This patch is fixing two issues for both CS and non-CS.
1) For external-call-internal, the head samples of the the internal function should be recorded.
2) avoid ignoring LBR after meeting the interrupt branch for CS profile
LBR parser is shared between CS and non-CS, we found it's error-prone while dealing with artificial branch inside LBR parser. Since artificial branch is mainly used for CS profile unwinding, this patch tries to simplify LBR parser by decoupling artificial branch code from it, the concept of artificial branch is removed and split into two transitional branches(internal-to-external, external-to-internal). Then we leave all the processing of external branch to unwinder.
Specifically for unwinder, remembering that we introduce external frame in https://reviews.llvm.org/D115550. We can just take external address as a regular address and reuse current unwind function(unwindCall, unwindReturn). For a normal case, the external frame will match an external LBR, and it will be filtered out by `unwindLinear` without losing any context.
The data also shows that the interrupt or standalone LBR pattern(unpaired case) does exist, we choose to handle it by clearing the call stack and keeping unwinding. Here we leverage checking in `unwindLinear`, because a standalone LBR, no matter its type, since it doesn’t have other part to pair, it will eventually cause a wrong linear range, like [external, internal], [internal, external]. Then set the state to invalid there.
Reviewed By: hoy, wenlei
Differential Revision: https://reviews.llvm.org/D118177
Before we have an issue with artificial LBR whose source is a return, recalling that "an internal code(A) can return to external address, then from the external address call a new internal code(B), making an artificial branch that looks like a return from A to B can confuse the unwinder". We just ignore the LBRs after this artificial LBR which can miss some samples. This change aims at fixing this by correctly unwinding them instead of ignoring them.
List some typical scenarios covered by this change.
1) multiple sequential call back happen in external address, e.g.
```
[ext, call, foo] [foo, return, ext] [ext, call, bar]
```
Unwinder should avoid having foo return from bar. Wrong call stack is like [foo, bar]
2) the call stack before and after external call should be correctly unwinded.
```
{call stack1} {call stack2}
[foo, call, ext] [ext, call, bar] [bar, return, ext] [ext, return, foo ]
```
call stack 1 should be the same to call stack2. Both shouldn't be truncated
3) call stack should be truncated after call into external code since we can't do inlining with external code.
```
[foo, call, ext] [ext, call, bar] [bar, call, baz] [baz, return, bar ] [bar, return, ext]
```
the call stack of code in baz should not include foo.
### Implementation:
We leverage artificial frame to fix#2 and #3: when we got a return artificial LBR, push an extra artificial frame to the stack. when we pop frame, check if the parent is an artificial frame to pop(fix#2). Therefore, call/ return artificial LBR is just the same as regular LBR which can keep the call stack.
While recording context on the trie, artificial frame is used as a tag indicating that we should truncate the call stack(fix#3).
To differentiate #1 and #2, we leverage `getCallAddrFromFrameAddr`. Normally the target of the return should be the next inst of a call inst and `getCallAddrFromFrameAddr` will return the address of call inst. Otherwise, getCallAddrFromFrameAddr will return to 0 which is the case of #1.
Reviewed By: hoy, wenlei
Differential Revision: https://reviews.llvm.org/D115550
wlei