Fix gamess miscompare

Jan Hubicka
Wed Nov 17 21:06:47 GMT 2021

this patch fixes bug in streaming in modref access tree that now cause a failure
of gamess benchmark.  The bug is quite old (present in GCC11 release) but it
needs quite interesting series of events to manifest. In particular
 1) At lto time ISRA turns some parameters passed by reference to scalar
 2) At lto time modref computes summaries for old parameters and then updates
    them but does so quite stupidly believing that the load from parameters
    are now unkonwn loads (rather than optimized out).
    This renders summary not very useful since it thinks every memory aliasing
    int is now accssed (as opposed as parameter dereference)
 3) At stream in we notice too early that summary is useless, set every_access
    flag and drop the list.  However while reading rest of the summary we
    overwrite the flag back to 0 which makes us to lose part of summary.
 4) right selection of partitions needs to be done to avoid late modref from
    recalculating and thus fixing the summary.

This patch fixes the stream in bug, however we also should fix updating of
summaries.  Martin, would be possible to extend get_original_index by "deref"
parameter that would be set to true when refernce was turned to scalar?

Bootstrapped/regtested x86_64-linux. Comitted.


2021-11-17  Jan Hubicka  <>

	PR ipa/103246
	* ipa-modref.c (read_modref_records): Fix streaminig in of every_access

diff --git a/gcc/ipa-modref.c b/gcc/ipa-modref.c
index 9ceecdd479f..c94f0589d44 100644
--- a/gcc/ipa-modref.c
+++ b/gcc/ipa-modref.c
@@ -3460,10 +3460,10 @@ read_modref_records (lto_input_block *ib, struct data_in *data_in,
 	  size_t every_access = streamer_read_uhwi (ib);
 	  size_t naccesses = streamer_read_uhwi (ib);
-	  if (nolto_ref_node)
-	    nolto_ref_node->every_access = every_access;
-	  if (lto_ref_node)
-	    lto_ref_node->every_access = every_access;
+	  if (nolto_ref_node && every_access)
+	    nolto_ref_node->collapse ();
+	  if (lto_ref_node && every_access)
+	    lto_ref_node->collapse ();
 	  for (size_t k = 0; k < naccesses; k++)

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