x86-64: Add alloca.

Makefile

@@ -177,6 +177,10 @@ VPATH+=win32/lib
 endif
 ifeq ($(ARCH),i386)
 LIBTCC1_OBJS+=alloca86.o alloca86-bt.o
+else
+ifeq ($(ARCH),x86-64)
+LIBTCC1_OBJS+=alloca86_64.o
+endif
 endif
 
 %.o: %.c

lib/alloca86_64.S

@@ -0,0 +1,33 @@
+/* ---------------------------------------------- */
+/* alloca86_64.S */
+
+#include "../config.h"
+
+.globl alloca
+
+alloca:
+    pop     %rdx
+    mov     %rdi,%rax
+    add     $3,%rax
+    and     $-4,%rax
+    jz      p3
+
+#ifdef TCC_TARGET_PE
+    /* XXX: not tested */
+p1:
+    cmp     $4096,%rax
+    jle     p2
+    sub     $4096,%rsp
+    sub     $4096,%rax
+    test    %rax,(%rsp)
+    jmp p1
+p2:
+#endif
+
+    sub     %rax,%rsp
+    mov     %rsp,%rax
+p3:
+    push    %rdx
+    ret
+
+/* ---------------------------------------------- */