Rodrigo Mesquita pushed to branch wip/romes/step-out-5 at Glasgow Haskell Compiler / GHC Commits: 2d7995dc by Rodrigo Mesquita at 2025-06-16T12:05:34+01:00 BRK_ALTS BCO - - - - - 6 changed files: - compiler/GHC/ByteCode/Asm.hs - compiler/GHC/ByteCode/Instr.hs - compiler/GHC/StgToByteCode.hs - rts/Disassembler.c - rts/Interpreter.c - rts/include/rts/Bytecodes.h Changes: ===================================== compiler/GHC/ByteCode/Asm.hs ===================================== @@ -858,7 +858,7 @@ assembleI platform i = case i of , Op np ] - BRK_INTERNAL active -> emit_ bci_BRK_INTERNAL [SmallOp active] + BRK_ALTS active -> emit_ bci_BRK_ALTS [SmallOp active] #if MIN_VERSION_rts(1,0,3) BCO_NAME name -> do np <- lit1 (BCONPtrStr name) ===================================== compiler/GHC/ByteCode/Instr.hs ===================================== @@ -272,9 +272,10 @@ data BCInstr !UnitId -- breakpoint info module unit id !Word16 -- breakpoint info index (RemotePtr CostCentre) - -- An internal breakpoint should only be set by the compiler or RTS - -- See Note [BRK_INTERNAL] - | BRK_INTERNAL !Word16 {- enabled? -} + + -- An internal breakpoint for triggering a break on any case alternative + -- See Note [Debugger: BRK_ALTS] + | BRK_ALTS !Word16 {- enabled? -} #if MIN_VERSION_rts(1,0,3) -- | A "meta"-instruction for recording the name of a BCO for debugging purposes. @@ -471,7 +472,7 @@ instance Outputable BCInstr where <+> text "<tick_module>" <+> text "<tick_module_unitid>" <+> ppr tickx <+> text "<info_module>" <+> text "<info_module_unitid>" <+> ppr infox <+> text "<cc>" - ppr (BRK_INTERNAL active) = text "BRK_INTERNAL" <+> ppr active + ppr (BRK_ALTS active) = text "BRK_ALTS" <+> ppr active #if MIN_VERSION_rts(1,0,3) ppr (BCO_NAME nm) = text "BCO_NAME" <+> text (show nm) #endif @@ -597,7 +598,7 @@ bciStackUse OP_INDEX_ADDR{} = 0 bciStackUse SWIZZLE{} = 0 bciStackUse BRK_FUN{} = 0 -bciStackUse BRK_INTERNAL{} = 0 +bciStackUse BRK_ALTS{} = 0 -- These insns actually reduce stack use, but we need the high-tide level, -- so can't use this info. Not that it matters much. ===================================== compiler/GHC/StgToByteCode.hs ===================================== @@ -1372,7 +1372,10 @@ doCase d s p scrut bndr alts | ubx_tuple_frame = SLIDE 0 2 `consOL` alt_final0 | otherwise = alt_final0 alt_final - = BRK_INTERNAL 0 `consOL` alt_final1 + | gopt Opt_InsertBreakpoints (hsc_dflags hsc_env) + -- See Note [Debugger: BRK_ALTS] + = BRK_ALTS 0 `consOL` alt_final1 + | otherwise = alt_final1 add_bco_name <- shouldAddBcoName let @@ -1392,6 +1395,37 @@ doCase d s p scrut bndr alts _ -> panic "schemeE(StgCase).push_alts" in return (PUSH_ALTS alt_bco scrut_rep `consOL` scrut_code) +{- +Note [Debugger: BRK_ALTS] +~~~~~~~~~~~~~~~~~~~~~~~~~ +As described in Note [Debugger: Step-out] in rts/Interpreter.c, to implement +the stepping-out debugger feature we traverse the heap at runtime, identify +the continuation BCO, and explicitly enable that BCO's breakpoint thus ensuring +we stop exactly when we return to the continuation. + +However, a case continuation BCO (pushed by PUSH_ALTS), which computes what +case alternative BCO to take, never gets a user-facing breakpoint tick (BRK_FUN): + + 1) It's not useful to a user stepping through the program to always have a + breakpoint after the scrutinee is evaluated but before the case alternative + is selected. The associated source span would also be slightly awkward to choose. + + 2) It's not easy to add a source-tick before the case alternatives because in + essentially all internal representations they are given as a list of Alts + rather than an expression. + +To provide the debugger a way to enable at runtime the case continuation +breakpoints regardless, we introduce at the start of every PUSH_ALTS BCO a +BRK_ALTS instruction. + +The BRK_ALTS instruction, if enabled (by its single arg), ensures we stop at +the breakpoint heading the case alternative we take. Under the hood, this means +that when BRK_ALTS is enabled we set TSO_STOP_NEXT_BREAKPOINT just before +selecting the alternative. + +It's important that BRK_ALTS (just like BRK_FUN) is the first instruction of +the BCO, since that's where the debugger will look to enable it at runtime. +-} -- ----------------------------------------------------------------------------- -- Deal with tuples ===================================== rts/Disassembler.c ===================================== @@ -94,8 +94,8 @@ disInstr ( StgBCO *bco, int pc ) debugBelch("\n"); pc += 6; break; - case bci_BRK_INTERNAL: - debugBelch ("BRK_INTERNAL %d\n", BCO_NEXT); + case bci_BRK_ALTS: + debugBelch ("BRK_ALTS %d\n", BCO_NEXT); break; case bci_SWIZZLE: { W_ stkoff = BCO_GET_LARGE_ARG; ===================================== rts/Interpreter.c ===================================== @@ -643,10 +643,12 @@ interpretBCO (Capability* cap) // ACTIVATE the breakpoint by tick index ((StgInt*)breakPoints->payload)[tick_index] = 0; } - else if ((bco_instrs[0] & 0xFF) == bci_BRK_INTERNAL) { + else if ((bco_instrs[0] & 0xFF) == bci_BRK_ALTS) { // ACTIVATE BRK_ALTS by setting its only argument to ON bco_instrs[1] = 1; } + // else: if there is no BRK instruction perhaps we should keep + // traversing; that said, the continuation should always have a BRK } else /* type == STOP_FRAME */ { /* No continuation frame to further stop at: Nothing to do */ @@ -1588,14 +1590,11 @@ run_BCO: goto nextInsn; } - case bci_BRK_INTERNAL: + /* See Note [Debugger: BRK_ALTS] */ + case bci_BRK_ALTS: { - printf("BREAK INTERNAL\n"); StgWord16 active = BCO_NEXT; - printf("BREAK INTERNAL: %d\n", active); if (active) { - printf("BREAK INTERNAL is active\n"); - // Break on next! cap->r.rCurrentTSO->flags |= TSO_STOP_NEXT_BREAKPOINT; } ===================================== rts/include/rts/Bytecodes.h ===================================== @@ -214,7 +214,7 @@ #define bci_OP_INDEX_ADDR_32 242 #define bci_OP_INDEX_ADDR_64 243 -#define bci_BRK_INTERNAL 244 +#define bci_BRK_ALTS 244 /* If you need to go past 255 then you will run into the flags */ View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/2d7995dca4abd94a3f36a14e8262f67a... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/2d7995dca4abd94a3f36a14e8262f67a... 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