https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110991
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |ASSIGNED
Target| |x86_64-*-*
Target Milestone|--- |14.0
Assignee|unassigned at gcc dot gnu.org |rguenth at gcc dot
gnu.org
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
So the difference is that GCC 14 vectorizes the loop and that vectorized loop
is not completely unrolled because
Loop 1 likely iterates at most 2 times.
Estimating sizes for loop 1
BB: 3, after_exit: 0
size: 1 _34 = vect_vec_iv_.15_33 + { 252, 252, 252, 252 };
size: 0 vect_a.16_35 = VIEW_CONVERT_EXPR<vector(4) signed
char>(vect_vec_iv_.15_33);
size: 1 vect_iftmp.17_36 = vect_a.16_35 << 3;
size: 1 mask__23.18_38 = vect_a.16_35 < { 0, 0, 0, 0 };
size: 1 vect_iftmp.19_40 = VEC_COND_EXPR <mask__23.18_38, { 1, 1, 1, 1 },
vect_iftmp.17_36>;
size: 1 ivtmp_44 = ivtmp_43 + 1;
Induction variable computation will be folded away.
size: 2 if (ivtmp_44 < 3)
Exit condition will be eliminated in peeled copies.
Exit condition will be eliminated in last copy.
Constant conditional.
BB: 9, after_exit: 1
size: 7-3, last_iteration: 7-3
Loop size: 7
Estimated size after unrolling: 8
Not unrolling loop 1: size would grow.
when we still have a loop there's nothing that can fully elide things.
Without vectorization we have
Loop 2 likely iterates at most 11 times.
Estimating sizes for loop 2
BB: 10, after_exit: 0
size: 0 a.2_13 = (signed char) a.6_22;
Induction variable computation will be folded away.
size: 2 if (a.2_13 < 0)
Constant conditional.
BB: 13, after_exit: 1
BB: 12, after_exit: 0
size: 1 _26 = a.6_22 + 255;
Induction variable computation will be folded away.
size: 1 ivtmp_27 = ivtmp_4 - 1;
Induction variable computation will be folded away.
size: 2 if (ivtmp_27 != 0)
Exit condition will be eliminated in peeled copies.
Exit condition will be eliminated in last copy.
Constant conditional.
BB: 11, after_exit: 0
size: 1 iftmp.0_12 = a.2_13 << 3;
Induction variable computation will be folded away.
size: 7-7, last_iteration: 7-7
Loop size: 7
Estimated size after unrolling: 1
unrolling relies on constant_after_peeling which relies on SCEV which
doesn't handle vector IVs.
I have a patch improving it to
size: 7-4, last_iteration: 7-4
Loop size: 7
Estimated size after unrolling: 6
IIRC I also had a patch more appropriately "propagating" constness at some
point.
