our use of equivalences on range-on-entry calculations cause an issue
through a PHI node when a back edge is involved. ie
a = VARYING
<...>
bb5
b = PHI <UNDEFINED(2), a (5)>
bb6
if (a != 0)
goto bb5
since the value of b is undefined on the edge 2->5, we ignore it. The
range of a on the edge 6->5 is ~[0,0]
we calculate the range of b to be ~[0,0]. we also provide an
equivalency between a and b.
Unfortunately the on-entry code looks at equivalencies, and says, "hey,
a and b are equivalent, so we can use the range of b instead"
So it now thinks a is ~[0,0] and folds away the condition.
The problem is that b can be considered equivalent to a, but the
converse is not true, because there is a path (2->5) upon which a is not
equivalent to b. we have no way to represent a one way equivalence at
the moment. This patch avoid using that relation in range-on-entry
calculations.
Perhaps next release I'll add a specific kind of one way equivalence for
this kind of situation.
Bootstraps on x86_64-pc-linux-gnu with no regressions. OK?
Andrew
commit ecf19b6eb6f8e17d8d148e3c6627bd2151766420
Author: Andrew MacLeod <amacl...@redhat.com>
Date: Fri Dec 16 16:53:31 2022 -0500
Don't use PHI equivalences in range-on-entry.
If there is only one argument to a PHI which is defined, an equivalency is
created between the def and the argument. It is safe to consider the def
equal to the argument, but it is dangerous to assume the argument is also
equivalent to the def as there may be branches which change the range on the
path to the PHI on that argument
This patch avoid using that relation in range-on-entry calculations.
PR tree-optimization/108139
gcc/
* gimple-range-cache.cc (ranger_cache::fill_block_cache): Do not
use equivalences originating from PHIS.
gcc/testsuite/
* gcc.dg/pr108139.c: New.
diff --git a/gcc/gimple-range-cache.cc b/gcc/gimple-range-cache.cc
index ce5a0c8155e..9848d140242 100644
--- a/gcc/gimple-range-cache.cc
+++ b/gcc/gimple-range-cache.cc
@@ -1235,6 +1235,13 @@ ranger_cache::fill_block_cache (tree name, basic_block bb, basic_block def_bb)
if (!m_gori.has_edge_range_p (equiv_name))
continue;
+ // PR 108139. It is hazardous to assume an equivalence with
+ // a PHI is the same value. The PHI may be an equivalence
+ // via UNDEFINED arguments which is really a one way equivalence.
+ // PHIDEF == name, but name may not be == PHIDEF.
+ if (is_a<gphi *> (SSA_NAME_DEF_STMT (equiv_name)))
+ continue;
+
// Check if the equiv definition dominates this block
if (equiv_bb == bb ||
(equiv_bb && !dominated_by_p (CDI_DOMINATORS, bb, equiv_bb)))
diff --git a/gcc/testsuite/gcc.dg/pr108139.c b/gcc/testsuite/gcc.dg/pr108139.c
new file mode 100644
index 00000000000..6f224e3ce62
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr108139.c
@@ -0,0 +1,18 @@
+/* { dg-do compile { target int128 } } */
+/* { dg-options "-O1 -ftree-vrp -fdump-tree-vrp" } */
+
+int a, b;
+int main() {
+ int c;
+ if (a > 1)
+ a++;
+ while (a)
+ if (c == b)
+ c = a;
+ return 0;
+}
+
+
+/* { dg-final { scan-tree-dump-not "Folding predicate" "vrp2" } } */
+
+
