https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102622
Aldy Hernandez <aldyh at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |amacleod at redhat dot com,
| |jakub at gcc dot gnu.org,
| |rguenth at gcc dot gnu.org
--- Comment #12 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
I had to download the Intel SDE to reproduce it, but was finally able to narrow
it down to 4 jump threading paths:
./xg++ -B./ a.c -O3 -L/usr/lib/gcc/x86_64-redhat-linux/11/
-march=skylake-avx512 -fdump-tree-all-details -fdisable-tree-ethread
-fdisable-tree-thread1 -fdisable-tree-thread2 -fdisable-tree-thread3
-fdisable-tree-thread4 -fdbg-cnt=registered_jump_thread:2-4:19-20
$ grep 'thread' a.c.* |grep Registering
a.c.113t.vrp-thread1: [2] Registering jump thread: (9, 11) incoming edge;
(11, 10) normal;
a.c.113t.vrp-thread1: [4] Registering jump thread: (5, 7) incoming edge; (7,
8) normal;
a.c.197t.vrp-thread2: [19] Registering jump thread: (15, 16) incoming edge;
(16, 18) normal;
a.c.197t.vrp-thread2: [20] Registering jump thread: (21, 16) incoming edge;
(16, 18) normal;
Things start getting challenging after the vectorizer and cunroll run. Such
that by vrp-thread2, ranger figures out that the 2->3 edge is unreachable, _21
must be 0, and shit rolls downhill from there:
unsigned long ivtmp.31;
short int * vectp_arr_32.21;
vector(8) short int * vectp_arr_32.20;
unsigned short tmp.19;
short int tmp.18;
int D.4484;
bool var_22_lsm_flag.15;
int var_22_lsm.14;
int D.4481;
bool var_20_lsm_flag.13;
int var_20_lsm.12;
unsigned int f;
int e;
short int d;
short int _2;
unsigned int _3;
bool _4;
long long int _6;
int _7;
bool _21;
unsigned short ivtmp_24;
int _30(D);
bool _33;
unsigned short ivtmp_41;
int _43(D);
short int * _55;
<signed-boolean:1> _64;
int _66;
unsigned long _74;
<signed-boolean:1> _75;
vector(8) <signed-boolean:1> vect_cst__76;
void * _81;
<bb 2> [local count: 8685306]:
_2 = (short int) b_15(D);
_3 = (unsigned int) _2;
_4 = _3 != 0;
_33 = _3 < b_15(D);
_21 = _4 | _33;
_64 = (<signed-boolean:1>) _21;
_75 = -_64;
vect_cst__76 = {_75, _75, _75, _75, _75, _75, _75, _75};
if (_75 == 0)
goto <bb 4>; [100.00%]
else
goto <bb 3>; [20.00%]
<bb 3> [local count: 1737061]:
MEM <vector(8) short int> [(short int *)&arr_32] = { 0, 0, 0, 0, 0, 0, 0, 0
};
<bb 4> [local count: 8685306]:
if (_21 != 0)
goto <bb 5>; [50.00%]
else
goto <bb 6>; [50.00%]
<bb 5> [local count: 2171327]:
arr_32[8] = 0;
<bb 6> [local count: 4342653]:
if (_21 != 0)
goto <bb 7>; [50.00%]
else
goto <bb 8>; [50.00%]
[snip]
[snip]
The reason ranger concludes that 2->3 is unreachable is from analyzing block 2:
<bb 2> [local count: 8685306]:
_2 = (short int) b_15(D);
_3 = (unsigned int) _2;
_4 = _3 != 0;
_33 = _3 < b_15(D);
_21 = _4 | _33;
_64 = (<signed-boolean:1>) _21;
_75 = -_64;
vect_cst__76 = {_75, _75, _75, _75, _75, _75, _75, _75};
if (_75 == 0)
goto <bb 4>; [100.00%]
else
goto <bb 3>; [20.00%]
On the 2->3 edge, _75 == -1 because this is a 1-bit signed integer. Solving
back we have:
_75 = - _64; ==> [-1,-1] = -_64;
-1 for a 1-bit signed integer is TYPE_MIN_VALUE, and NEG(TYPE_MIN_VALUE) is
unrepresentable. So, _75 cannot be -1, thus the edge is unexecutable.
Questions:
a) Is -(-1) representable in 1-bit signed?
b) Could we somehow avoid creating the 1-bit signed in the vectorizer, since
they are a source of endless exception?
Thanks.
