https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102183
Bug ID: 102183
Summary: sccvn compare predicated result issue in
vn_nary_op_insert_into
Product: gcc
Version: tree-ssa
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: dizhao at os dot amperecomputing.com
Target Milestone: ---
In tree-ssa-sccvn.c, function vn_nary_op_insert_into, the following code might
lead to inserting entra vn_pval entry:
vn_pval *nval = vno->u.values;
vn_pval **next = &vno->u.values;
bool found = false;
for (vn_pval *val = (*slot)->u.values; val; val = val->next)
{
if (expressions_equal_p (val->result, vno->u.values->result)) // Here
{
...
/* Append value. */
*next = (vn_pval *) obstack_alloc (&vn_tables_obstack,
sizeof (vn_pval)
+ val->n * sizeof (int));
(*next)->next = NULL;
(*next)->result = val->result;
...
next = &(*next)->next;
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, "Appending predicate to value.\n");
continue;
}
/* Copy other predicated values. */
*next = (vn_pval *) obstack_alloc (&vn_tables_obstack,
sizeof (vn_pval)
+ (val->n-1) * sizeof (int));
memcpy (*next, val, sizeof (vn_pval) + (val->n-1) * sizeof (int));
(*next)->next = NULL;
next = &(*next)->next;
}
I think 'expressions_equal_p' here should check if current val->result equals
to the result in original vno? But vno->u.values will be changed if the "Copy
other predicated values" branch is taken first (by setting *next). Here's an
example test code:
#include <stdio.h>
unsigned int c = 1;
void
f (unsigned int a, unsigned int b)
{
if (a >= b)
c = 2;
else
c = 9;
for (unsigned i = 0; i < 100; i++)
{
if (a < b)
{
printf ("AAAA");
if (a >= b)
printf ("XXXXX");
}
else
printf ("BBBB");
}
}
In detailed fre1 dump file, the new predicate value is not appended to old
entry:
Processing block 0: BB2
Value numbering stmt = if (a_5(D) >= b_6(D))
Recording on edge 2->3 a_5(D) ge_expr b_6(D) == true
Recording on edge 2->4 a_5(D) ge_expr b_6(D) == false
Recording on edge 2->3 a_5(D) lt_expr b_6(D) == false //===>old one
Recording on edge 2->4 a_5(D) lt_expr b_6(D) == true //===>old one
...
Processing block 5: BB6
Value numbering stmt = if (a_5(D) < b_6(D))
Recording on edge 6->7 a_5(D) lt_expr b_6(D) == true //===>new one
Recording on edge 6->9 a_5(D) lt_expr b_6(D) == false
Appending predicate to value.
After changed to 'if (expressions_equal_p (val->result, nval->result))', the
new predicate value can be appended to old one:
Processing block 0: BB2
Value numbering stmt = if (a_5(D) >= b_6(D))
Recording on edge 2->3 a_5(D) ge_expr b_6(D) == true
Recording on edge 2->4 a_5(D) ge_expr b_6(D) == false
Recording on edge 2->3 a_5(D) lt_expr b_6(D) == false //===>old one
Recording on edge 2->4 a_5(D) lt_expr b_6(D) == true //===>old one
...
Processing block 5: BB6
Value numbering stmt = if (a_5(D) < b_6(D))
Recording on edge 6->7 a_5(D) lt_expr b_6(D) == true //===>new one
Appending predicate to value.
Recording on edge 6->9 a_5(D) lt_expr b_6(D) == false
Appending predicate to value.
(I think inserting extra vn_pval entries might cause losing optimize
opportunities.)
