On Thu, Mar 10, 2022 at 12:32 PM Xi Ruoyao <xry...@mengyan1223.wang> wrote:
>
> 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.
The issue is that what the CONST_INT actually means depends on the
mode, an "overflow" to a positive number will eventually change what
is lower and what is the upper bound(?)
> > 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
So maybe you can figure out a fix that makes it work for this case as well.
> > > -- >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
