Fix PR83323 (crafty miscompile by unroll-and-jam)

[Michael Matz <matz at suse dot de>]

Hi,

the predicate for feasiblity of unroll-and-jam was a bit nonsensical.  
Properly check for non-head-controlled loops, and also check all BBs of 
the outer loop for any instructions that would inhibit fusion (before we 
never checked header or latch).

Richi also asked me to reuse the -floop-unrool-and-jam option (currently 
aliased to graphites loop-nest-optimize, like a couple other old options), 
instead of adding a new one with slightly different spelling.  This patch 
does that in addition.

Fixes the added testcase and crafty from cpu2000.  Regstrapping on 
x86_64-linux in progress.  Okay if that passes?


Ciao,
Michael.
---------------------------
    Fix PR83323
    
    	* gimple-loop-jam (unroll_jam_possible_p): Correct test for
    	head-controlled loops and loop BBs.
    	* common.opt (funroll-and-jam): Remove, instead ...
    	(floop-unroll-and-jam): ... reuse this option.
    	* opts.c (default_options_table): Use OPT_floop_unroll_and_jam.
    	* doc/invoke.texi (-funroll-and-jam): Move docu to ...
    	(-floop-unroll-and-jam): ... this option.
    
    testsuite/
    	* gcc.dg/pr83323.c: New test.

diff --git a/gcc/common.opt b/gcc/common.opt
index 6fab2ab..57b3cd7 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1512,8 +1512,8 @@ Common Alias(floop-nest-optimize)
 Enable loop nest transforms.  Same as -floop-nest-optimize.
 
 floop-unroll-and-jam
-Common Alias(floop-nest-optimize)
-Enable loop nest transforms.  Same as -floop-nest-optimize.
+Common Report Var(flag_unroll_jam) Optimization
+Perform unroll-and-jam on loops.
 
 fgnu-tm
 Common Report Var(flag_tm)
@@ -2695,10 +2695,6 @@ fsplit-loops
 Common Report Var(flag_split_loops) Optimization
 Perform loop splitting.
 
-funroll-and-jam
-Common Report Var(flag_unroll_jam) Optimization
-Perform unroll-and-jam on loops.
-
 funwind-tables
 Common Report Var(flag_unwind_tables) Optimization
 Just generate unwind tables for exception handling.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 50740c5..1413095 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -437,7 +437,7 @@ Objective-C and Objective-C++ Dialects}.
 -ftree-reassoc  -ftree-sink  -ftree-slsr  -ftree-sra @gol
 -ftree-switch-conversion  -ftree-tail-merge @gol
 -ftree-ter  -ftree-vectorize  -ftree-vrp  -funconstrained-commons @gol
--funit-at-a-time  -funroll-all-loops  -funroll-loops -funroll-and-jam @gol
+-funit-at-a-time  -funroll-all-loops  -funroll-loops @gol
 -funsafe-math-optimizations  -funswitch-loops @gol
 -fipa-ra  -fvariable-expansion-in-unroller  -fvect-cost-model  -fvpt @gol
 -fweb  -fwhole-program  -fwpa  -fuse-linker-plugin @gol
@@ -8511,11 +8511,9 @@ at @option{-O} and higher.
 @item -ftree-loop-linear
 @itemx -floop-strip-mine
 @itemx -floop-block
-@itemx -floop-unroll-and-jam
 @opindex ftree-loop-linear
 @opindex floop-strip-mine
 @opindex floop-block
-@opindex floop-unroll-and-jam
 Perform loop nest optimizations.  Same as
 @option{-floop-nest-optimize}.  To use this code transformation, GCC has
 to be configured with @option{--with-isl} to enable the Graphite loop
@@ -9789,8 +9787,8 @@ for one side of the iteration space and false for the other.
 Move branches with loop invariant conditions out of the loop, with duplicates
 of the loop on both branches (modified according to result of the condition).
 
-@item -funroll-and-jam
-@opindex funroll-and-jam
+@item -floop-unroll-and-jam
+@opindex floop-unroll-and-jam
 Apply unroll and jam transoformations on feasible loops.  In a loop
 nest this unrolls the outer loop by some factor and fuses the resulting
 multiple inner loops.
diff --git a/gcc/gimple-loop-jam.c b/gcc/gimple-loop-jam.c
index 32f813b..8ed1bef 100644
--- a/gcc/gimple-loop-jam.c
+++ b/gcc/gimple-loop-jam.c
@@ -152,7 +152,7 @@ merge_loop_tree (struct loop *loop, struct loop *old)
   free (bbs);
 }
 
