https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107043
Aldy Hernandez <aldyh at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |aldyh at gcc dot gnu.org
--- Comment #3 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> int g1(int n)
> {
> int n1 = n & 0x8000;
> if (n1 == 0)
> return 1;
> // n>>15 will be xxxxxx1 here.
> return (n >> 15) & 0x1;
> }
Interestingly, getting this one requires us to track something completely
different, the bits that are *definitely* set.
[Right now we track the nonzero mask, which is a misnomer because we're not
tracking bits are nonzero but the bits that *may* be nonzero. Or more
precisely the inverse of the bits that are known to be 0. For example, a
"nonzero" mask of 0xfffffff0 means the least significant 8 bits are known to be
zero, and the rest of the bits are unknown. So we're tracking the "and mask"
of a number? Or the maybe_nonzero bits? The reason for the name is because
legacy VRP had this name.]
To get the above, we'd need to track the bits that are definitely 1 (the "or
mask" of a number?). For example, on the 2->4 edge we'd need to know that n_3
has the 0x8000 bit set:
<bb 2> :
n1_4 = n_3(D) & 32768;
if (n1_4 == 0)
goto <bb 3>; [INV]
else
goto <bb 4>; [INV]
<bb 3> :
goto <bb 5>; [INV]
<bb 4> :
_1 = n_3(D) >> 15;
_5 = _1 & 1;
<bb 5> :
# _2 = PHI <1(3), _5(4)>
return _2;
What we're looking for is solving n_3:
[not-zero] = n_3 & 32768
which should give us:
[-INF,-1][32768, +INF] ORMASK [0x8000]
or whatever the hell we want to call it. I hate these names. Please someone,
come up with a name that makes sense to us all!
Andrew M and I had a plan for this earlier this cycle, but got sidetracked by
floats. What we'd need is a way to track or-mask's in addition to and-masks.
There's actual infrastructure missing here, but it should be as easy as what we
did for "nonzero" tracking in commit 4e82205b68024f5c1a9006fe2b62e1a0fa7f1245
(plus supporting patches). Basically we need to add a slot for the or-mask in
the irange, add union/intersect code, and then add some glue in range-ops to
solve:
1 = x & mask
x = y | mask
etc etc.
Thanks for the testcase, it's quite useful.
