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Clarify contents of pctab

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Felix Geisendörfer
2021-04-07 20:27:43 +02:00
parent fecb0c4b98
commit b022d6e6b6
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stack-traces.md
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@ -147,7 +147,7 @@ There are various use cases for stack traces in Go, but they all end up hitting
Each frame lookup begins with the current `pc` which is passed to [`findfunc()`](https://github.com/golang/go/blob/go1.16.3/src/runtime/symtab.go#L671) which looks up the meta data for the function that contains the `pc`. Historically this was done using `O(log N)` binary search, but [nowadays](https://go-review.googlesource.com/c/go/+/2097/) there is a hash-map-like index of [`findfuncbucket`](https://github.com/golang/go/blob/go1.16.3/src/runtime/symtab.go#L671) structs that usually directly guides us to the right entry using an `O(1)` algorithm. So at this point the overall complexity is still the same as frame pointer unwinding, but it's worth noting that the constant overheads are already significantly higher.
The [_func](https://github.com/golang/go/blob/9baddd3f21230c55f0ad2a10f5f20579dcf0a0bb/src/runtime/runtime2.go#L825) meta data that we just retrieved contains a `pcsp` offset into the `pctab` table that maps program counters to stack pointer deltas. To decode this information, we call [`funcspdelta()`](https://github.com/golang/go/blob/go1.16.3/src/runtime/symtab.go#L903) which does a `O(N)` linear search over all program counters of the function until it finds the (`pc`, `sp delta`) pair were are looking for. For stacks with recursive call cycles, a tiny program counter cache is used to avoid doing lots of duplicated work.
The [_func](https://github.com/golang/go/blob/9baddd3f21230c55f0ad2a10f5f20579dcf0a0bb/src/runtime/runtime2.go#L825) meta data that we just retrieved contains a `pcsp` offset into the `pctab` table that maps program counters to stack pointer deltas. To decode this information, we call [`funcspdelta()`](https://github.com/golang/go/blob/go1.16.3/src/runtime/symtab.go#L903) which does a `O(N)` linear search over all program counters that change the `sp delta` of the function until it finds the closest (`pc`, `sp delta`) pair. For stacks with recursive call cycles, a tiny program counter cache is used to avoid doing lots of duplicated work.
Now that that we have the stack pointer delta, we we are almost ready to locate the next `return address (pc)` value of the caller and do the same lookup for it until we reach the "bottom" of the stack. But before that, we need to check if the current `pc` is part of one or more inlined function calls. This is done by checking the `_FUNCDATA_InlTree` data for the current `_func` and doing another linear search over the (`pc`, `inline index`) pairs in that table. Any inlined call found this way gets virtual stack frame `pc` added to the list. Then we continue with `return address (pc)` as mentioned in the beginning of the paragraph.