add xrt::elf raw elf data constructor

[HimanshuChoudhary-Xilinx]

Problem solved by the commit

Added new xrt::elf constructor to take in-memory buffer as input

Bug / issue (if any) fixed, which PR introduced the bug, how it was discovered

NA

How problem was solved, alternative solutions (if any) and why they were rejected

added new constructor to take memory elf buffer

Risks (if any) associated the changes in the commit

None

What has been tested and how, request additional testing if necessary

taken multiple efl and check we are able to get correct xrt::elf object

 // Sample usage
  aiebu::aiebu_assembler as(aiebu::aiebu_assembler::buffer_type::blob_instr_dpu, v1, v2, patch_data);
  auto e = as.get_elf();

  std::istringstream elf_stream;
  elf_stream.rdbuf()->pubsetbuf(e.data(), e.size());
  xrt::elf elf{elf_stream};
  xrt::module mod{elf};

Documentation impact (if any)

Yes, we have to update XRT doc

[gbuildx]

HimanshuChoudhary-Xilinx - is not a collaborator
Can XRT admins please validate PR

[gbuildx]

Can one of the admins verify this patch?

[gbuildx]

HimanshuChoudhary-Xilinx - is not a collaborator
Can XRT admins please validate PR

[gbuildx]

HimanshuChoudhary-Xilinx - is not a collaborator
Can XRT admins please validate PR

[sonals]

ok to test

[chvamshi-xilinx]

ok to test/retest is no longer working. We need to go to actions and run all jobs again. Started run just now

[sonals]

We are copying the vector here. Can we write a simple adapter for std::vector to make it look like std::istream?

[dezhiAmd]

Someone did this:
https://stackoverflow.com/questions/16875321/stdistringstream-from-stdstring-without-copying

[HimanshuChoudhary-Xilinx]

fixed

[sonals]

The const_cast looks evil here. Can you add a comment why we are doing it and the fact that elf_stream is input stream (read only) so it cannot accidentally change data.

[HimanshuChoudhary-Xilinx]

since data is const type so data.data() return const char*
https://cplusplus.com/reference/vector/vector/data/
If the vector object is const-qualified, the function returns a pointer to const value_type. Otherwise, it returns a pointer to value_type.
while
streambuf* pubsetbuf (char* s, streamsize n);
https://cplusplus.com/reference/streambuf/streambuf/pubsetbuf/

[stsoe]

A constructor from std::vector isn't great IMO. What if data is not held in a vector? A std::span would be better, but that's c++20. I would opt for const char*, size_t but you are still faced with the problem of how to create the istream without copying the in-memory buffer.

[HimanshuChoudhary-Xilinx]

@stsoe
Updated the PR with changes discussed offline.