-/* BB exits the outer loop of an unroll-and-jam situation.
+/* BB is part of the outer loop of an unroll-and-jam situation.
    Check if any statements therein would prevent the transformation.  */
 
 static bool
@@ -160,9 +160,10 @@ bb_prevents_fusion_p (basic_block bb)
 {
   gimple_stmt_iterator gsi;
   /* BB is duplicated by outer unrolling and then all N-1 first copies
-     move into the body of the fused inner loop.  The last copy remains
-     the exit block of the outer loop and is still outside the inner loop
-     also after fusion.  We can't allow this for some effects of BB:
+     move into the body of the fused inner loop.  If BB exits the outer loop
+     the last copy still doess so, and the first N-1 copies are cancelled
+     by loop unrolling, so also after fusion it's the exit block.
+     But there might be other reasons that prevent fusion:
        * stores or unknown side-effects prevent fusion
        * loads don't
        * computations into SSA names: these aren't problematic.  Their
@@ -204,6 +205,19 @@ unroll_jam_possible_p (struct loop *outer, struct loop *loop)
   if (outer->inner != loop || loop->next)
     return false;
 
+  /* Prevent head-controlled inner loops, that we usually have.
+     The guard block would need to be accepted
+     (invariant condition either entering or skipping the loop),
+     without also accepting arbitrary control flow.  When unswitching
+     ran before us (as with -O3) this won't be a problem because its
+     outer loop unswitching will have moved out the invariant condition.
+
+     If we do that we need to extend fuse_loops() to cope with this
+     by threading through the (still invariant) copied condition
+     between the two loop copies.  */
+  if (!dominated_by_p (CDI_DOMINATORS, outer->latch, loop->header))
+    return false;
+
   /* The number of iterations of the inner loop must be loop invariant
      with respect to the outer loop.  */
   if (!number_of_iterations_exit (loop, single_exit (loop), &niter,
@@ -218,23 +232,8 @@ unroll_jam_possible_p (struct loop *outer, struct loop *loop)
   n = get_loop_body_with_size (outer, bbs, n_basic_blocks_for_fn (cfun));
 
   for (i = 0; i < n; i++)
-    {
-      if (bbs[i]->loop_father == outer
-	  && bbs[i] != outer->latch && bbs[i] != outer->header
-	  && (!loop_exits_from_bb_p (outer, bbs[i])
-	      || bb_prevents_fusion_p (bbs[i])))
-	break;
-      /* XXX Note that the above disallows head-controlled inner loops,
-         that we usually have.  The guard block would need to be accepted
-	 (invariant condition either entering or skipping the loop),
-	 without also accepting arbitrary control flow.  When unswitching
-	 ran before us (as with -O3) this won't be a problem because its
-	 outer loop unswitching will have moved out the invariant condition.
-	 
-	 If we do that we need to extend fuse_loops() to cope with this
-	 by threading through the (still invariant) copied condition
-	 between the two loop copies.  */
-    }
+    if (bbs[i]->loop_father == outer && bb_prevents_fusion_p (bbs[i]))
+      break;
   free (bbs);
   if (i != n)
     return false;
diff --git a/gcc/opts.c b/gcc/opts.c
index 98fbf53..a157b5f 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -536,7 +536,7 @@ static const struct default_options default_options_table[] =
     { OPT_LEVELS_1_PLUS_NOT_DEBUG, OPT_finline_functions_called_once, NULL, 1 },
     { OPT_LEVELS_3_PLUS, OPT_fsplit_loops, NULL, 1 },
     { OPT_LEVELS_3_PLUS, OPT_funswitch_loops, NULL, 1 },
-    { OPT_LEVELS_3_PLUS, OPT_funroll_and_jam, NULL, 1 },
+    { OPT_LEVELS_3_PLUS, OPT_floop_unroll_and_jam, NULL, 1 },
     { OPT_LEVELS_3_PLUS, OPT_fgcse_after_reload, NULL, 1 },
     { OPT_LEVELS_3_PLUS, OPT_ftree_loop_vectorize, NULL, 1 },
     { OPT_LEVELS_3_PLUS, OPT_ftree_slp_vectorize, NULL, 1 },
diff --git a/gcc/testsuite/gcc.dg/pr83323.c b/gcc/testsuite/gcc.dg/pr83323.c
new file mode 100644
index 0000000..6111745
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr83323.c
@@ -0,0 +1,23 @@
+/* { dg-do run } */
+/* { dg-options "-O3 -floop-unroll-and-jam --param unroll-jam-min-percent=0" } */
+int x[1024], y[1024];
+
+void __attribute__((noipa)) foo ()
+{
+  for (int i = 0; i < 1024; ++i)
+    {
+      x[i] = 0;
+      for (int j = 0; j < 1024; ++j)
+        if (!y[j])
+          x[i] = 1;
+    }
+}
+
+int main()
+{
+  y[1023] = 1;
+  foo ();
+  if (x[1] != 1)
+    __builtin_abort ();
+  return 0;
+}