From 43c47ac44cbdb00c67cb3ed4c751738602d775ca Mon Sep 17 00:00:00 2001
From: bakpakin <calsrose@gmail.com>
Date: Tue, 20 Sep 2022 15:36:03 -0500
Subject: [PATCH] Address #1037 - move stack hack after arg writing logic to
 avoid clobber.

---
 examples/ffi/so.c | 1 +
 src/core/ffi.c    | 8 ++++++--
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/examples/ffi/so.c b/examples/ffi/so.c
index 2549ed06..c60a76e1 100644
--- a/examples/ffi/so.c
+++ b/examples/ffi/so.c
@@ -154,6 +154,7 @@ void void_ret_fn(int x) {
 
 EXPORTER
 int intintint_fn_2(intintint iii, int i) {
+    fprintf(stderr, "iii.a = %d, iii.b = %d, iii.c = %d, i = %d\n", iii.a, iii.b, iii.c, i);
     return i * (iii.a + iii.b + iii.c);
 }
 
diff --git a/src/core/ffi.c b/src/core/ffi.c
index c7ba5185..c93c6ec4 100644
--- a/src/core/ffi.c
+++ b/src/core/ffi.c
@@ -1093,8 +1093,8 @@ static Janet janet_ffi_win64(JanetFFISignature *signature, void *function_pointe
         ret_mem = alloca(type_size(signature->ret.type));
         regs[0].integer = (uint64_t) ret_mem;
     }
-    uint64_t *stack = alloca(signature->stack_count * 8);
-    stack -= 2; /* hack to get proper stack placement */
+    size_t stack_size = signature->stack_count * 8;
+    uint64_t *stack = alloca(stack_size);
     for (uint32_t i = 0; i < signature->arg_count; i++) {
         int32_t n = i + 2;
         JanetFFIMapping arg = signature->args[i];
@@ -1113,6 +1113,10 @@ static Janet janet_ffi_win64(JanetFFISignature *signature, void *function_pointe
         }
     }
 
+    /* hack to get proper stack placement and avoid clobbering from logic above - shift stack down, otherwise we have issues.
+     * Technically, this writes into 16 bytes of unallocated stack memory */
+    if (stack_size) memmove(stack - 2, stack, stack_size);
+
     switch (signature->variant) {
         default:
             janet_panicf("unknown variant %d", signature->variant);