riscv32::swap: fix stack push/pop asm

src/arch/riscv32.rs

@@ -174,9 +174,9 @@ pub unsafe fn swap(arg: usize, new_sp: StackPointer,
         # from the stack; unwinder uses them to find the CFA of the caller,
         # and so they have to have the correct value immediately after the
         # call instruction that invoked the trampoline.
-        sw      fp, -12(sp)
-        sw      ra, -16(sp)
         addi    sp, sp, -16
+        sw      fp, 4(sp)
+        sw      ra, 0(sp)
         .cfi_offset fp, 4
         .cfi_offset ra, 0
 
@@ -191,13 +191,12 @@ pub unsafe fn swap(arg: usize, new_sp: StackPointer,
         or      a1, zero, sp
         # Load stack pointer of the new context.
         or      sp, zero, a2
-        # Deallocate the 16 bytes
-        addi    sp, sp, 16
 
         # Restore frame pointer and link register of the new context.
         # Load frame and instruction pointers of the new context.
-        lw      fp, -12(sp)
-        lw      ra, -16(sp)
+        lw      fp, 4(sp)
+        lw      ra, 0(sp)
+        addi    sp, sp, 16
 
         # Return into the new context.
         jr      ra