Move shadow space reservation to x86_64 makecontext

src/fiber.cr

@@ -108,21 +108,9 @@ class Fiber
 
     fiber_main = ->(f : Fiber) { f.run }
 
-    # FIXME: This line shouldn't be necessary (#7975)
-    stack_ptr = nil
-    {% if flag?(:win32) %}
-      # align stack bottom to 16 bytes
-      @stack_bottom = Pointer(Void).new(@stack_bottom.address & ~0x0f_u64)
-
-      # It's the caller's responsibility to allocate 32 bytes of "shadow space" on the stack right
-      # before calling the function (regardless of the actual number of parameters used)
-
-      stack_ptr = @stack_bottom - sizeof(Void*) * 6
-    {% else %}
-      # point to first addressable pointer on the stack (@stack_bottom points past
-      # the stack because the stack grows down):
-      stack_ptr = @stack_bottom - sizeof(Void*)
-    {% end %}
+    # point to first addressable pointer on the stack (@stack_bottom points past
+    # the stack because the stack grows down):
+    stack_ptr = @stack_bottom - sizeof(Void*)
 
     # align the stack pointer to 16 bytes:
     stack_ptr = Pointer(Void*).new(stack_ptr.address & ~0x0f_u64)

src/fiber/context/x86_64-microsoft.cr

@@ -6,14 +6,15 @@ class Fiber
     # A great explanation on stack contexts for win32:
     # https://web.archive.org/web/20220527113808/https://cfsamson.gitbook.io/green-threads-explained-in-200-lines-of-rust/supporting-windows
 
-    # 8 registers + 3 qwords for NT_TIB + 1 parameter + 10 128bit XMM registers
-    @context.stack_top = (stack_ptr - (12 + 10*2)).as(Void*)
+    # 4 shadow space + (8 registers + 3 qwords for NT_TIB + 1 parameter) + 10 128bit XMM registers
+    @context.stack_top = (stack_ptr - (4 + 12 + 10*2)).as(Void*)
     @context.resumable = 1
 
     # actual stack top, not including guard pages and reserved pages
     LibC.GetNativeSystemInfo(out system_info)
     stack_top = @stack_bottom - system_info.dwPageSize
 
+    stack_ptr -= 4                    # shadow space (or home space) before return address
     stack_ptr[0] = fiber_main.pointer # %rbx: Initial `resume` will `ret` to this address
     stack_ptr[-1] = self.as(Void*)    # %rcx: puts `self` as first argument for `fiber_main`