[llvm-dev] XRay threshold (bug?)

[Serge Rogatch via llvm-dev]

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
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