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diff --git a/compiler/rustc_codegen_cranelift/docs/dwarf.md b/compiler/rustc_codegen_cranelift/docs/dwarf.md new file mode 100644 index 000000000..502b1b036 --- /dev/null +++ b/compiler/rustc_codegen_cranelift/docs/dwarf.md @@ -0,0 +1,153 @@ +# Line number information + +Line number information maps between machine code instructions and the source level location. + +## Encoding + +The line number information is stored in the `.debug_line` section for ELF and `__debug_line` +section of the `__DWARF` segment for Mach-O object files. The line number information contains a +header followed by the line program. The line program is a program for a virtual machine with +instructions like set line number for the current machine code instruction and advance the current +machine code instruction. + +## Tips + +You need to set either `DW_AT_low_pc` and `DW_AT_high_pc` **or** `DW_AT_ranges` of a +`DW_TAG_compilation_unit` to the range of addresses in the compilation unit. After that you need +to set `DW_AT_stmt_list` to the `.debug_line` section offset of the line program. Otherwise a +debugger won't find the line number information. On macOS the debuginfo relocations **must** be +section relative and not symbol relative. +See [#303 (comment)](https://github.com/bjorn3/rustc_codegen_cranelift/issues/303#issuecomment-457825535) +for more information. + +# Function debuginfo + +## Tips + +`DW_TAG_subprogram` requires `DW_AT_name`, `DW_AT_low_pc` and `DW_AT_high_pc` **or** `DW_AT_ranges`. +Otherwise gdb will silently skip it. When `DW_AT_high_pc` is a length instead of an address, the +DWARF version must be at least 4. + +<details> +<summary>IRC log of #gdb on irc.freenode.org at 2020-04-23</summary> + +``` +(13:46:11) bjorn3: i am writing a backend for a compiler that uses DWARF for debuginfo. for some reason gdb seems to completely ignore all DW_TAG_subprogram, while lldb works fine. any idea what the problem could be? +(13:47:49) bjorn3: this is the output of llvm-dwarfdump: https://gist.github.com/bjorn3/8a34e333c80f13cb048381e94b4a3756 +(13:47:50) osa1: luispm: why is that problem not exists in 'commands'? (the target vs. host) +(13:52:16) luispm: osa1, commands is a bit more high level. It executes isolated commands. Breakpoint conditions need to be evaluated in the context of a valid expression. That expression may involve variables, symbols etc. +(13:52:36) luispm: osa1, Oh, i see your point now. Commands is only executed on the host. +(13:53:18) luispm: osa1, The commands are not tied to the execution context of the debugged program. The breakpoint conditions determine if execution must stop or continue etc. +(13:55:00) luispm: bjorn3, Likely something GDB thinks is wrong. Does enabling "set debug dwarf*" show anything? +(13:56:01) bjorn3: luispm: no +(13:56:12) bjorn3: for more context: https://github.com/bjorn3/rustc_codegen_cranelift/pull/978 +(13:58:16) osa1 verliet de ruimte (quit: Quit: osa1). +(13:58:28) bjorn3: luispm: wait, for b m<TAB> it shows nothing, but when stepping into a new function it does +(13:58:45) bjorn3: it still doesn't show anything for `info args` though +(13:58:50) bjorn3: No symbol table info available. +(14:00:50) luispm: bjorn3, Is that expected given the nature of the binary? +(14:01:17) bjorn3: b main<TAB> may show nothing as I only set DW_AT_linkage_name and not DW_AT_name +(14:01:24) bjorn3: info args should work though +(14:03:26) luispm: Sorry, I'm not sure what's up. There may be a genuine bug