Gcc 8.1, -O2 optimisation build failure

[Wei Liu <wei.liu2@citrix.com>]

Hello,

We have a program which fails to build with gcc 8.1 -m32 -O2 (version
Debian 8.1.0-12). We couldn't figure out how the optimiser came up with
the idea that array bounds could become negative. Any help would be
appreciated. The attached code has been simplified from the original to
reproduce the issue.

The same code snippet builds find with 8.1 debug build and older
versions of gcc.

$ gcc  -m32 -march=i686 -std=gnu99 -Wall -O2   -Werror   -c -o t.o t.c
t.c: In function 'func':
t.c:41:9: error: 'memcpy' offset [-204, -717] is out of the bounds [0, 216] of object 'ctrl' with type 'struct kdd_ctrl' [-Werror=array-bounds]
         memcpy(buf, ((uint8_t *)&ctrl.c32) + offset, len);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
t.c:27:21: note: 'ctrl' declared here
     struct kdd_ctrl ctrl;

<code>
#include <stdint.h>
#include <string.h>

struct kdd_ctrl_32 {
    uint8_t _[84];
};

struct kdd_ctrl_64 {
    uint8_t _[216];
};

struct kdd_ctrl {
    union {
        struct kdd_ctrl_32 c32;
        struct kdd_ctrl_64 c64;
    };
};

typedef struct {
    uint8_t buf[17 + 65536];
    uint32_t length_req;
    uint64_t addr;
} kdd_state;

void func(kdd_state *s)
{
    struct kdd_ctrl ctrl;
    uint8_t *buf = s->buf + 17 + 57;
    uint32_t len = s->length_req;
    uint64_t addr = s->addr;

    uint32_t offset = addr;

    /* 32-bit control-register space starts at 0x[2]cc, for 84 bytes */
    if (offset > 0x200)
        offset -= 0x200;
    offset -= 0xcc;
    if (offset > sizeof ctrl.c32 || offset + len > sizeof ctrl.c32) {
        len = 0;
    } else {
        memcpy(buf, ((uint8_t *)&ctrl.c32) + offset, len);
    }
}
</code>

Regards,
Wei.