One thing that would make testing infinitely easier is if there were a simulator of a 24-bit byte architecture that we could use in our test suite. It wouldn’t have to simulate the CPU or anything like that – it would just have to let LLDB attach (maybe via the gdb-remote protocol?) and vend information about a very simple core file… enough information that should allow LLDB to backtrace. Then we would just have to validate the resulting backtrace to make sure we haven’t regressed the fundamentals of unusual byte sizes.
Sean > On Aug 29, 2014, at 9:17 AM, Todd Fiala <tfi...@google.com> wrote: > > > "You can't possibly do that, so many other architectures have 8-bit bytes, > > and so this proposal would make them harder to enhance, for the benefit of > > (currently) just kalimba" > > From a design principle, I would probably vote for having non-8-bit-bytes > added as a layer on top of memory accesses that is only used for systems that > need it. I'd strongly prefer not to have the generality of n-bit (or > multiple of 8-bit bytes) permute throughout the lldb code base. I can see > that becoming challenging from a maintainability standpoint (testing, code > complexity, etc.) > > I could imagine a layer you build on top of memory accesses that knows how to > run out and grab the right underlying cpu_arch-specific bytes to satisfy the > request. And only wiring that in to low level memory access maybe in the > memory read/write virtual methods (like Process::DoReadMemory() and friends). > > That's just my take. Maintainability and testability being the key driver > here. > > > On Fri, Aug 29, 2014 at 5:11 AM, Matthew Gardiner <m...@csr.com > <mailto:m...@csr.com>> wrote: > Based on some recent investigation, it looks as if I won't need to modify the > CoreDefinition structure of ArchSpec.cpp. In a local change, I've added > specification for the kalimba variants in the SubArchType of llvm::Triple. So > it's now possible for me to implement > > uint32_t ArchSpec::GetCodeByteSize() const > uint32_t ArchSpec::GetDataByteSize() const > > by inspection of subarch field of the triple contained in ArchSpec. > > However, I'd still appreciate some feedback on a more conceptual level > regarding this proposal. > > thanks > Matt > > Matthew Gardiner wrote: > Hi folks, > > One of the challenges that I need to resolve regarding debugging kalimba > processors, is that certain variants have different notions of the size (in > bits) of a byte, compared to a lot of more mainstream processors. What I'm > referring to is the size of a minimum addressable unit, when the processor > accesses memory. For example, on a kalimba architecture version 3, a "byte" > (minimum addressable unit) from the data bus is 24-bits, so if the processor > reads from address 8001 it reads 24-bits, and from address 8002 the next > 24-bits are read, and so on... (this also means that for this variant a char, > int, long, pointer are 24-bits in size). For kalimba architecture version 4, > however, we have the minimum addressable unit being 8-bits, and > correspondingly more "conventional" sizes for primitive types. > > I imagine that this will effect the kalimba lldb port is various ways. The > most obvious one, and hence the one I'd like to solve first, is that way in > which raw memory read/write are implemented. As an example when I ask lldb to > read 4 "bytes" (addressable units worth of data) from a kalimba with 8-bit > bytes I expect to see this: > > (lldb) memory read --count 4 0x0328 > 0x00000328: 00 07 08 08 .... > (lldb) > > However if target processor has 24-bit bytes then I expect the same query to > yield the following answer: > > (lldb) memory read --count 4 0x0328 > 0x00000328: 000708 080012 095630 023480 > .... > (lldb) > > Just considering the above scenario leads me to believe that my first > challenge is arranging for the remote protocol implementation (currently > Process/gdb-remote et al) to assume Nx host bytes (N being a target-specific > value) for each target byte accessed, and for the memory read and formatting > code (above) to behave correctly, given the discrepancy between host and > target byte sizes. I guess I'll see many other challenges - for example, > frame variable decode, stack unwind etc. (but since *those* challenges > require work on clang/llvm backend, and CSR have no llvm person yet, I want > to concentrate on raw memory access first...) > > For an added complication (since kalimba is a harvard architecture) certain > kalimba variants have differing addressable unit sizes for memory on the code > bus and data bus. Kalimba Architecture 5 has 8-bit addressable code, and > 24-bit addressable data... > > My initial idea for how to start to address the above challenge is to augment > the CoreDefinition structure in ArchSpec.cpp as follows: > > struct CoreDefinition > { > ByteOrder default_byte_order; > uint32_t addr_byte_size; > uint32_t min_opcode_byte_size; > uint32_t max_opcode_byte_size; > + uint32_t code_byte_size; > + uint32_t data_byte_size; > llvm::Triple::ArchType machine; > ArchSpec::Core core; > const char * const name; > }; > > Where code_byte_size and data_byte_size would specify the size in host > (8-bit) bytes the sizes of the minimum addressable units on the referenced > architectures. So, e.g. > For kalimba 3, with 24-bit data bytes and 32-bit code bytes we'd have > data_byte_size=3 and code_byte_size=4 > For kalimba 4, with 8-bit data bytes and 8-bit code bytes we'd have > data_byte_size=1 and code_byte_size=1 > > So, then I'd update the g_core_definitions array within ArchSpec.cpp > accordingly, such that all non-kalimbas would have 1 as the setting for the > new datas and the kalimba entries would have those fields made to match the > architectures. > > The ArchSpec class would then require the following accessors: uint32_t > GetCodeByteSize() and uint32_t GetDataByteSize(); to supply client code with > the required hints to correctly implement memory accesses. > > My next plan would be to "massage" the code in the execution flow from an > (lldb) memory read invocation through to the gdb-remote comms until I see the > memory read examples I illustrated above, working for 8-bit and 24-bit data > kalimba targets. > > I'd appreciate all comments and opinions as to what I've described above from > the lldb community. Basically, I'm curious as to what people think of the > whole concept, e.g. > > "You can't possibly do that, so many other architectures have 8-bit bytes, > and so this proposal would make them harder to enhance, for the benefit of > (currently) just kalimba" > "Yes, that's a good idea, lldb can accommodate the most unusual of > architectures" > > And I'm also interested in technical comments, e.g. should an instance of > CoreDefinition be added to ArchSpec, or is just adding the extra byte-size > attributes sufficient... or if anyone thinks that modifying gdb-remote is a > bad idea, and that I should be creating kalimba process abstractions (and > factor out the common code)? > > thanks > Matt > > > > Member of the CSR plc group of companies. CSR plc registered in England and > Wales, registered number 4187346, registered office Churchill House, > Cambridge Business Park, Cowley Road, Cambridge, CB4 0WZ, United Kingdom > More information can be found at www.csr.com <http://www.csr.com/>. 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