#15179: Unwinding info for stg_ap_v_info is wrong -------------------------------------+------------------------------------- Reporter: niteria | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.5 (Debugging) | Resolution: | Keywords: Operating System: Linux | Architecture: Type of failure: Debugging | Unknown/Multiple information is incorrect | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by niteria): The beginning of `stg_ap_v_info` looks like this: {{{ INFO_TABLE_RET(stg_ap_v, RET_SMALL, W_ info_ptr, ) { W_ info; W_ arity; unwind Sp = Sp + WDS(1); IF_DEBUG(apply,foreign "C" debugBelch("stg_ap_v_ret... "); foreign "C" printClosure(R1 "ptr")); IF_DEBUG(sanity,foreign "C" checkStackFrame(Sp+WDS(1)"ptr")); again: if (GETTAG(R1)==1) { Sp_adj(1); jump %GET_ENTRY(R1-1) [R1]; } ... }}} This looks reasonable, and you'd expect the beginning to unwind to the next word on the stack based on the manual annotation here. The Parsed Cmm also looks reasonable: {{{ ==================== Parsed Cmm ==================== [section ""cstring" . c8_str" { c8_str: I8[] [115,116,103,95,97,112,95,118,95,114,101,116] }, stg_ap_v_ret() // [] { info_tbl: [(c13, label: stg_ap_v_info rep:tag:30 StackRep [])] stack_info: arg_space: 8 updfr_space: Just 8 } {offset c13: // c1 //tick srcAutoApply.cmm:(8,1)-(95,1) unwind Sp = Just Sp + 1 * 8; // CmmUnwind goto c4; // CmmBranch c4: // c1 if (R1 & ((1 << 3) - 1) == 1) goto c5; else goto c7; // CmmCondBranch c5: // c1 //tick srcAutoApply.cmm:(17,35)-(20,5) //tick srcAutoApply.cmm:18:12-26 Sp = Sp + 1 * 8; // CmmAssign call (I64[R1 - 1])(R1) args: 8, res: 0, upd: 8; // CmmCall ... }}} and it survives in this form till the very end of the Cmm pipeline, that is into Optimised Cmm: {{{ ==================== Optimised Cmm ==================== stg_ap_v_ret() // [] { [(c13, stg_ap_v_info: const 0; const 30;)] } {offset c13: // c1 //tick srcAutoApply.cmm:(8,1)-(95,1) unwind Sp = Just Sp + 8; // CmmUnwind goto c4; // CmmBranch c4: // c1 if (R1 & 7 == 1) goto c5; else goto c7; // CmmCondBranch c5: // c1 //tick srcAutoApply.cmm:(17,35)-(20,5) //tick srcAutoApply.cmm:18:12-26 Sp = Sp + 8; // CmmAssign call (I64[R1 - 1])(R1) args: 8, res: 0, upd: 8; // CmmCall ... }}} The Native code preserves the c13 label which is where we have our unwind info: {{{ ==================== Native code ==================== .section .text .align 8 .align 8 .loc 1 8 1 .quad 0 .quad 30 .globl stg_ap_v_info .type stg_ap_v_info, @object stg_ap_v_info: _c13: .loc 1 8 1 _n1b: jmp _c4 .Lc13_end: _c4: .loc 1 8 1 movq %rbx,%vI_n1d andl $7,%vI_n1d cmpq $1,%vI_n1d je _c5 jmp _c7 .Lc4_end: _c5: .loc 1 8 1 addq $8,%rbp jmp *-1(%rbx) .Lc5_end: ... ==================== Asm code ==================== .file 1 "AutoApply.cmm" .section .text .align 8 .align 8 .loc 1 8 1 .quad 0 .quad 30 .globl stg_ap_v_info .type stg_ap_v_info, @object stg_ap_v_info: _c13: .loc 1 8 1 _n1b: .Lc13_end: _c4: .loc 1 8 1 movq %rbx,%rax andl $7,%eax cmpq $1,%rax je _c5 .Lc4_end: _c7: .loc 1 8 1 andq $-8,%rbx movq (%rbx),%rax cmpl $58,-8(%rax) jb _u14 .Lc7_end: }}} Debug Infos keeps our unwind info attached to c13: {{{ ==================== Debug Infos ==================== proc c13 (stg_ap_v_info) srcAutoApply.cmm:(8,1)-(95,1) removed [{_n1b: Sp=Just Sp+8}] [blk c4 (_c4) srcAutoApply.cmm:(8,1)-(95,1) pos 0 [], blk c7 (_c7) srcAutoApply.cmm:(8,1)-(95,1) pos 1 [], blk u18 (_u18) srcAutoApply.cmm:(8,1)-(95,1) pos 2 [], blk u14 (_u14) srcAutoApply.cmm:(8,1)-(95,1) pos 4 [], blk u17 (_u17) srcAutoApply.cmm:(8,1)-(95,1) pos 