Fix amd64 restore callback stack pointer

Restore the sysv_amd64 callback stack-pointer fix so STACKMAN_OP_RESTORE receives the active switched stack pointer, not the pre-switch value.

Update both the assembly and inline GCC x86_64 paths for consistent callback behavior.

Add a regression assertion in tests/test.c and document the fix in CHANGELOG.md under Unreleased.

Author: kristjanvalur

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+- Fixed amd64 (sysv_x86_64 GCC) restore callback stack-pointer argument to pass the active switched stack pointer
+- Added assertion coverage in `tests/test.c` to detect incorrect restore callback stack pointer on amd64
+
 ## [1.2.0] - 2025-11-16
 
 ### Added

stackman/platforms/switch_x86_64_gcc.S

@@ -93,7 +93,7 @@ LABEL(stackman_switch)
 	addq %rax,  %rbp
 # 0 "" 2
 #NO_APP
-	movq	%rbx, %rdx
+	movq	%rsp, %rdx
 	movl	$1, %esi
 	movq	%r15, %rdi
 	call	*%r14

stackman/platforms/switch_x86_64_gcc.h

@@ -67,7 +67,7 @@ void *STACKMAN_SWITCH_INASM_NAME(stackman_cb_t callback, void *context)
 	__asm__ ("movq %[result], %%rsp" :: [result] "r" (stack_pointer2));
 	__asm__ ("addq %[arg],  %%rbp" :: [arg] "r" (diff));
 
-	stack_pointer = callback(context, STACKMAN_OP_RESTORE, stack_pointer);
+	stack_pointer = callback(context, STACKMAN_OP_RESTORE, stack_pointer2);
 	/* restore non-volatile registers from stack */
 	__asm__ volatile (
         "ldmxcsr %[sr]\n\t"

tests/test.c

@@ -102,6 +102,8 @@ void *jmp_cb(void* context, int opcode, void *sp)
 	if (opcode == (int)STACKMAN_OP_SAVE) {
 		return c->stack_near;
 	} else {
+		/* RESTORE must receive the active switched stack pointer. */
+		assert(sp == c->stack_near);
 		restore_stack(c->stack_near, c->buf, c->size);
 		return sp;
 	}