https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116766
Bug ID: 116766
Summary: Missed loop vectorization case with gather/scatter
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: dusan.stojko...@rt-rk.com
Target Milestone: ---
Created attachment 59138
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59138&action=edit
A dump file of vect pass with details enabled
Hello this report is about a loop which GCC doesn't vectorize.
This is the loop in question:
void
foo(int *in, int *out1, int *out2, int n)
{
for (int i = 0; i < n; i+=2)
{
out1[i/2] = in[i];
out2[i/2] = in[i+1];
}
}
GCC isn't able to vectorize this example:
ARM: https://godbolt.org/z/Eq7vs7ssx
RISC-V: https://godbolt.org/z/Y1sr4cr8K
Clang vectorizes this loop for ARM as well as RISC-V:
ARM: https://godbolt.org/z/YvaMqTz1h
RISC-V: https://godbolt.org/z/x1MxGc7Gx
In the vect optimization pass there are two misses regarding array indexing:
analyze_innermost: t.c:6:19: missed: failed: evolution of base is not affine.
analyze_innermost: t.c:7:19: missed: failed: evolution of base is not affine.
There are also these misses:
t.c:6:23: missed: possible alias involving gather/scatter between *_3 and *_7
t.c:4:23: missed: bad data dependence.
t.c:6:23: missed: possible alias involving gather/scatter between *_3 and *_7
t.c:4:23: missed: bad data dependence.
t.c:6:23: missed: possible alias involving gather/scatter between *_3 and *_7
t.c:4:23: missed: bad data dependence.
t.c:6:23: missed: possible alias involving gather/scatter between *_3 and *_7
t.c:4:23: missed: bad data dependence.
t.c:6:23: missed: possible alias involving gather/scatter between *_3 and *_7
t.c:4:23: missed: bad data dependence.
t.c:6:23: missed: possible alias involving gather/scatter between *_3 and *_7
t.c:4:23: missed: bad data dependence.
t.c:6:23: missed: possible alias involving gather/scatter between *_3 and *_7
t.c:4:23: missed: bad data dependence.
t.c:6:23: missed: possible alias involving gather/scatter between *_3 and *_7
t.c:4:23: missed: bad data dependence.
t.c:6:23: missed: possible alias involving gather/scatter between *_3 and *_7
t.c:4:23: missed: bad data dependence.
t.c:4:23: missed: couldn't vectorize loop
void foo (int * in, int * out1, int * out2, int n)
{
int i;
long unsigned int _1;
long unsigned int _2;
int * _3;
int _4;
long unsigned int _5;
long unsigned int _6;
int * _7;
int _8;
sizetype _9;
sizetype _10;
int * _11;
int * _12;
int _13;
<bb 2> [local count: 118111600]:
if (n_17(D) > 0)
goto <bb 5>; [89.00%]
else
goto <bb 4>; [11.00%]
<bb 5> [local count: 105119324]:
<bb 3> [local count: 955630224]:
# i_26 = PHI <i_23(6), 0(5)>
_1 = (long unsigned int) i_26;
_2 = _1 * 4;
_3 = in_18(D) + _2;
_4 = i_26 >> 1;
_5 = (long unsigned int) _4;
_6 = _5 * 4;
_7 = out1_19(D) + _6;
_8 = *_3;
*_7 = _8;
_9 = _1 + 1;
_10 = _9 * 4;
_11 = in_18(D) + _10;
_12 = out2_21(D) + _6;
_13 = *_11;
*_12 = _13;
i_23 = i_26 + 2;
if (n_17(D) > i_23)
goto <bb 6>; [89.00%]
else
goto <bb 7>; [11.00%]
<bb 6> [local count: 850510900]:
goto <bb 3>; [100.00%]
<bb 7> [local count: 105119324]:
<bb 4> [local count: 118111600]:
return;
}
