Simplify LZMA decoding by using ArrayRef::take_back

[kwk]

Summary

Follow-up for D66791#inline-616303

[labath]

lldb/source/Host/common/LZMA.cpp

99

I think this should be + to preserve semantics, though I am not sure why no tests fail because of that. I guess none of the lzma streams we have have non-zero backward_size..

[kwk]

Fixup

[kwk]

lldb/source/Host/common/LZMA.cpp

99

Sorry, tests are still running... and you're right.

[labath]

LGTM. I don't know if there's a reasonable way to add a test with a non-zero backward_size, but if you can think of something, please add one.

lldb/source/Host/common/LZMA.cpp

return llvm::createStringError(
         llvm::inconvertibleErrorCode(),
         "size of xz-compressed blob (%lu bytes) is smaller than the "
         "LZMA_STREAM_HEADER_SIZE (%lu bytes)",
         InputBuffer.size(), LZMA_STREAM_HEADER_SIZE);
   }
 
   // Decode xz footer.
   lzma_ret xzerr = lzma_stream_footer_decode(
-      &opts, InputBuffer.data() + InputBuffer.size() - LZMA_STREAM_HEADER_SIZE);
+      &opts, InputBuffer.take_back(LZMA_STREAM_HEADER_SIZE).data());
   if (xzerr != LZMA_OK) {
     return llvm::createStringError(llvm::inconvertibleErrorCode(),
                                    "lzma_stream_footer_decode()=%s",
                                    convertLZMACodeToString(xzerr));
   }
   if (InputBuffer.size() < (opts.backward_size + LZMA_STREAM_HEADER_SIZE)) {
     return llvm::createStringError(
         llvm::inconvertibleErrorCode(),
         "xz-compressed buffer size (%lu bytes) too small (required at "
         "least %lu bytes) ",
         InputBuffer.size(), (opts.backward_size + LZMA_STREAM_HEADER_SIZE));
   }
 
   // Decode xz index.
   lzma_index *xzindex;
   uint64_t memlimit(UINT64_MAX);
   size_t inpos = 0;
-  xzerr =
-      lzma_index_buffer_decode(&xzindex, &memlimit, nullptr,
-                               InputBuffer.data() + InputBuffer.size() -
-                                   LZMA_STREAM_HEADER_SIZE - opts.backward_size,
-                               &inpos, InputBuffer.size());
+  xzerr = lzma_index_buffer_decode(
+      &xzindex, &memlimit, nullptr,
+      InputBuffer.take_back(LZMA_STREAM_HEADER_SIZE + opts.backward_size)
+          .data(),
   if (xzerr != LZMA_OK) {
     return llvm::createStringError(llvm::inconvertibleErrorCode(),
                                    "lzma_index_buffer_decode()=%s",
                                    convertLZMACodeToString(xzerr));
   }
 
   // Get size of uncompressed file to construct an in-memory buffer of the
   // same size on the calling end (if needed).