Both comments predated the marker, and now neither does

lib/build.ml

@@ -744,18 +744,20 @@ let executable ?(opts = default) ?(csrcs = []) ?(lflags = []) ?(pnames = [])
      by this backend — and the module that would redefine through them arrives
      with the lane that writes it.
 
-     That lane inherits one thing this comment should say out loud rather than
+     [X86.redefinition] is that lane, and it has landed, so the paragraph this
-     leave for it to find. [x86.ml]'s header licenses its own calling
+     comment used to end with — that no counterpart existed — is no longer
-     convention on the grounds that a dev build is compiled entirely by it and
+     true. What remains true is why it had to be written here rather than
-     a release build entirely by LLVM, so the two never meet in one process.
+     borrowed from LLVM. [x86.ml]'s header licenses its own calling convention
-     Publishing a cell an LLVM-built module can store into is the first thing
+     on the grounds that a dev build is compiled entirely by it and a release
-     that could make that false: the two conventions agree on scalars and
+     build entirely by LLVM, so the two never meet in one process. Publishing
-     disagree on every aggregate, so an [Emit.redefinition] module dlopened
+     a cell an LLVM-built module can store into is the first thing that could
-     into an [--x86] host would be correct until the first redefined function
+     make that false: the two conventions agree on scalars and disagree on
-     took or returned a struct. Nothing in the toolchain does that today —
+     every aggregate, so an [Emit.redefinition] module dlopened into an
-     [flan reload] and [flan dev] both build host and module through LLVM —
+     [--x86] host is correct until the first redefined function takes or
-     and the fix when something does is to emit the module through this
+     returns a struct. That pair is now refused at [dlopen] by a marker symbol
-     backend too, not to grow a classifier. *)
+     each backend defines and each backend's module references — see [shared]
+     below — rather than left to die at the call. The answer was to emit the
+     module through this backend too, and never to grow a classifier. *)
   (* [--debug] used to be in this list too. It is not any more: [x86.ml] emits
      a compile unit, a subprogram per function and a line table, all written
      out as bytes because [.loc] cannot work against a file whose instructions
@@ -911,12 +913,17 @@ let shared ?(opts = default) ~ir ~out () : timing =
      than it is is worse than none. It catches a caller holding one option
      record and reaching for the wrong builder. It does not catch a caller
      holding two and picking the wrong one — the crossed pair that was measured
-     segfaulting passes this check and still segfaults, because it hands an
+     segfaulting passes this check, because it hands an LLVM record to the LLVM
-     LLVM record to the LLVM builder and simply loads the result into an x86
+     builder and simply loads the result into an x86 host. [flan reload] is
-     host. [flan reload] is precisely that caller. The complete answer is a
+     precisely that caller, and it is why this guard was never the whole
-     marker symbol the host defines and a module references, so the loader
+     answer. The whole answer is the marker symbol: a dev build defines
-     refuses the pair at dlopen rather than the processor refusing it at a
+     [flan.abi.x86] or [flan.abi.llvm] according to which backend emitted it,
-     call. See docs/handoffs/HANDOFF-x86-aggregates.md. *)
+     each backend's redefinition module holds a pointer to its own, and the
+     loader has to resolve that pointer while it maps the object — so a crossed
+     pair is refused at [dlopen], naming both backends, before any new body
+     runs. That landed; this check is the cheap first line rather than the only
+     one. See docs/handoffs/HANDOFF-x86-aggregates.md and
+     docs/handoffs/HANDOFF-x86-abi-marker.md. *)
   if opts.x86 then
     failwith
       "--x86: Build.shared is the LLVM redefinition path, and an --x86 host \