there. +(14:03:41) luispm: tromey (not currently in the channel, but maybe later today) may have more input. +(14:04:08) bjorn3: okay, thanks luispm! +(14:04:27) luispm: In the worst case, reporting a bug may prompt someone to look into that as well. +(14:04:48) luispm: Or send an e-mail to the gdb@sourceware.org mailing list. +(14:05:11) bjorn3: I don't know if it is a bug in gdb, or just me producing (slightly) wrong DWARF +(14:39:40) irker749: gdb: tom binutils-gdb.git:master * 740480b88af / gdb/ChangeLog gdb/darwin-nat.c gdb/inferior.c gdb/inferior.h: Remove iterate_over_inferiors +(15:22:45) irker749: gdb: tromey binutils-gdb.git:master * ecc6c6066b5 / gdb/ChangeLog gdb/dwarf2/read.c gdb/unittests/lookup_name_info-selftests.c: Fix Ada crash with .debug_names +(15:23:13) bjorn3: tromey: ping +(15:23:29) tromey: bjorn3: hey +(15:24:16) bjorn3: I am writing a backend for a compiler which uses DWARF for debuginfo. I unfortunately can't get gdb to show arguments. lldb works fine. +(15:25:13) bjorn3: it just says: No symbol table info available. +(15:25:21) bjorn3: any idea what it could be? +(15:25:34) bjorn3: dwarfdump output: https://gist.github.com/bjorn3/8a34e333c80f13cb048381e94b4a3756 +(15:26:48) bjorn3: more context: https://github.com/bjorn3/rustc_codegen_cranelift/pull/978 +(15:28:05) tromey: offhand I don't know, but if you can send me an executable I can look +(15:28:17) bjorn3: how should I send it? +(15:29:26) tromey: good question +(15:29:41) tromey: you could try emailing it to tromey at adacore.com +(15:29:47) tromey: dunno if that will work or not +(15:30:26) bjorn3: i will try +(15:37:27) bjorn3: tromey: i sent an email with the subject "gdb args not showing" +(15:38:29) tromey: will check now +(15:38:40) bjorn3: thanks! +(15:42:51) irker749: gdb: tdevries binutils-gdb.git:master * de82891ce5b / gdb/ChangeLog gdb/block.c gdb/block.h gdb/symtab.c gdb/testsuite/ChangeLog gdb/testsuite/gdb.base/decl-before-def-decl.c gdb/testsuite/gdb.base/decl-before-def-def.c gdb/testsuite/gdb.base/decl-before-def.exp: [gdb/symtab] Prefer def over decl (inter-CU case) +(15:42:52) irker749: gdb: tdevries binutils-gdb.git:master * 70bc38f5138 / gdb/ChangeLog gdb/symtab.c gdb/testsuite/ChangeLog gdb/testsuite/gdb.base/decl-before-def.exp: [gdb/symtab] Prefer def over decl (inter-CU case, with context) +(15:43:36) tromey: bjorn3: sorry, got distracted. I have the file now +(15:45:35) tromey: my first thing when investigating was to enable complaints +(15:45:37) tromey: so I did +(15:45:40) tromey: set complaints 1000 +(15:45:42) tromey: then +(15:45:51) tromey: file -readnow mini_core_hello_world +(15:46:00) tromey: gdb printed just one style of complaint +(15:46:07) tromey: During symbol reading: missing name for subprogram DIE at 0x3f7 +(15:46:18) tromey: (which is really pretty good, most compilers manage to generate a bunch) +(15:46:29) tromey: and then the gdb DWARF reader says +(15:46:34) tromey: /* Ignore functions with missing or empty names. These are actually +(15:46:34) tromey: illegal according to the DWARF standard. */ +(15:46:34) tromey: if (name == NULL) +(15:46:34) tromey: { +(15:46:37) tromey: complaint (_("missing name for subprogram DIE at %s"), +(15:46:40) tromey: sect_offset_str (die->sect_off)); +(15:46:47) tromey: I wonder if that comment is correct though +(15:47:34) tromey: I guess pedantically maybe it is, DWARF 5 3.3.1 says +(15:47:43) tromey: The subroutine or entry point entry has a DW_AT_name attribute whose value is +(15:47:43) tromey: a null-terminated string containing