5 [], blk u15 (_u15) srcAutoApply.cmm:(8,1)-(95,1) pos 35 [], blk u16 (_u16) srcAutoApply.cmm:(8,1)-(95,1) pos 36 [], blk c5 (_c5) srcAutoApply.cmm:(8,1)-(95,1) pos 34 [], blk cb (_cb) srcAutoApply.cmm:(91,18)-(93,9) pos 3 [{_n1f: MachSp=Just MachSp+8}, {_n1g: MachSp=Just MachSp}], blk cp (_cp) srcAutoApply.cmm:(27,18)-(36,9) pos 27 [] [blk ce (_ce) srcAutoApply.cmm:(27,18)-(36,9) pos 28 [], blk ck (_ck) srcAutoApply.cmm:(27,18)-(36,9) pos 29 [], blk cl (_cl) srcAutoApply.cmm:(27,18)-(36,9) pos 30 [], blk cm (_cm) srcAutoApply.cmm:(27,18)-(36,9) pos 31 [], blk cc (_cc) srcAutoApply.cmm:(27,18)-(36,9) pos 32 []], blk cG (_cG) srcAutoApply.cmm:(43,18)-(56,9) pos 20 [] [blk cs (_cs) srcAutoApply.cmm:(43,18)-(56,9) pos 21 [], blk ct (_ct) srcAutoApply.cmm:(43,18)-(56,9) pos 22 [], blk cu (_cu) srcAutoApply.cmm:(43,18)-(56,9) pos 23 [], blk cB (_cB) srcAutoApply.cmm:(43,18)-(56,9) pos 24 [], blk cC (_cC) srcAutoApply.cmm:(43,18)-(56,9) pos 25 [], blk cD (_cD) srcAutoApply.cmm:(43,18)-(56,9) pos 26 [], blk cq (_cq) srcAutoApply.cmm:(43,18)-(56,9) pos 18 [], blk co (_co) srcAutoApply.cmm:54:596-601 pos 19 [], blk cO (_cO) srcAutoApply.cmm:54:128-215 pos 16 []], }}} But c4 is the first label we generate the debug info for: {{{ ==================== Asm code ==================== .Lsection_info: .section .debug_info,"",@progbits _n1H: .long .Ln1H_end-_n1H-4 .short 3 .long .Lsection_abbrev .byte 8 .byte 1 .asciz "/tmp/ghc802745_0/ghc_1.cmm" .asciz "The Glorious Glasgow Haskell Compilation System 8.5.20180514" .long 24 .asciz "/data/users/bnitka/ghc-unwind-asm-2/" .quad stg_ap_v_info .quad .Lstg_ap_v_info_end .long .Lsection_line .Lstg_ap_v_info_die: .byte 2 .asciz "stg_ap_v_ret" .asciz "stg_ap_v_info" .byte 255 .quad stg_ap_v_info .quad .Lstg_ap_v_info_end .byte 1 .byte 156 .Lc4_die: .byte 5 .asciz "_c4" .quad _c4 .quad .Lc4_end .byte 6 .asciz "AutoApply.cmm" .long 8 .short 1 .long 95 .short 2 .byte 6 .asciz "AutoApply.cmm" .long 24 .short 8 .long 24 .short 33 .byte 6 .asciz "AutoApply.cmm" .long 25 .short 10 .long 25 .short 26 .byte 6 .asciz "AutoApply.cmm" .long 17 .short 35 .long 20 .short 6 .byte 6 .asciz "AutoApply.cmm" .long 18 .short 12 .long 18 .short 27 .byte 0 }}} Going back to the Debug Info's dump you can see that c13 has `"removed"` in it, which refers to the `dblPosition` field which keeps the relative position of the block in a procedure. Later when we see that `dblPosition` is `"removed"` we don't generate a debug info for it. `blockToDwarf` is where we decide not to use removed blocks. [0] But the block is clearly there in the Cmm and Asm output, so there's some mismatch here. The part of code that declares them missing is `cmmDebugLabels` [1]. It does so because the block only consists of "meta" instructions. One way to avoid this problem is to say that `unwind` is not a meta instruction. Another workaround is to add unwind info under `again:` label, that puts the `unwind` pseudo-instruction in the c4 block which has some real code. I'm speculating here, but I think the reason that c13 block didn't disappear in assembly is because we generate `.loc` from the tick. I don't know where that happens yet, so a pointer would be helpful. I don't know how to fix this in a robust way yet. [0] https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/nativeG... [1] https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/cmm/Deb... -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15179#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler