Don't create a swift async unwind plan when in a function prologue

[jasonmolenda]

Don't create a swift async unwind plan when in a function prologue

If a swift async function does a standard ABI function call into
a normal function, at the start, we will still have the caller's
async register values (e.g. deref frame pointer shows us an
AsyncContext address with high-nibble flags set).

This patch changes SwiftLanguageRuntime to detect when we are in the
middle of a function prologue, and not return an async unwind plan
in that case.

[jasonmolenda]

@swift-ci please test

[fredriss]

Shouldn't this be else if (sc.symbol)? Or is there a reason to test function and symbol?

[kastiglione]

I'll piggyback off this question and ask: Does anyone know off hand when this call to CalculateSymbolContext would return only one of function/symbol, but not the other?

[jasonmolenda]

You're right. Function is based on debug information; Symbol is based on the raw symbol table (here's a list of symbol names and addresses). Symbol may have a prologue description based on the instruction profiler. But the line table based size is going to be the more reliable one. We use the Symbol as a backup if we couldn't find a Function.

[kastiglione]

Which order should be used: Function; Symbol, or Symbol; Function?

@adrian-prantl mentioned that the line table data for is_prologue_end may not be reliable. WIth Function::GetPrologueByteSize first, as it is now, then is_prologue_end is used in part to determine the end. It may be fine, but if we think that data isn't good enough, then these checks could start with Symbol::GetPrologueByteSize.

[jasonmolenda]

Which order should be used: Function; Symbol, or Symbol; Function?

Function; Symbol.

@adrian-prantl mentioned that the line table data for is_prologue_end may not be reliable. WIth Function::GetPrologueByteSize first, as it is now, then is_prologue_end is used in part to determine the end. It may be fine, but if we think that data isn't good enough, then these checks could start with Symbol::GetPrologueByteSize.

We already use GetPrologueByteSize for step (determining how far a step should advance into a function before we tell the user we're stopped there) and for breakpoint migration (put a breakpoint on a function name, lldb advances the breakpoint past the prologue). My assumption is that GetPrologueByteSize behaves correctly.

[jasonmolenda]

@swift-ci please test

[kastiglione]

Thanks for the lesson.