[PATCH] D86502: [CSSPGO] A Clang switch -fpseudo-probe-for-profiling for pseudo-probe instrumentation.

Wed Nov 18 10:46:27 PST 2020

hoy added a comment.

In D86502#2245734 <https://reviews.llvm.org/D86502#2245734>, @wmi wrote:

> In D86502#2245578 <https://reviews.llvm.org/D86502#2245578>, @hoy wrote:
>
>> In D86502#2245460 <https://reviews.llvm.org/D86502#2245460>, @wmi wrote:
>>
>>>> The early instrumentation also allows the inliner to duplicate probes for inlined instances. When a probe along with the other instructions of a callee function are inlined into its caller function, the GUID of the callee function goes with the probe. This allows samples collected on inlined probes to be reported for the original callee function.
>>>
>>> Just get a question from reading the above. Suppose `A` only has one BB and the BB has one PseudoProbe in it. If function `A` is inlined into `B1` and `B2` and both `B1` and `B2` inlined into `C`, the PseudoProbe from `A` will have two copies in `C` both carrying GUID of `A`. How the samples collected from `A` inlined into `B1` inlined into `C` are categorized differently from `A` inlined into `B2` inlined into `C`, especially when debug information is not enabled (so no inline stack information in the binary)?
>>
>> This is a very good question. Inlined functions are differentiated by their original callsites. A pseudo probe is allocated for each callsite in the `SampleProfileProbe` pass. Nested inlining will produce a stack of pseudo probes, similar with the Dwarf inline stack. The work is not included in the first set of patches.
>
> Thanks, then how does the pseudo probe for a callsite after inline to represent the inline scope it covers?

It is represented by the dwarf inline stack for that inlined pseudo probe. I'm about to send out a patch for it.

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