Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC Commits: 8a69196e by Rodrigo Mesquita at 2025-07-08T07:39:47-04:00 debugger/rts: Allow toggling step-in per thread The RTS global flag `rts_stop_next_breakpoint` globally sets the interpreter to stop at the immediate next breakpoint. With this commit, single step mode can additionally be set per thread in the TSO flag (TSO_STOP_NEXT_BREAKPOINT). Being able to toggle "stop at next breakpoint" per thread is an important requirement for implementing "stepping out" of a function in a multi-threaded context. And, more generally, having a per-thread flag for single-stepping paves the way for multi-threaded debugging. That said, when we want to enable "single step" mode for the whole interpreted program we still want to stop at the immediate next breakpoint, whichever thread it belongs to. That's why we also keep the global `rts_stop_next_breakpoint` flag, with `rts_enableStopNextBreakpointAll` and `rts_disableStopNextBreakpointAll` helpers. Preparation for #26042 - - - - - 73d3f864 by Rodrigo Mesquita at 2025-07-08T07:39:47-04:00 docs: Case continuation BCOs This commit documents a subtle interaction between frames for case BCOs and their parents frames. Namely, case continuation BCOs may refer to (non-local) variables that are part of the parent's frame. The note expanding a bit on these details is called [Case continuation BCOs] - - - - - d7aeddcf by Rodrigo Mesquita at 2025-07-08T07:39:47-04:00 debugger: Implement step-out feature Implements support for stepping-out of a function (aka breaking right after returning from a function) in the interactive debugger. It also introduces a GHCi command :stepout to step-out of a function being debugged in the interpreter. The feature is described as: Stop at the first breakpoint immediately after returning from the current function scope. Known limitations: because a function tail-call does not push a stack frame, if step-out is used inside of a function that was tail-called, execution will not be returned to its caller, but rather its caller's first non-tail caller. On the other hand, it means the debugger follows the more realistic execution of the program. In the following example: .. code-block:: none f = do a b <--- (1) set breakpoint then step in here c b = do ... d <--- (2) step-into this tail call d = do ... something <--- (3) step-out here ... Stepping-out will stop execution at the `c` invokation in `f`, rather than stopping at `b`. The key idea is simple: When step-out is enabled, traverse the runtime stack until a continuation BCO is found -- and enable the breakpoint heading that BCO explicitly using its tick-index. The details are specified in `Note [Debugger: Step-out]` in `rts/Interpreter.c`. Since PUSH_ALTS BCOs (representing case continuations) were never headed by a breakpoint (unlike the case alternatives they push), we introduced the BRK_ALTS instruction to allow the debugger to set a case continuation to stop at the breakpoint heading the alternative that is taken. This is further described in `Note [Debugger: BRK_ALTS]`. Fixes #26042 - - - - - 5d9adf51 by Rodrigo Mesquita at 2025-07-08T07:39:47-04:00 debugger: Filter step-out stops by SrcSpan To implement step-out, the RTS looks for the first continuation frame on the stack and explicitly enables its entry breakpoint. However, some continuations will be contained in the function from which step-out was initiated (trivial example is a case expression). Similarly to steplocal, we will filter the breakpoints at which the RTS yields to the debugger based on the SrcSpan. When doing step-out, only stop if the breakpoint is /not/ contained in the function from which we initiated it. This is especially relevant in monadic statements such as IO which is compiled to a long chain of case expressions. See Note [Debugger: Filtering step-out stops] - - - - - 46 changed files: - compiler/GHC/ByteCode/Asm.hs - compiler/GHC/ByteCode/Instr.hs - compiler/GHC/ByteCode/Types.hs - compiler/GHC/Driver/Config.hs - compiler/GHC/Runtime/Eval.hs - compiler/GHC/Runtime/Eval/Types.hs - compiler/GHC/StgToByteCode.hs - docs/users_guide/ghci.rst - ghc/GHCi/UI.hs - libraries/ghc-heap/GHC/Exts/Heap/Closures.hs - libraries/ghc-heap/GHC/Exts/Heap/FFIClosures_ProfilingDisabled.hsc - libraries/ghc-heap/GHC/Exts/Heap/FFIClosures_ProfilingEnabled.hsc - libraries/ghc-heap/tests/parse_tso_flags.hs - + libraries/ghci/GHCi/Debugger.hs - libraries/ghci/GHCi/Message.hs - libraries/ghci/GHCi/Run.hs - libraries/ghci/ghci.cabal.in - rts/Disassembler.c - rts/Interpreter.c - rts/Interpreter.h - rts/RtsSymbols.c - rts/StgMiscClosures.cmm - rts/include/rts/Bytecodes.h - rts/include/rts/Constants.h - rts/include/rts/storage/Closures.h - + testsuite/tests/ghci.debugger/scripts/T26042b.hs - + testsuite/tests/ghci.debugger/scripts/T26042b.script - + testsuite/tests/ghci.debugger/scripts/T26042b.stdout - + testsuite/tests/ghci.debugger/scripts/T26042c.hs - + testsuite/tests/ghci.debugger/scripts/T26042c.script - + testsuite/tests/ghci.debugger/scripts/T26042c.stdout - + testsuite/tests/ghci.debugger/scripts/T26042d.hs - + testsuite/tests/ghci.debugger/scripts/T26042d.script - + testsuite/tests/ghci.debugger/scripts/T26042d.stdout - + testsuite/tests/ghci.debugger/scripts/T26042e.hs - + testsuite/tests/ghci.debugger/scripts/T26042e.script - + testsuite/tests/ghci.debugger/scripts/T26042e.stdout - + testsuite/tests/ghci.debugger/scripts/T26042f.hs - + testsuite/tests/ghci.debugger/scripts/T26042f.script - + testsuite/tests/ghci.debugger/scripts/T26042f1.stderr - + testsuite/tests/ghci.debugger/scripts/T26042f1.stdout - + testsuite/tests/ghci.debugger/scripts/T26042f2.stdout - + testsuite/tests/ghci.debugger/scripts/T26042g.hs - + testsuite/tests/ghci.debugger/scripts/T26042g.script - + testsuite/tests/ghci.debugger/scripts/T26042g.stdout - testsuite/tests/ghci.debugger/scripts/all.T The diff was not included because it is too large. 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