[llvm-dev] RFC: Debug info for Cuda

Hal Finkel via llvm-dev llvm-dev at lists.llvm.org
Tue Nov 7 08:04:35 PST 2017

Hi, Alexey,

Thanks a bunch for working on this. A couple quick questions below...

On 11/06/2017 12:37 PM, Alexey Bataev 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.

Is this a change? It looks like this is already the behavior of the 
driver's CUDA target code:

   if (Args.hasFlag(options::OPT_cuda_noopt_device_debug,
                    options::OPT_no_cuda_noopt_device_debug, false)) {
     // ptxas does not accept -g option if optimization is enabled, so
     // we ignore the compiler's -O* options if we want debug info.
   } else if (Arg *A = Args.getLastArg(options::OPT_O_Group)) {
     // Map the -O we received to -O{0,1,2,3}.

>     •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.

Can you clarify what "unable to translate" mean? Does it refuse to 
compile the code, drop all debug info, drop all debug info except for 
line-table information, something else?

Thanks again,

> 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.

Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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