BigObj COFF Object Files: Binary Structure Explained

I recently implemented BigObj COFF file parsing in cgo ( golang/go#24341 ). In the process, I quickly discovered that Microsoft doesn&rsquo;t document the binary format anywhere. Their official documentation is the only reference they have to BigObj as far as I can tell, and it doesn&rsquo;t say anything about the binary format. I didn&rsquo;t see any other blogs or resources covering this topic…