[LLVMdev] RFC: Binary format for instrumentation based profiling data

[Xinliang David Li]

On Thu, Mar 13, 2014 at 2:14 PM, Diego Novillo <dnovillo at google.com> wrote:

> On Thu, Mar 13, 2014 at 11:51 AM, Bob Wilson <bob.wilson at apple.com> wrote:
> >
> > On Mar 13, 2014, at 5:48 AM, Diego Novillo <dnovillo at google.com> wrote:
> >>
> >> How are counters represented? Are these line numbers together with the
> >> counter? Basic blocks? Edges?
> >
> > There are no line numbers, basic blocks, or edges. It is just a sequence
> of counters that the front-end knows how to map to the code (the same as
> with our current textual file format).
>
> Sorry, you lost me.  How exactly does the FE map them to the code? In
> the sample profiler, each instrumented line consists of a line offset,
> a discriminator (to distinguish distinct control flow paths on the
> same line) and the counter. We match them by computing the absolute
> line number from the offset and assign the counter to the
> corresponding basic block.
>

For GCC, it is CFG based matching -- it requires exact match of sources
between instrumentation and annotation (the counters are laid out in some
CFG order).

David



> I think we should be able to use the same pass in
> lib/Transforms/Scalar/SampleProfile.cpp to read profiles generated
> from instrumentation. The information is basically the same, so a bit
> of generalization of that code should be all we need to pass those
> counters down into the analysis module.
>
>
> Diego.
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