[llvm-dev] RFC: Debug info for Cuda

[Justin Lebar via llvm-dev]

(Reply-all'ing this time.)

This all seems pretty reasonable to me, although I guess as we say, the
proof is in the patches.  :)

On Mon, Nov 6, 2017 at 10:43 AM Alexey Bataev via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi everybody,
>
> As you know, Cuda/NVPTX target has very limited support of the debug info
> in Clang/LLVM. Currently, LLVM supports only emission of the line numbers
> debug info.
>
> This is caused by limitations of the Cuda/NVPTX codegen. Clang/LLVM
> translates the source code to LLVM IR, which is then lowered to PTX
> (parallel thread execution) intermediate file. This PTX file represents
> special kind of the assembler code in text format, which contains the code
> itself + (possibly) debug info. Then this PTX file is compiled by ptxas
> tool into the CUDA binary representation.
>
>
> Debug info representation in PTX file.
> ========================
>
> According to PTX Writer's Guide to Interoperability, Debug information (
> http://docs.nvidia.com/cuda/ptx-writers-guide-to-interoperability/index.html#debug-information)
> , debug information must be encoded in DWARF (Debug With Arbitrary Record
> Format). The responsibility for generating debug information is split
> between the PTX producer and the PTX-to-SASS backend. The PTX producer is
> responsible for emitting binary DWARF into the PTX file, using the .section
> and .b8-.b16-.b32-and-.b64 directives in PTX. This should contain the
> .debug_info and .debug_abbrev sections, and possibly optional sections
> .debug_pubnames and .debug_aranges. These sections are standard DWARF2
> sections that refer to labels and registers in the PTX.
>
> The PTX-to-SASS backend is responsible for generating the .debug_line
> section from the .file and .loc directives in the PTX file. This section
> maps source lines to SASS addresses. The PTX-to-SASS backend also generates
> the .debug_frame section.
>
> LLVM is able to emit debug info in DWARF. But ptxas compiler has some
> limitations, that make it hard to adapt LLVM for correct emission of the
> debug info in PTX files.
>
>
> Limitations/features of the PTX format/ptxas compiler.
> ==================================
>
> a) Supports DWARF-2 only.
> b) Labels are allowed only in code section (only in functions).
> c) Does not support label arithmetic in DWARF sections.
>     “.b32 L1 – L2” as the size of the section is not allowed, so the
> sections sizes should be calculated explicitly.
> d) Debug info must point to the sections, not to labels inside these
> sections.
>     “.b32 .debug_abbrevs”
> e) Sections itself must be enclosed into braces
>     “.section .debug_info {…}”
> f) Frame info is non-register based
>     Based on function local “__local_depot” array, that represents the
> stack frame.
> g) All variables must have non-standard DW_AT_address_class attribute so
> the debuger had the info about address class of the variable - global or
> local. DWARF standard does support this attribute, but it can be appiled to
> pointer/reference types only, not variables.
> h) The first label in the function must follow the debug location macro.
> In LLVM, it is followed by the debug location macro.
> i) .debug_frame section is emitted by txas compiler.
>     DW_AT_frame_base must be set to dwarf::DW_FORM_data1
> dwarf::DW_OP_call_frame_cfa value.
> j) Strings cannot be referenced by the labels, instead they must be
> inlined in the sections in form of array of chars.
>
> Some changes in LLVM are required to support all these limitation/features
> in the output PTX files.
>
> Required changes in LLVM.
> ==================
>
> •include/llvm/CodeGen/AsmPrinter.h.
>     •Add “virtual MCSymbol *getFunctionFrameSymbol(const MachineFunction
> *MF) const” for non-register-based frame info.
>     •Override “NVPTXMCAsmPrinter.cpp” to return the name of the
> “__local_depot” frame storage.
> •Add ”cuda-gdb” specific tuning.
>     •Inlined strings must be used in sections, not string references.
>     •Label arithmetic is replaced by the absolute section size evaluation.
>     •Use “AsmPrinter::doInitialization()” instead of NVPTX-custom manual
> initialization.
>     •Local variables address emitted as “__local_depot” + <var offset>.
> •Add NVPTX specific “NVPTXMCAsmStreamer” class.
>     •Requires moving to includes of “MCAsmStreamer” class declaration.
>     •Overrides emission of the labels (names of the section are emitted
> instead).
>     •Overrides emission of the sections (emit braces)
>     •Overrides string emission (as sequence of bytes, not as strings)
>     •Overrides emission of files/locations debug info
>
> Required changes in Clang.
> =================
>
> •Add option “-gcuda-gdb” to driver.
>     •Emit cuda-gdb compatible debug info (DWARF-2 by default + CudaGDB
> tuning).
> •Add options “-g --dont-merge-basicblocks --return-at-end” to “ptxas” call.
>     •ptxas is able to translate debug information only if -O0 optimization
> level is used. It means, that we can use optimization level in LLVM > O0,
> but still have to use O0 when calling ptxas compiler.
>
>
> This approach was implemented in https://github.com/clang-ykt to support
> debug info emission for NVPTX target when generating code for OpenMP
> offloading constructs. You can try to use it.
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