noticed that XRay in llvm\lib\CodeGen\XRayInstrumentation.cpp compares its threshold against Function::size() . However, Function::size() returns the number of basic blocks (as I understand, such as cycle bodies, if/else bodies, switch-case bodies, etc.), rather than the number of instructions.

Was your intent to count the instructions instead? The name of the parameter -fxray-instruction-threshold=N suggests this, as well as XRay documentation at http://llvm.org/docs/XRay.html .
If so, I see two options:

  1. Count the number of MachineInstr`s in MachineFunction : this gives better estimate for the number of assembly instructions on the target. So a user can check in disassembly that the threshold works more or less correctly.
  2. Count the number of Instruction`s in a Function : AFAIK, this gives correct number of IR instructions, which the user can check in IR listing. However, this number may be far (several times for small functions) from the number of assembly instructions finally emitted.

My team is in favor of option 1 (proposed patch here https://reviews.llvm.org/D34027 ), because we think that having the closer estimate for the number of assembly instructions emitted is more important than to have a clear definition of the metric.

What do you think?

Cheers,
Serge

Hi Serge,

The old version was counting the number of basic blocks by iterating through them, because the list doesn’t keep the count of items in it. So the complexity was linear in the number of basic blocks.
The new version counts the number of instructions also by iterating them. So the complexity is linear in the number of instructions.
This effectively adds another pass over the whole program code, more specifically, over all the MachineInstrs, because the above iteration is performed for each function. However, this pass is as light as possible because it doesn't do anything for each instruction except counting it. I believe this pass is performed only when -fxray-instrument` option is specified, because XRayInstrumentation::runOnMachineFunction() is only run in that case.

Cheers,
Serge