[PATCH] Make last DCE remove empty loops

[PATCH] Make last DCE remove empty loops

[Richard Biener]

The following makes the last DCE pass CD-DCE and in turn the
last CD-DCE pass a DCE one.  That ensues we remove empty loops
that become empty between the two.  I've also moved the tail-call
pass after DCE since DCE can only improve things here.

The two testcases were the only ones scanning cddce3 so I've
changed them to scan the dce7 pass that's now in this place.
The testcases scanning dce7 also work when that's in the earlier
position.

Bootstrapped and tested on x86_64-unknown-linux-gnu, I'm going
to push this tomorrow if there are no comments.

Richard.

	PR tree-optimization/84646
	* tree-ssa-dce.cc (pass_dce::set_pass_param): Add param
	wheter to run update-address-taken.
	(pass_dce::execute): Honor it.
	* passes.def: Exchange last DCE and CD-DCE invocations.
	Swap pass_tail_calls and the last DCE.

	* g++.dg/tree-ssa/pr106922.C: Continue to scan earlier DCE dump.
	* gcc.dg/tree-ssa/20030808-1.c: Likewise.
---
 gcc/passes.def                             |  8 ++++----
 gcc/testsuite/g++.dg/tree-ssa/pr106922.C   |  4 ++--
 gcc/testsuite/gcc.dg/tree-ssa/20030808-1.c |  6 +++---
 gcc/tree-ssa-dce.cc                        | 15 +++++++++++++--
 4 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/gcc/passes.def b/gcc/passes.def
index 193b5794749..462e9afad61 100644
--- a/gcc/passes.def
+++ b/gcc/passes.def
@@ -346,8 +346,8 @@ along with GCC; see the file COPYING3.  If not see
       NEXT_PASS (pass_ccp, true /* nonzero_p */);
       NEXT_PASS (pass_warn_restrict);
       NEXT_PASS (pass_dse);
-      NEXT_PASS (pass_cd_dce, true /* update_address_taken_p */);
-      /* After late CD DCE we rewrite no longer addressed locals into SSA
+      NEXT_PASS (pass_dce, true /* update_address_taken_p */);
+      /* After late DCE we rewrite no longer addressed locals into SSA
 	 form if possible.  */
       NEXT_PASS (pass_forwprop);
       NEXT_PASS (pass_sink_code, true /* unsplit edges */);
@@ -355,12 +355,12 @@ along with GCC; see the file COPYING3.  If not see
       NEXT_PASS (pass_fold_builtins);
       NEXT_PASS (pass_optimize_widening_mul);
       NEXT_PASS (pass_store_merging);
-      NEXT_PASS (pass_tail_calls);
       /* If DCE is not run before checking for uninitialized uses,
 	 we may get false warnings (e.g., testsuite/gcc.dg/uninit-5.c).
 	 However, this also causes us to misdiagnose cases that should be
 	 real warnings (e.g., testsuite/gcc.dg/pr18501.c).  */
-      NEXT_PASS (pass_dce);
+      NEXT_PASS (pass_cd_dce, false /* update_address_taken_p */);
+      NEXT_PASS (pass_tail_calls);
       /* Split critical edges before late uninit warning to reduce the
          number of false positives from it.  */
       NEXT_PASS (pass_split_crit_edges);
diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr106922.C b/gcc/testsuite/g++.dg/tree-ssa/pr106922.C
index 2aec4975aa8..4b6a4ad7f6c 100644
--- a/gcc/testsuite/g++.dg/tree-ssa/pr106922.C
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr106922.C
@@ -1,5 +1,5 @@
 // { dg-require-effective-target c++20 }
-// { dg-options "-O2 -fdump-tree-cddce3" }
+// { dg-options "-O2 -fdump-tree-dce7" }
 
 template <typename> struct __new_allocator {
   void deallocate(int *, int) { operator delete(0); }
@@ -87,4 +87,4 @@ void testfunctionfoo() {
   }
 }
 
-// { dg-final { scan-tree-dump-not "m_initialized" "cddce3" } }
+// { dg-final { scan-tree-dump-not "m_initialized" "dce7" } }
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/20030808-1.c b/gcc/testsuite/gcc.dg/tree-ssa/20030808-1.c
index 456f6f27128..7d4a1383ca4 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/20030808-1.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/20030808-1.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O1 -fdump-tree-cddce3" } */
+/* { dg-options "-O1 -fdump-tree-dce7" } */
       
 extern void abort (void);
 
@@ -33,8 +33,8 @@ delete_dead_jumptables ()
 /* There should be no loads of ->code.  If any exist, then we failed to
    optimize away all the IF statements and the statements feeding
    their conditions.  */
-/* { dg-final { scan-tree-dump-times "->code" 0 "cddce3"} } */
+/* { dg-final { scan-tree-dump-times "->code" 0 "dce7"} } */
    
 /* There should be no IF statements.  */
-/* { dg-final { scan-tree-dump-times "if " 0 "cddce3"} } */
+/* { dg-final { scan-tree-dump-times "if " 0 "dce7"} } */
 
diff --git a/gcc/tree-ssa-dce.cc b/gcc/tree-ssa-dce.cc
index 54e5d8c2923..187d58bdd37 100644
--- a/gcc/tree-ssa-dce.cc
+++ b/gcc/tree-ssa-dce.cc
@@ -2005,14 +2005,25 @@ class pass_dce : public gimple_opt_pass
 {
 public:
   pass_dce (gcc::context *ctxt)
-    : gimple_opt_pass (pass_data_dce, ctxt)
+    : gimple_opt_pass (pass_data_dce, ctxt), update_address_taken_p (false)
   {}
 
   /* opt_pass methods: */
   opt_pass * clone () final override { return new pass_dce (m_ctxt); }
+  void set_pass_param (unsigned n, bool param) final override
+    {
+      gcc_assert (n == 0);
+      update_address_taken_p = param;
+    }
   bool gate (function *) final override { return flag_tree_dce != 0; }
-  unsigned int execute (function *) final override { return tree_ssa_dce (); }
+  unsigned int execute (function *) final override
+    {
+      return (tree_ssa_dce ()
+	      | (update_address_taken_p ? TODO_update_address_taken : 0));
+    }
 
+private:
+  bool update_address_taken_p;
 }; // class pass_dce
 
 } // anon namespace
-- 
2.35.3