On Fri, 7 Mar 2025 20:46:32 GMT, Mikael Vidstedt <[email protected]> wrote:
> Background (from JBS):
>
> Recent versions of Xcode, most likely starting with Xcode 15, do not produce
> deterministic binaries/libraries out of the box. In particular, the UUID data
> in the LC_UUID load command is not stable.
>
> After spending some significant time trying to understand why, it turns out
> that the linker has started including the last modified time stamp of the
> object files in the (debug?) data and that time stamp data is included in the
> computed UUID "hash". So even if the rest of the data is the same between two
> builds, the build time (or, more accurately, the time when the object file
> gets written to the file system) now affects the UUID and therefore the data
> of the resulting .dylib files.
>
> It seems (unconfirmed) like this may be an artifact of Xcode 15 using a new
> linker ("ld-prime") which may or may not be why the behavior has changed.
>
> After spending even more time trying to understand what to do about it I
> noticed this in the ld man page:
>
> -reproducible
> By default output content will be deterministic, but small
> changes in input files such as a compilation time might affect certain data
> structures in the linked binary. This option instructs ld to create a
> reproducible output binary by ignoring certain input properties or using
> alternative algorithms.
>
> And, adding that flag (-Wl,-reproducible to clang) does indeed seem to solve
> the issue.
>
> Testing:
>
> * Manual verification with both Xcode14.3.1 and Xcode 15.4
> * tier1,builds-tier[2-5]
Marked as reviewed by erikj (Reviewer).
-------------
PR Review: https://git.openjdk.org/jdk/pull/23949#pullrequestreview-2670834186
