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From: "dwmw2 at infradead dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/55177] missed optimizations with __builtin_bswap
Date: Fri, 23 Jan 2015 14:08:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55177

--- Comment #17 from David Woodhouse <dwmw2 at infradead dot org> ---
Er, yes. Sorry, I originally tried it with uint16_t but it wasn't even using
movbe for the pointer_abuse() function then, so I made it 32-bit instead.
Badly. Come to think of it, the lack of movbe in the 16-bit pointer_abuse()
case is potentially another bug worth fixing.

Here's the corrected test case.

/* gcc -march=atom -O2 -c load_be.c -Wstrict-aliasing -o- -S */
struct pkt {
    unsigned char data[10];
};

unsigned or(struct pkt *p)
{
    return (p->data[0] << 24) | (p->data[1] << 16) | (p->data[2] << 8) |
p->data[3];
}

unsigned add(struct pkt *p)
{
    return (p->data[0] << 24) + (p->data[1] << 16) + (p->data[2] << 8) +
p->data[3];
}

unsigned pointer_abuse(struct pkt *p)
{
    return __builtin_bswap16(*(unsigned short *)p->data);
}