Fix register metadata for Win64 ABI

rbx is normally preserved in the Win64 ABI (as for Unix), but OCaml 5
uses it to save the OCaml stack pointer for direct (noalloc) calls. The
fact rbx is clobbered at C call was incorrectly missing in the Win64
register descriptions.

2 files changed, 6 insertions, 3 deletions

diff --git a/Changes b/Changes
index ff889243d3..95a1dbd8d1 100644
--- a/Changes
+++ b/Changes
@@ -619,6 +619,9 @@ Working version
   (Jérôme Vouillon, review by Nicolás Ojeda Bär, Stephen Dolan and
    Hugo Heuzard)
 
+- #11846: Mark rbx as destroyed at C call for Win64 (mingw-w64 and Cygwin64).
+  (David Allsopp, review by KC Sivaramakrishnan)
+
 - #11850: When stopping before the `emit` phase (using `-stop-after`), an empty
   temporary assembly file is no longer left in the file system.
   (Nicolás Ojeda Bär, review by Gabriel Scherer and Xavier Leroy)
diff --git a/asmcomp/amd64/proc.ml b/asmcomp/amd64/proc.ml
index 56377b322c..8a46c05f48 100644
--- a/asmcomp/amd64/proc.ml
+++ b/asmcomp/amd64/proc.ml
@@ -291,12 +291,12 @@ let destroyed_at_c_call =
      by the code sequence used for C calls in emit.mlp, so it
      is marked as destroyed. *)
   if win64 then
-    (* Win64: rbx, rsi, rdi, r12-r15, xmm6-xmm15 preserved *)
+    (* Win64: rsi, rdi, r12-r15, xmm6-xmm15 preserved *)
     Array.of_list(List.map phys_reg
-      [0;4;5;6;7;10;11;12;
+      [0;1;4;5;6;7;10;11;12;
        100;101;102;103;104;105])
   else
-    (* Unix: rbx, r12-r15 preserved *)
+    (* Unix: r12-r15 preserved *)
     Array.of_list(List.map phys_reg
       [0;1;2;3;4;5;6;7;10;11;
        100;101;102;103;104;105;106;107;