[lldb][swift] Filter Q funclets when setting breakpoints #9112
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These funclets only serve to `task_dealloc` previously allocated tasks when returning from an async call, and immediately `task_switch` to the next await-suspend funclet (which contains real user code). By not filtering out these funclets, any breakpoint on a line with an async call will cause execution to pause 3 times: once before the call, twice when "returning" from the call, which makes for a confusing experience. The patch does the filtering on `BreakpointResolver::SetSCMatchesByLine`, which is the common code between BreakpointResolverFileLine and BreakpointResolverFileRegex. We also considered changing the debug line information in any of the many different lowering stages the swift compiler, but this turned out to be very complex to do in a targeted way; more often than not, a handful of early-IR coroutine instructions get expanded into multiple function clones, all inheriting the same debug line information. The current approach also has the advantaged of being easily reversible if we decide to do so.
adrian-prantl
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Aug 15, 2024
felipepiovezan
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Feb 11, 2025
With [1], stepping should work reliably even in the multithreaded case. With [2], the issue of multiple breakpoints on the same should no longer affect this test. [1]: swiftlang#9839 [2]: swiftlang#9112
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rdar://123391705
These funclets only serve to
task_deallocpreviously allocated tasks when returning from an async call, and immediatelytask_switchto the next await-suspend ("Y") funclet (which contains real user code).By not filtering out these funclets, any breakpoint on a line with an async call will cause execution to pause 3 times: once before the call, twice when "returning" from the call, which makes for a confusing experience.
The patch does the filtering on
BreakpointResolver::SetSCMatchesByLine, which is the common code between BreakpointResolverFileLine and BreakpointResolverFileRegex.We also considered changing the debug line information in any of the many different lowering stages the swift compiler, but this turned out to be very complex to do in a targeted way; more often than not, a handful of early-IR coroutine instructions get expanded into multiple function clones, all inheriting the same debug line information. The current approach also has the advantaged of being easily reversible if we decide to do so.