https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114322
Bug ID: 114322
Summary: [14 Regression] SCEV analysis failed for bases like
A[(i+x)*stride] since r14-9193-ga0b1798042d033
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: hliu at amperecomputing dot com
Target Milestone: ---
Compile the following case with: gcc simp.c -Ofast -mcpu=neoverse-n1 -S \
-fdump-tree-ifcvt -fdump-tree-vect-details-scev
int
foo (short *A, int x, int stride)
{
int sum = 0;
if (stride > 1)
{
#pragma GCC unroll 1
for (int i = 0; i < 1024; ++i)
sum += A[(i + x) * stride];
}
return sum;
}
The gimple in the loop is:
<bb 3>:
# sum_19 = PHI <sum_15(6), 0(5)>
# i_20 = PHI <i_16(6), 0(5)>
# ivtmp_37 = PHI <ivtmp_36(6), 1024(5)>
_1 = x_12(D) + i_20;
_2 = _1 * stride_11(D);
_3 = (long unsigned int) _2;
_4 = _3 * 2;
_5 = A_13(D) + _4;
_6 = *_5;
_7 = (int) _6;
sum_15 = _7 + sum_19;
Before the commit (i.e., from pr114074 bug fix), it can be vectorized:
Creating dr for *_5
analyze_innermost: (analyze_scalar_evolution
(loop_nb = 1)
(scalar = _5)
(get_scalar_evolution
(scalar = _5)
(scalar_evolution = {A_13(D) + (long unsigned int) (stride_11(D) * x_12(D)) *
2, +, (long unsigned int) stride_11(D) * 2}_1))
)
success.
(analyze_scalar_evolution
(loop_nb = 1)
(scalar = _5)
(get_scalar_evolution
(scalar = _5)
(scalar_evolution = {A_13(D) + (long unsigned int) (stride_11(D) * x_12(D)) *
2, +, (long unsigned int) stride_11(D) * 2}_1))
)
(instantiate_scev
(instantiate_below = 5 -> 3)
(evolution_loop = 1)
(chrec = {A_13(D) + (long unsigned int) (stride_11(D) * x_12(D)) * 2, +,
(long unsigned int) stride_11(D) * 2}_1)
(res = {A_13(D) + (long unsigned int) (stride_11(D) * x_12(D)) * 2, +, (long
unsigned int) stride_11(D) * 2}_1))
base_address: A_13(D) + (sizetype) (stride_11(D) * x_12(D)) * 2
offset from base address: 0
constant offset from base address: 0
step: (ssizetype) ((long unsigned int) stride_11(D) * 2)
base alignment: 2
base misalignment: 0
offset alignment: 128
step alignment: 2
base_object: *A_13(D) + (sizetype) (stride_11(D) * x_12(D)) * 2
Access function 0: {0B, +, (long unsigned int) stride_11(D) * 2}_1
After the commit, loop vectorized failed due to SCEV failure with *_5:
Creating dr for *_5
analyze_innermost: (analyze_scalar_evolution
(loop_nb = 1)
(scalar = _5)
(get_scalar_evolution
(scalar = _5)
(scalar_evolution = _5))
)
(analyze_scalar_evolution
(loop_nb = 1)
(scalar = _5)
(get_scalar_evolution
(scalar = _5)
(scalar_evolution = _5))
)
simp.c:11:10: missed: failed: evolution of base is not affine.
..
(res = scev_not_known))
To my understanding, '(i + x) * stride' is signed integer calculation, in which
overflow is undefined behavior and the case should be vectorized.