the subroutine or entry point name. +(15:48:14) bjorn3: i tried set complaints, but it returned complaints for system files. i didn't know about file -readnow. +(15:48:21) tromey: cool +(15:48:26) bjorn3: i will try adding DW_AT_name +(15:48:45) tromey: without readnow unfortunately you get less stuff, because for whatever reason gdb has 2 separate DWARF scanners +(15:49:02) tromey: sort of anyway +(15:49:43) tromey: this seems kind of pedantic of gdb, like if there's a linkage name but no DW_AT_name, then why bail? +(15:50:01) tromey: also what about anonymous functions +(15:50:17) tromey: but anyway this explains the current situation and if you don't mind adding DW_AT_name, then that's probably simplest +(15:51:47) bjorn3: i added DW_AT_name. +(15:51:54) bjorn3: now it says cannot get low and high bounds for subprogram DIE at ... +(15:52:01) tromey: ugh +(15:52:10) bjorn3: i will add DW_AT_low_pc and DW_AT_high_pc +(15:52:15) tromey: /* Ignore functions with missing or invalid low and high pc attributes. */ +(15:52:37) tromey: you can also use DW_AT_ranges +(15:52:55) tromey: if you'd prefer +(15:53:08) bjorn3: already using DW_AT_ranges for DW_TAG_compilation_unit +(15:53:19) bjorn3: for individual functions, there are no gaps +(15:57:07) bjorn3: still the same error with DW_AT_low_pc and DW_AT_high_pc +(15:57:24) bjorn3: tromey: ^ +(15:58:08) tromey: hmmm +(15:58:30) bjorn3: should i send the new executable? +(15:58:31) tromey: send me another executable & I will debug +(15:58:33) tromey: yep +(15:59:23) bjorn3: sent as repy of the previous mail +(16:03:23) tromey: the low PC has DW_FORM_addr, but the high PC has DW_FORM_udata, which seems weird +(16:03:50) mjw: no +(16:03:54) tromey: no? +(16:04:00) mjw: I suggested that for the DWARF standard... +(16:04:05) mjw: sorry +(16:04:58) mjw: The idea was that instead of two relocations and two address wide fields, you have one address and a constant offset. +(16:05:05) tromey: ahh, I see the code now +(16:05:07) tromey: I forgot about this +(16:05:18) tromey: if (cu->header.version >= 4 && attr_high->form_is_constant ()) +(16:05:18) tromey: high += low; +(16:05:36) mjw: that second offset doesn't need a relocation and can often be packed in something small, like an uleb128 +(16:05:51) mjw: using udata might not be ideal though, but is allowed +(16:05:51) tromey: bjorn3: the problem is that this CU claims to be DWARF 3 but is using a DWARF 4 feature +(16:05:58) mjw: aha +(16:05:59) bjorn3: which one? +(16:06:03) ryoshu: hi +(16:06:08) tromey: high_pc (udata) 107 (+0x00000000000011b0 <_ZN21mini_core_hello_world5start17hec55b7ca64fc434eE>) +(16:06:08) tromey: +(16:06:12) ryoshu: just soft ping, I have a queue of patches :) +(16:06:22) tromey: using this as a length requires DWARF 4 +(16:06:36) tromey: for gdb at least it's fine to always emit DWARF 4 +(16:06:44) bjorn3: trying dwarf 4 now +(16:06:48) tromey: I think there are some DWARF 5 features still in the works but DWARF 4 should be solid AFAIK +(16:07:03) tromey: fini +(16:07:08) tromey: lol wrong window +(16:07:56) mjw: Maybe you can accept it for DWARF < 4. But if I remember correctly it might be that people might have been using udata as if it was an address... +(16:08:13) tromey: yeah, I vaguely recall this as well, though I'd expect there to be a comment +(16:08:21) mjw: Cannot really remember why it needed version >= 4. Maybe there was no good reason? +(16:08:32) bjorn3: tromey: it works!!!! thanks for all the help! +(16:08:41) tromey: my pleasure bjorn3 +``` + +</details> |