varalloc: system calls do modify the stack pointer

CHANGELOG.md

@@ -15,6 +15,11 @@
   ([PR #888](https://github.com/jasmin-lang/jasmin/pull/888));
   fixes [#887](https://github.com/jasmin-lang/jasmin/issues/887)).
 
+- Fix compilation of functions with system calls but not making other use of
+  the stack
+  ([PR #892](https://github.com/jasmin-lang/jasmin/pull/892));
+  fixes [#870](https://github.com/jasmin-lang/jasmin/issues/870)).
+
 ## Other changes
 
 - The deprecated legacy interface to the LATEX pretty-printer has been removed

compiler/src/varalloc.ml

@@ -425,8 +425,8 @@ let alloc_stack_fd callstyle pd get_info gtbl fd =
 
   let sao_alloc = List.iter (Hv.remove lalloc) fd.f_args; lalloc in
 
-  let sao_modify_rsp = 
-    sao_size <> 0 || ra_on_stack ||
+  let sao_modify_rsp =
+    sao_size <> 0 || has_syscall fd.f_body ||
       Sf.exists (fun fn -> (get_info fn).sao_modify_rsp) sao_calls in
   let sao = {
     sao_calls;

compiler/tests/success/common/bug_870.jazz

@@ -0,0 +1,5 @@
+export
+fn rand(reg ptr u8[32] io) -> reg ptr u8[32] {
+  io = #randombytes(io);
+  return io;
+}