On Wed, 3 Oct 2007, Linus Torvalds wrote:
>
> - the bug happens on this:
>
> char c = *p++;
>
> - which has been compiled into
>
> 8b 3a mov (%edx),%edi
Btw, this definitely doesn't happen for me, either on x86-64 or plain x86.
The x86 thing I tested was Fedora 8 testing (ie not even some stable
setup), so I wonder what experimental compiler you have.
Your compiler generates
movl -16(%ebp),%edx
movl (%edx),%edi /* this is _totally_ bogus! */
incl %edx
movl %edx,-16(%ebp)
movl %edi,%ecx
testb %cl,%cl
je ...
while I get (gcc version 4.1.2 20070925 (Red Hat 4.1.2-28)):
movl -16(%ebp), %eax # p,
movzbl (%eax), %edi #, c /* not bogus! */
movl %edi, %edx # c,
testb %dl, %dl #
je .L64 #,
incl %eax #
movsbl %dl,%ebx #, D.12414
movl %eax, -16(%ebp) #, p
where the difference (apart from doing the increment differently and
different register allocation) is that I have a "movzbl" (correct), while
you have a "movl" (pure and utter crap).
I *suspect* that the compiler bug is along the lines of:
(a) start off with movzbl
(b) notice that the higher bits don't matter, because nobody subsequently
uses them
(c) turn the thing into just a byte move.
(d) make the totally incorrect optimization of using a full 32-bit move
in order to avoid a partial register access stall
and the thing is, that final optimization can actually speed things up
(although it can also slow things down for any access that crosses a cache
sector boundary - 8/16 bytes), but it's seriously bogus, exactly because
it can cause an invalid access to the three next bytes that may not even
exist.
Linus
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