rtl-optimization/117467 - limit ext-dce memory use

The following puts in a hard limit on ext-dce because it might end
up requiring memory on the order of the number of basic blocks
times the number of pseudo registers.  The limiting follows what
GCSE based passes do and thus I re-use --param max-gcse-memory here.

This doesn't in any way address the implementation issues of the pass,
but it reduces the memory-use when compiling the
module_first_rk_step_part1.F90 TU from 521.wrf_r from 25GB to 1GB.

	PR rtl-optimization/117467
	PR rtl-optimization/117934
	* ext-dce.cc (ext_dce_execute): Do nothing if a memory
	allocation estimate exceeds what is allowed by
	--param max-gcse-memory.

1 files changed, 16 insertions, 0 deletions

diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc
index 6cf6418..e257e3b 100644
--- a/gcc/ext-dce.cc
+++ b/gcc/ext-dce.cc
@@ -34,6 +34,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "df.h"
 #include "print-rtl.h"
 #include "dbgcnt.h"
+#include "diagnostic-core.h"
 
 /* These should probably move into a C++ class.  */
 static vec<bitmap_head> livein;
@@ -1110,6 +1111,21 @@ static bool ext_dce_rd_confluence_n (edge) { return true; }
 void
 ext_dce_execute (void)
 {
+  /* Limit the amount of memory we use for livein, with 4 bits per
+     reg per basic-block including overhead that maps to one byte
+     per reg per basic-block.  */
+  uint64_t memory_request
+    = (uint64_t)n_basic_blocks_for_fn (cfun) * max_reg_num ();
+  if (memory_request / 1024 > (uint64_t)param_max_gcse_memory)
+    {
+      warning (OPT_Wdisabled_optimization,
+	       "ext-dce disabled: %d basic blocks and %d registers; "
+	       "increase %<--param max-gcse-memory%> above %wu",
+	       n_basic_blocks_for_fn (cfun), max_reg_num (),
+	       memory_request / 1024);
+      return;
+    }
+
   /* Some settings of SUBREG_PROMOTED_VAR_P are actively harmful
      to this pass.  Clear it for those cases.  */
   maybe_clear_subreg_promoted_p ();