On Wed, Nov 26, 2014 at 06:50:29PM +0100, Jakub Jelinek wrote:
> On Wed, Nov 26, 2014 at 06:39:44PM +0100, Marek Polacek wrote:
> > Both C11 and C++14 standards specify that integral promotions are
> > performed on both operands of a shift-expression. This we do just
> > fine. But then we convert the right operand to integer_type_node.
> > Not only is this unnecessary, it can also be harfmul, because for
> > e.g.
> > void
> > foo (unsigned int x)
> > {
> > if (-1 >> x != -1)
> > bar ();
> > }
> > with (int) x we lose info that x is nonnegative, which means that
> > tree_expr_nonnegative_p cannot fold this expr. Another problem
> > with the conversion is that we weren't able to detect e.g. shift
> > by 0x100000000ULL, since after the conversion this is 0.
>
> Have you checked what the expander does with it? Say trying to
> shift something by __int128 count or similar might upset it.
I tried
int
main ()
{
__int128 x = 1;
__int128 y = 200;
__int128 z;
asm ("" : "+g" (x), "+g" (y));
z = x << y;
asm ("" : "+g" (z));
return z;
}
and that works (even with ubsan) as expected. I haven't checked .expand
(I don't think I'd be able to judge the difference anyway), but on the
testcase above the assembly is the same with -O, with -O0 I see:
movq %rbx, -24(%rbp)
movq %rax, -48(%rbp)
movq %rdx, -40(%rbp)
- movq -48(%rbp), %rax
- movl %eax, %ecx
movq -32(%rbp), %rax
movq -24(%rbp), %rdx
+ movzbl -48(%rbp), %ecx
shldq %rax, %rdx
salq %cl, %rax
testb $64, %cl
> Wonder about if we don't make similar assumptions in the middle-end.
> It might make a difference (sometimes positive, sometimes negative)
> for vectorization too.
I don't really know; I assume the testsuite would have catched it if
something went awry.
> > 2014-11-26 Marek Polacek <pola...@redhat.com>
> >
> > PR c/63862
> > c-family/
> > * c-ubsan.c (ubsan_instrument_shift): Change the type of a MINUS_EXPR
> > to op1_utype.
>
> This part is ok regardless of the rest.
Thanks,
Marek
