Fix stack probing in the singlepass compiler

Stack probes must be done before the stack pointer is adjusted. This
ensures that the stack pointer is still within the bounds of the stack
when inspected by the signal handler.

lib/compiler-singlepass/src/codegen.rs

@@ -510,6 +510,18 @@ impl<'a, M: Machine> FuncGen<'a, M> {
 
         // Allocate save area, without actually writing to it.
         static_area_size = self.machine.round_stack_adjust(static_area_size);
+
+        // Stack probe.
+        //
+        // `rep stosq` writes data from low address to high address and may skip the stack guard page.
+        // so here we probe it explicitly when needed.
+        for i in (sig.params().len()..n)
+            .step_by(NATIVE_PAGE_SIZE / 8)
+            .skip(0)
+        {
+            self.machine.zero_location(Size::S64, locations[i]);
+        }
+
         self.machine.adjust_stack(static_area_size as _);
 
         // Save callee-saved registers.
@@ -586,17 +598,6 @@ impl<'a, M: Machine> FuncGen<'a, M> {
             Location::GPR(self.machine.get_vmctx_reg()),
         );
 
-        // Stack probe.
-        //
-        // `rep stosq` writes data from low address to high address and may skip the stack guard page.
-        // so here we probe it explicitly when needed.
-        for i in (sig.params().len()..n)
-            .step_by(NATIVE_PAGE_SIZE / 8)
-            .skip(1)
-        {
-            self.machine.zero_location(Size::S64, locations[i]);
-        }
-
         // Initialize all normal locals to zero.
         let mut init_stack_loc_cnt = 0;
         let mut last_stack_loc = Location::Memory(self.machine.local_pointer(), i32::MAX);