Merge PR #762: align the stack of Linux fibers to the System V ABI (deadlynch)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/platform/linux.c

@@ -270,7 +270,8 @@ int plat_intr_resume(PlatIntr *t, uint64_t to) {
 // ---- fibers: a stack switch of our own. glibc's swapcontext saves and restores the signal mask with a system call
 // on every switch, and the game switches fibers often. fiber_swap saves the callee-saved registers, the MXCSR and the
 // x87 control word on the current stack, stores the stack pointer in *save and loads the other one. A new fiber's
-// stack is laid out as fiber_swap leaves one, returning into fiber_entry, which calls fn(arg) (r12 and r13).
+// stack is laid out as fiber_swap leaves one, returning into fiber_entry, which realigns the stack and calls fn(arg)
+// (r12 and r13).
 struct PlatFiber {
     void *sp;
     void *stack;   // mapped stack (NULL for a thread's own context)
@@ -312,6 +313,11 @@ __asm__(".text\n"
         ".type plat_fiber_entry, @function\n"
         ".p2align 4\n"
         "plat_fiber_entry:\n"
+        // fiber_swap's ret lands here with rsp = top - 8, as at the entry of a called function. The call below must
+        // be made with rsp 16-byte aligned so that fn starts with rsp = 8 mod 16, as the System V ABI demands; without
+        // the realignment every 16-byte local of fn is off by 8, which the VEX instructions of a v3 build tolerate
+        // but the SSE ones of a v2 or k10 build (movdqa, pcmpeqb with a memory operand) fault on.
+        "    andq $-16, %rsp\n"
         "    movq %r13, %rdi\n"
         "    callq *%r12\n"
         "    ud2\n"