Now also documented at https://docs.microsoft.com/en-us/windows/desktop/Debug/pe-format#section-data :

"The presence of an entry of type IMAGE_DEBUG_TYPE_REPRO indicates the PE file is built in a way to achieve determinism or reproducibility. If the input does not change, the output PE file is guaranteed to be bit-for-bit identical no matter when or where the PE is produced. Various date/time stamp fields in the PE file are filled with part or all the bits from a calculated hash value that uses PE file content as input, and therefore no longer represent the actual date and time when a PE file or related specific data within the PE is produced. The raw data of this debug entry may be empty, or may contain a calculated hash value preceded by a four-byte value that represents the hash value length."

Looks like link.exe writes a 0-sized entry with /debug and a long hash without. Just always doing the 0-sized things for lld-link is probably fine.

I'll try to make a patch.

Looks like llvm even knows the value already, see https://reviews.llvm.org/D20885?id=59297#inline-176515 , https://reviews.llvm.org/D20885 , r271549

https://reviews.llvm.org/D51652

https://reviews.llvm.org/rL341486 / https://reviews.llvm.org/rL341485