On Thu, 2022-03-10 at 09:01 +0100, Richard Biener wrote:
> On Wed, Mar 9, 2022 at 5:12 PM Xi Ruoyao <xry...@mengyan1223.wang>
> wrote:
> >
> > On Wed, 2022-03-09 at 15:55 +0100, Richard Biener wrote:
> >
> > > isn't it better to make targetm.const_anchor unsigned?
> > > The & and ~ are not subject to overflow rules.
> >
> > It's not enough: if n is the minimum value of HOST_WIDE_INT and
> > const_anchor = 0x8000 (the value for MIPS), we'll have a signed
> > 0x7fff
> > in *upper_base. Then the next line, "*upper_offs = n -
> > *upper_base;"
> > will be a signed overflow again.
> >
> > How about the following?
>
> Hmm, so all this seems to be to round CST up and down to a multiple of
> CONST_ANCHOR.
> It works on CONST_INT only which is sign-extended, so if there is
> overflow the resulting
> anchor is broken as far as I can see.
On MIPS addiu/daddiu do 2-complement addition, so the overflowed result
is still usable.
> So instead of papering over this issue
> the function should return false when n is negative since then
> n & ~(targetm.const_anchor - 1) is also not n rounded down to a
> multiple of const_anchor.
This function does work for negative n, like:
void g (int, int);
void
f (void)
{
g(0x8123ffff, 0x81240001);
}
It should produce:
li $4,-2128347136 # 0xffffffff81240000
daddiu $5,$4,1
daddiu $4,$4,-1
jal g
But return false for negative n will cause regression for this case,
producing:
li $5,-2128347136 # 0xffffffff81240000
li $4,-2128412672 # 0xffffffff81230000
ori $5,$5,0x1
ori $4,$4,0xffff
jal g
That being said, it indeed does not work for:
void g (int, int);
void f ()
{
g (0x7fffffff, 0x80000001);
}
It produces:
li $5,-2147483648 # 0xffffffff80000000
li $4,2147418112 # 0x7fff0000
daddiu $5,$5,1
ori $4,$4,0xffff
jal g
Should be:
li $5,-2147483648 # 0xffffffff80000000
daddiu $5,$5,1
addiu $4,$5,-1
> > -- >8 --
> >
> > With a non-zero const_anchor, the behavior of this function relied on
> > signed overflow.
> >
> > gcc/
> >
> > PR rtl-optimization/104843
> > * cse.cc (compute_const_anchors): Use unsigned HOST_WIDE_INT for
> > n to perform overflow arithmetics safely.
> > ---
> > gcc/cse.cc | 8 ++++----
> > 1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/gcc/cse.cc b/gcc/cse.cc
> > index a18b599d324..052fa0c3490 100644
> > --- a/gcc/cse.cc
> > +++ b/gcc/cse.cc
> > @@ -1169,12 +1169,12 @@ compute_const_anchors (rtx cst,
> > HOST_WIDE_INT *lower_base, HOST_WIDE_INT *lower_offs,
> > HOST_WIDE_INT *upper_base, HOST_WIDE_INT *upper_offs)
> > {
> > - HOST_WIDE_INT n = INTVAL (cst);
> > -
> > - *lower_base = n & ~(targetm.const_anchor - 1);
> > - if (*lower_base == n)
> > + unsigned HOST_WIDE_INT n = UINTVAL (cst);
> > + unsigned HOST_WIDE_INT lb = n & ~(targetm.const_anchor - 1);
> > + if (lb == n)
> > return false;
> >
> > + *lower_base = lb;
> > *upper_base =
> > (n + (targetm.const_anchor - 1)) & ~(targetm.const_anchor - 1);
> > *upper_offs = n - *upper_base;
> > --
> > 2.35.1
> >
> >
> > >
--
Xi Ruoyao <xry...@mengyan1223.wang>
School of Aerospace Science and Technology, Xidian University
