Xcode 11: stack_not_16_byte_aligned_error with AVX code

Question

Created Aug ’19

Replies 16

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Participants 8

There seems to be a regression in Xcode 11 beta, when AVX is enabled. The C library doesn't make the same assertions regarding alignment as the compiler.

Here is a C-reduced test case:

 #include 
#include 
#include 
#include 
#include 
 
typedef struct {
    char  d[16];
    void *e;
    struct { char b[5536]; } f;
} i;
 
void g(void) {
    struct addrinfo hints, *k;
    memset(&hints, 0, sizeof hints);
    getaddrinfo(NULL, NULL, &hints, &k);
}
 
int main(void) {
    puts("Hello world");
    fflush(stdout);
    close(open("/dev/null", O_RDONLY));
    i context;
    context.e = open;
    printf("%p\n", context.d);
    g();
    return 0;
}

When compiled with AVX optimizations (e.g. -mavx or more commonly -march=native), this crashes even before `main()` is executed:

cc -mavx -O2 a.c && ./a.out

lldb ./a.out
run
  * thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
frame #0: 0x00007fff65e73316 libdyld.dylib`stack_not_16_byte_aligned_error libdyld.dylib`stack_not_16_byte_aligned_error:
 ->  0x7fff65e73316 <+0>: movdqa %xmm0, (%rsp)

Without AVX optimizations, or with Xcode 10, this doesn't happen.

This bug affects real-world applications such as libsodium and dsvpn.

A workaround is to use `-ffreestanding`.

Boost

Answer 1

kandre OP

Oct ’19

i just want to say that this has be EXTREMELY helpfull

thank you @snowcat @Kevindb

-------------------

additional background in my case:

i was attempting to build linux headers with GCC-9 and was getting the same problem as with xcode (gcc-9 was built with xcode 11 command line tools).... the solution was to build with normal xcode and set `MACOSX_DEPLOYMENT_TARGET=10.14` to build linux headers properly

note:

i also tried building GCC-8 with `8.3.0-xcode-bug-_Atomic-fix.patch` and see if i can still use a GCC-based toolchain... ill update here if gcc-8 is a valid workaround.....

0

	#include
	#include
	#include
	#include
	#include

	typedef struct {
	char d[16];
	void *e;
	struct { char b[5536]; } f;
	} i;

	void g(void) {
	struct addrinfo hints, *k;
	memset(&hints, 0, sizeof hints);
	getaddrinfo(NULL, NULL, &hints, &k);
	}

	int main(void) {
	puts("Hello world");
	fflush(stdout);
	close(open("/dev/null", O_RDONLY));
	i context;
	context.e = open;
	printf("%p\n", context.d);
	g();
	return 0;
	}