[llvm-dev] addition of vendor dwarf operator extension.

Tue Dec 3 21:32:43 PST 2019

Below is a snippet of the dwarf info that we generate,

0x00000057:   DW_TAG_variable
                DW_AT_name      ("__gbloffset__")
                DW_AT_type      (0x000001e5 "int")
                DW_AT_external  (true)
                DW_AT_decl_file ("…")
                DW_AT_decl_line (8)
                DW_AT_location  (DW_OP_addr 0) // pre linkage
                DW_AT_linkage_name      ("_gblsection__")
                DW_AT_endianity (DW_END_big)

0x00000170:   DW_TAG_variable
                DW_AT_name      ("VAR1")
                DW_AT_type      (0x0000010b "fixed.dec.display.72")
                DW_AT_decl_file ("…")
                DW_AT_decl_line (10)
                DW_AT_location  (DW_OP_addr 0x0, DW_OP_call4 0x57, DW_OP_deref_size, 4,
DW_OP_dup, DW_OP_constu 0xff, DW_OP_lit0, DW_OP_shl, DW_OP_and, DW_OP_lit24, DW_OP_shl, DW_OP_swap, DW_OP_dup, DW_OP_constu 0xff, DW_OP_lit8, DW_OP_shl, DW_OP_and, DW_OP_lit8, DW_OP_shl, DW_OP_swap, DW_OP_dup, DW_OP_constu 0xff, DW_OP_lit16, DW_OP_shl, DW_OP_and, DW_OP_lit8, DW_OP_shr, DW_OP_swap, DW_OP_constu 0xff, DW_OP_lit24, DW_OP_shl, DW_OP_and, DW_OP_lit24, DW_OP_shr, DW_OP_swap, DW_OP_or, DW_OP_or, DW_OP_or, DW_OP_plus)
                DW_AT_linkage_name      ("VAR1")

In above code for location calculation of “VAR1” we change the endianity of “__gbloffset__” var, it is 32 bit value on 64-bit machine.

In above case DW_OP_byte_swap will replace near about 38 operations(for 32 bit value), that’s 38-bytes per usage savings on mem footprint and at runtime as well. I do not have any profiling data.

The use case for DW_OP_byte_swap is, for typed entry we know the size, for raw data, dwarf operator will consider the generic type and pop the top of the stack, byte swap it and push the data back on the stack.

Regards,

Chirag Patel
Software Engineer | Raincode Labs India
Tel: (+91) 080 41159811
Mob: (+91) 9049336744
www.raincodelabs.com<http://www.raincodelabs.com/>
[linkedin-button]<https://in.linkedin.com/in/chirag-patel->

From: David Blaikie <dblaikie at gmail.com>
Sent: 03 December 2019 21:31
To: Chirag Patel <Chirag at raincode.com>
Cc: Adrian Prantl <aprantl at apple.com>; Paul Robinson <paul.robinson at sony.com>; Jonas Devlieghere <jdevlieghere at apple.com>; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] addition of vendor dwarf operator extension.

Does this have a significant impact on total DWARF size or performance that you've observed/have data about?

On Tue, Dec 3, 2019 at 12:06 AM Chirag Patel <Chirag at raincode.com<mailto:Chirag at raincode.com>> wrote:
Hello,

Thanks for prompt reply.

Motivation,
In case of debugging of bi-endian dwarf and value known at runtime some of the raw values needs to be byte swapped to change the endianity. We have a llvm based compiler which generates such dwarf, TAG_variable marked as big endian and debugging on little endian host using lldb. The listed operator on dwarf issues does that, one operator instead of list of operators.

Proposal,
Addition of listed dwarf operator as vender extension in llvm/lldb till it gets released.

Regards,

Chirag Patel
Software Engineer | Raincode Labs India
Tel: (+91) 080 41159811
Mob: (+91) 9049336744
www.raincodelabs.com<http://www.raincodelabs.com/>
[linkedin-button]<https://in.linkedin.com/in/chirag-patel->

From: David Blaikie <dblaikie at gmail.com<mailto:dblaikie at gmail.com>>
Sent: 02 December 2019 19:51
To: Chirag Patel <Chirag at raincode.com<mailto:Chirag at raincode.com>>; Adrian Prantl <aprantl at apple.com<mailto:aprantl at apple.com>>; Paul Robinson <paul.robinson at sony.com<mailto:paul.robinson at sony.com>>; Jonas Devlieghere <jdevlieghere at apple.com<mailto:jdevlieghere at apple.com>>
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] addition of vendor dwarf operator extension.

There are some vendor extensions already in LLVM/LLDB, though I guess the main question would be: what's the motivation?

On Mon, Dec 2, 2019 at 3:14 AM Chirag Patel via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hello all,

There is one enhancement request open for dwarfv5, http://dwarfstd.org/ShowIssue.php?issue=191107.1
The request is for addition of dwarf expression operator to swap the top of the dwarf stack, the response seems positive but it may take some time till v6.
I like to add that operator as vendor extension but I am not sure how to go about it for llvm/lldb. Currently I am using it as vendor specific extension on our local fork.

Does adding it as vender specific extension seems like a good idea? If yes please provide me the details about adding specific vendor.

Regards,

Chirag Patel
Software Engineer | Raincode Labs India
Tel: (+91) 080 41159811
Mob: (+91) 9049336744
www.raincodelabs.com<http://www.raincodelabs.com/>
[linkedin-button]<https://in.linkedin.com/in/chirag-patel->

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