[PATCH] Fix PR72772

[Richard Biener]

This fixes PR72772 by avoing placing a degenerate PHI in each
forwarder block loop init creates when creating simple preheaders.
The solution is to simply split the single loop entry edge which
is also way cheaper than using the forwarder block creation path.

You've seen a load of fallout fixes already, so this is the final
patch adjusting two testcases (for gcc.dg/tree-ssa/pr59597.c we
no longer register the unwanted threadings as the forwarders
no longer contain PHIs).

This patch will cause

+FAIL: gcc.dg/graphite/scop-dsyr2k.c scan-tree-dump-times graphite "number 
of SCoPs: 1" 1
+FAIL: gcc.dg/graphite/scop-dsyrk.c scan-tree-dump-times graphite "number 
of SCoPs: 1" 1

on x86_64 with -m32 as niter analysis is confused by us now generating
an optimized loop nest via threading that has all redundant checks 
removed.  We no longer can prove that the loops do not eventually
iterate zero times.  I will open a PR for this (it is a latent issue).
The tests would pass with VRP disabled but I choose to leave them
FAILing for now.

Bootstrapped / tested many times on x86_64-unknown-linux-gnu, re-doing
this after the latest fallout fix now.

Richard.

2016-08-05  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/72772
	* cfgloopmanip.c (create_preheader): Use split_edge if there
	is a single loop entry, avoiding degenerate PHIs.

	* gcc.dg/tree-ssa/ldist-24.c: New testcase.
	* gcc.dg/graphite/pr35356-1.c: Adjust.
	* gcc.dg/tree-ssa/pr59597.c: Likewise.

Index: gcc/cfgloopmanip.c
===================================================================
*** gcc/cfgloopmanip.c	(revision 239117)
--- gcc/cfgloopmanip.c	(working copy)
*************** has_preds_from_loop (basic_block block,
*** 1497,1503 ****
  basic_block
  create_preheader (struct loop *loop, int flags)
  {
!   edge e, fallthru;
    basic_block dummy;
    int nentry = 0;
    bool irred = false;
--- 1497,1503 ----
  basic_block
  create_preheader (struct loop *loop, int flags)
  {
!   edge e;
    basic_block dummy;
    int nentry = 0;
    bool irred = false;
*************** create_preheader (struct loop *loop, int
*** 1544,1552 ****
  
    mfb_kj_edge = loop_latch_edge (loop);
    latch_edge_was_fallthru = (mfb_kj_edge->flags & EDGE_FALLTHRU) != 0;
!   fallthru = make_forwarder_block (loop->header, mfb_keep_just, NULL);
!   dummy = fallthru->src;
!   loop->header = fallthru->dest;
  
    /* Try to be clever in placing the newly created preheader.  The idea is to
       avoid breaking any "fallthruness" relationship between blocks.
--- 1544,1557 ----
  
    mfb_kj_edge = loop_latch_edge (loop);
    latch_edge_was_fallthru = (mfb_kj_edge->flags & EDGE_FALLTHRU) != 0;
!   if (nentry == 1)
!     dummy = split_edge (single_entry);
!   else
!     {
!       edge fallthru = make_forwarder_block (loop->header, mfb_keep_just, NULL);
!       dummy = fallthru->src;
!       loop->header = fallthru->dest;
!     }
  
    /* Try to be clever in placing the newly created preheader.  The idea is to
       avoid breaking any "fallthruness" relationship between blocks.
Index: gcc/testsuite/gcc.dg/tree-ssa/ldist-24.c
===================================================================
*** gcc/testsuite/gcc.dg/tree-ssa/ldist-24.c	(revision 0)
--- gcc/testsuite/gcc.dg/tree-ssa/ldist-24.c	(working copy)
***************
*** 0 ****
--- 1,19 ----
+ /* { dg-do compile } */
+ /* { dg-options "-O3 -fdump-tree-ldist" } */
+ 
+ int foo (int flag, char *a)
+ {
+   short i, j;
+   short l = 0;
+   if (flag == 1)
+     l = 3;
+ 
+   for (i = 0; i < 4; i++)
+     {
+       for (j = l - 1; j > 0; j--)
+ 	a[j] = a[j - 1];
+       a[0] = i;
+     }
+ }
+ 
+ /* { dg-final { scan-tree-dump "memmove" "ldist" } } */
Index: gcc/testsuite/gcc.dg/graphite/pr35356-1.c
===================================================================
--- gcc/testsuite/gcc.dg/graphite/pr35356-1.c	(revision 239117)
+++ gcc/testsuite/gcc.dg/graphite/pr35356-1.c	(working copy)
@@ -34,4 +34,4 @@ if (n >= k + 1 && k >= 0) {
 
 */
 
-/* { dg-final { scan-tree-dump "if \\\(P_9 >= P_10 \\\+ 1 && P_10 >= 0\\\) \\\{" "graphite" } } */
+/* { dg-final { scan-tree-dump "if \\\(P_8 >= P_9 \\\+ 1 && P_9 >= 0\\\) \\\{" "graphite" } } */
Index: gcc/testsuite/gcc.dg/tree-ssa/pr59597.c
===================================================================
--- gcc/testsuite/gcc.dg/tree-ssa/pr59597.c	(revision 239164)
+++ gcc/testsuite/gcc.dg/tree-ssa/pr59597.c	(working copy)
@@ -54,5 +54,6 @@ main (int argc, char argv[])
   return crc;
 }
 
-/* { dg-final { scan-tree-dump "Cancelling" "vrp1" } } */
-
+/* { dg-final { scan-tree-dump-times "Registering jump thread" 3 "vrp1" } } */
+/* { dg-final { scan-tree-dump-not "joiner" "vrp1" } } */
+/* { dg-final { scan-tree-dump-times "Threaded jump" 3 "vrp1" } } */