rtl-optimization/117922 - disable fold-mem-offsets for highly connected CFG

The PR shows fold-mem-offsets taking ages and a lot of memory computing
DU/UD chains as that requires the RD problem.  The issue is not so much
the memory required for the pruned sets but the high CFG connectivity
(and that the CFG is cyclic) which makes solving the dataflow problem
expensive.

The following adds the same limit as the one imposed by GCSE and CPROP.

	PR rtl-optimization/117922
	* fold-mem-offsets.cc (pass_fold_mem_offsets::execute):
	Do nothing for a highly connected CFG.

1 files changed, 18 insertions, 0 deletions

diff --git a/gcc/fold-mem-offsets.cc b/gcc/fold-mem-offsets.cc
index a816006..c1c9447 100644
--- a/gcc/fold-mem-offsets.cc
+++ b/gcc/fold-mem-offsets.cc
@@ -35,6 +35,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "df.h"
 #include "tree-pass.h"
 #include "cfgrtl.h"
+#include "diagnostic-core.h"
 
 /* This pass tries to optimize memory offset calculations by moving constants
    from add instructions to the memory instructions (loads / stores).
@@ -841,6 +842,23 @@ do_commit_insn (rtx_insn *insn)
 unsigned int
 pass_fold_mem_offsets::execute (function *fn)
 {
+  /* Computing UD/DU chains for flow graphs which have a high connectivity
+     will take a long time and is unlikely to be particularly useful.
+
+     In normal circumstances a cfg should have about twice as many
+     edges as blocks.  But we do not want to punish small functions
+     which have a couple switch statements.  Rather than simply
+     threshold the number of blocks, uses something with a more
+     graceful degradation.  */
+  if (n_edges_for_fn (fn) > 20000 + n_basic_blocks_for_fn (fn) * 4)
+    {
+      warning (OPT_Wdisabled_optimization,
+	       "fold-mem-offsets: %d basic blocks and %d edges/basic block",
+	       n_basic_blocks_for_fn (cfun),
+	       n_edges_for_fn (cfun) / n_basic_blocks_for_fn (cfun));
+      return 0;
+    }
+
   df_set_flags (DF_EQ_NOTES + DF_RD_PRUNE_DEAD_DEFS + DF_DEFER_INSN_RESCAN);
   df_chain_add_problem (DF_UD_CHAIN + DF_DU_CHAIN);
   df_analyze ();