[PATCH][RFC] Fix UBSAN in postreload-gcse.c (PR rtl-optimization/87868).

[Martin Liška]

Hi.

The patch is adding a check overflow in  eliminate_partially_redundant_load.
Question is whether the usage of conditional compilation of 
__builtin_mul_overflow
is fine?

Thanks,
Martin

gcc/ChangeLog:

2018-11-06  Martin Liska  <mli...@suse.cz>

        PR rtl-optimization/87868
        * postreload-gcse.c (eliminate_partially_redundant_load): Set
        threshold to max_count if we would overflow.
        * profile-count.h: Make max_count a public constant.
---
 gcc/postreload-gcse.c | 14 ++++++++++++--
 gcc/profile-count.h   |  2 +-
 2 files changed, 13 insertions(+), 3 deletions(-)



diff --git a/gcc/postreload-gcse.c b/gcc/postreload-gcse.c
index b56993183d0..399970c368a 100644
--- a/gcc/postreload-gcse.c
+++ b/gcc/postreload-gcse.c
@@ -1170,8 +1170,18 @@ eliminate_partially_redundant_load (basic_block bb, rtx_insn *insn,
   if (ok_count.to_gcov_type ()
       < GCSE_AFTER_RELOAD_PARTIAL_FRACTION * not_ok_count.to_gcov_type ())
     goto cleanup;
-  if (ok_count.to_gcov_type ()
-      < GCSE_AFTER_RELOAD_CRITICAL_FRACTION * critical_count.to_gcov_type ())
+
+  gcov_type threshold;
+#if (GCC_VERSION >= 5000)
+  if (__builtin_mul_overflow (GCSE_AFTER_RELOAD_CRITICAL_FRACTION,
+			      critical_count.to_gcov_type (), &threshold))
+    threshold = profile_count::max_count;
+#else
+  threshold
+    = GCSE_AFTER_RELOAD_CRITICAL_FRACTION * critical_count.to_gcov_type ();
+#endif
+
+  if (ok_count.to_gcov_type () < threshold)
     goto cleanup;
 
   /* Generate moves to the loaded register from where
diff --git a/gcc/profile-count.h b/gcc/profile-count.h
index f4d0c340a0a..5d3bcc75f6d 100644
--- a/gcc/profile-count.h
+++ b/gcc/profile-count.h
@@ -641,8 +641,8 @@ public:
      type to hold various extra stages.  */
 
   static const int n_bits = 61;
-private:
   static const uint64_t max_count = ((uint64_t) 1 << n_bits) - 2;
+private:
   static const uint64_t uninitialized_count = ((uint64_t) 1 << n_bits) - 1;
 
   uint64_t m_val : n_bits;