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tree-optimization/105198 - wrong code with predictive commoning

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When predictive commoning looks for a looparound PHI it tries
to match the entry value definition (a load) up with the appropriate
member of the chain.  But it fails to consider stmts clobbering
the very same memory location inbetween the load and loop entry.

In theory we could be more clever on must aliases that would be
also picked up from a load (so not exactly stmt_kills_ref_p) and
use the stored value from that if it is an exact match.  But we
currently have no way to propagate this information inside predcom.

2022-04-08  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105198
	* tree-predcom.cc (find_looparound_phi): Check whether
	the found memory location of the entry value is clobbered
	inbetween the value we want to use and loop entry.

	* gcc.dg/torture/pr105198.c: New testcase.
This commit is contained in:
Richard Biener 2022-04-08 13:13:29 +02:00
parent 04ecadbeae
commit e5453bcc21
2 changed files with 59 additions and 5 deletions
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26
gcc/testsuite/gcc.dg/torture/pr105198.c Normal file
View file

@ -0,0 +1,26 @@
/* { dg-do run } */
/* { dg-additional-options "-fno-tree-pre -fpredictive-commoning" } */
static __attribute__ ((noipa)) void
next_set(int *x, int n, int k)
{
int j = k - 1;
int tmp = x[j]++;
while (j > 0)
{
if (x[j] < n - (k - 1 -j))
break;
j--;
x[j]++;
tmp = x[j];
}
if (tmp != 2 || j != 1 || x[0] != 0 || x[1] != 2 || x[2] != 5)
__builtin_abort ();
}
int main()
{
int x[3] = {0, 1, 4};
next_set(x, 5, 3);
return 0;
}

38
gcc/tree-predcom.cc
View file

@ -1377,7 +1377,6 @@ gphi *
pcom_worker::find_looparound_phi (dref ref, dref root)
{
tree name, init, init_ref;
gphi *phi = NULL;
gimple *init_stmt;
edge latch = loop_latch_edge (m_loop);
struct data_reference init_dr;
@ -1395,14 +1394,19 @@ pcom_worker::find_looparound_phi (dref ref, dref root)
if (!name)
return NULL;
tree entry_vuse = NULL_TREE;
gphi *phi = NULL;
for (psi = gsi_start_phis (m_loop->header); !gsi_end_p (psi); gsi_next (&psi))
{
phi = psi.phi ();
if (PHI_ARG_DEF_FROM_EDGE (phi, latch) == name)
gphi *p = psi.phi ();
if (PHI_ARG_DEF_FROM_EDGE (p, latch) == name)
phi = p;
else if (virtual_operand_p (gimple_phi_result (p)))
entry_vuse = PHI_ARG_DEF_FROM_EDGE (p, loop_preheader_edge (m_loop));
if (phi && entry_vuse)
break;
}
if (gsi_end_p (psi))
if (!phi || !entry_vuse)
return NULL;
init = PHI_ARG_DEF_FROM_EDGE (phi, loop_preheader_edge (m_loop));
@ -1430,6 +1434,30 @@ pcom_worker::find_looparound_phi (dref ref, dref root)
if (!valid_initializer_p (&init_dr, ref->distance + 1, root->ref))
return NULL;
/* Make sure nothing clobbers the location we re-use the initial value
from. */
if (entry_vuse != gimple_vuse (init_stmt))
{
ao_ref ref;
ao_ref_init (&ref, init_ref);
unsigned limit = param_sccvn_max_alias_queries_per_access;
tree vdef = entry_vuse;
do
{
gimple *def = SSA_NAME_DEF_STMT (vdef);
if (limit-- == 0 || gimple_code (def) == GIMPLE_PHI)
return NULL;
if (stmt_may_clobber_ref_p_1 (def, &ref))
/* When the stmt is an assign to init_ref we could in theory
use its RHS for the initial value of the looparound PHI
we replace in prepare_initializers_chain, but we have
no convenient place to store this info at the moment. */
return NULL;
vdef = gimple_vuse (def);
}
while (vdef != gimple_vuse (init_stmt));
}
return phi;
}