[PATCH v2] ipa-utils: avoid uninitialized probabilities on ICF [PR111559]

[Sergei Trofimovich]

From: Sergei Trofimovich <siarh...@google.com>

r14-3459-g0c78240fd7d519 "Check that passes do not forget to define profile"
exposed check failures in cases when gcc produces uninitialized profile
probabilities. In case of PR/111559 uninitialized profile is generated
by edges executed 0 times reported by IPA profile:

    $ gcc -O2 -fprofile-generate pr111559.c -o b -fopt-info
    $ ./b
    $ gcc -O2 -fprofile-use -fprofile-correction pr111559.c -o b -fopt-info

    pr111559.c: In function 'rule1':
    pr111559.c:6:13: error: probability of edge 3->4 not initialized
        6 | static void rule1(void) { if (p) edge(); }
          |             ^~~~~
    during GIMPLE pass: fixup_cfg
    pr111559.c:6:13: internal compiler error: verify_flow_info failed

The change conservatively ignores updates with uninitialized values and
uses initially assigned probabilities (`always` probability in case of
the example).

        PR ipa/111283
        PR gcov-profile/111559

gcc/
        * ipa-utils.cc (ipa_merge_profiles): Avoid producing
        uninitialized probabilities when merging counters with zero
        denominators.

gcc/testsuite/
        * gcc.dg/tree-prof/pr111559.c: New test.
---
 gcc/ipa-utils.cc                          |  6 +++++-
 gcc/testsuite/gcc.dg/tree-prof/pr111559.c | 16 ++++++++++++++++
 2 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.dg/tree-prof/pr111559.c

diff --git a/gcc/ipa-utils.cc b/gcc/ipa-utils.cc
index 956c6294fd7..7c53ae9dd45 100644
--- a/gcc/ipa-utils.cc
+++ b/gcc/ipa-utils.cc
@@ -651,13 +651,17 @@ ipa_merge_profiles (struct cgraph_node *dst,
                {
                  edge srce = EDGE_SUCC (srcbb, i);
                  edge dste = EDGE_SUCC (dstbb, i);
-                 dste->probability = 
+                 profile_probability merged =
                    dste->probability * dstbb->count.ipa ().probability_in
                                                 (dstbb->count.ipa ()
                                                  + srccount.ipa ())
                    + srce->probability * srcbb->count.ipa ().probability_in
                                                 (dstbb->count.ipa ()
                                                  + srccount.ipa ());
+                 /* We produce uninitialized probabilities when
+                    denominator is zero: https://gcc.gnu.org/PR111559.  */
+                 if (merged.initialized_p ())
+                   dste->probability = merged;
                }
              dstbb->count = dstbb->count.ipa () + srccount.ipa ();
            }
diff --git a/gcc/testsuite/gcc.dg/tree-prof/pr111559.c 
b/gcc/testsuite/gcc.dg/tree-prof/pr111559.c
new file mode 100644
index 00000000000..43202c6c888
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-prof/pr111559.c
@@ -0,0 +1,16 @@
+/* { dg-options "-O2" } */
+
+__attribute__((noipa)) static void edge(void) {}
+
+int p = 0;
+
+__attribute__((noinline))
+static void rule1(void) { if (p) edge(); }
+
+__attribute__((noinline))
+static void rule1_same(void) { if (p) edge(); }
+
+__attribute__((noipa)) int main(void) {
+    rule1();
+    rule1_same();
+}
-- 
2.42.0