[llvm-dev] XRay threshold (bug?)

Serge Rogatch via llvm-dev llvm-dev at lists.llvm.org
Thu Jun 8 04:55:57 PDT 2017

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 MachineInstr`s, 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.


On 8 June 2017 at 14:41, Dean Michael Berris <dean.berris at gmail.com> wrote:

> Hi Serge,
> Yes I like option 1 as well, if size() isn't actually the machine
> instruction count. My only concern might be the cost of counting the
> instructions, but if it's only when XRay instrumentation is on then this
> should be fine.
> Let me have a look at the patch.
> Cheers
> Sent from my iPhone
> On 8 Jun 2017, at 9:26 pm, Serge Rogatch via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> 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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