[PATCH] Allow loop header copying when first iteration condition is known.

Aldy Hernandez aldyh@redhat.com
Wed Nov 10 18:20:03 GMT 2021


As discussed in the PR, the loop header copying pass avoids doing so
when optimizing for size.  However, sometimes we can determine the
loop entry conditional statically for the first iteration of the loop.

This patch uses the path solver to determine the outgoing edge
out of preheader->header->xx.  If so, it allows header copying.  Doing
this in the loop optimizer saves us from doing gymnastics in the
threader which doesn't have the context to determine if a loop
transformation is profitable.

I am only returning true in entry_loop_condition_is_static for
a true conditional.  Technically a false conditional is also
provably static, but allowing any boolean value causes a regression
in gfortran.dg/vector_subscript_1.f90.

I would have preferred not passing around the query object, but the
layout of pass_ch and should_duplicate_loop_header_p make it a bit
awkward to get it right without an outright refactor to the
pass.

Tested on x86-64 Linux.

OK?

gcc/ChangeLog:

	PR tree-optimization/102906
	* tree-ssa-loop-ch.c (entry_loop_condition_is_static): New.
	(should_duplicate_loop_header_p): Call entry_loop_condition_is_static.
	(class ch_base): Add m_ranger and m_query.
	(ch_base::copy_headers): Pass m_query to
	entry_loop_condition_is_static.
	(pass_ch::execute): Allocate and deallocate m_ranger and
	m_query.
	(pass_ch_vect::execute): Same.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr102906.c: New test.
---
 gcc/testsuite/gcc.dg/tree-ssa/pr102906.c | 17 ++++++++
 gcc/tree-ssa-loop-ch.c                   | 51 ++++++++++++++++++++----
 2 files changed, 60 insertions(+), 8 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr102906.c

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr102906.c b/gcc/testsuite/gcc.dg/tree-ssa/pr102906.c
new file mode 100644
index 00000000000..1846f0b6dba
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr102906.c
@@ -0,0 +1,17 @@
+// { dg-do compile }
+// { dg-options "-Os -fdump-tree-ch-details" }
+
+extern unsigned int foo (int*) __attribute__((pure));
+
+unsigned int
+tr2 (int array[], int n)
+{
+  unsigned int sum = 0;
+  int x;
+  if (n > 0)
+    for (x = 0; x < n; x++)
+      sum += foo (&array[x]);
+  return sum;
+}
+
+// { dg-final { scan-tree-dump-not "Not duplicating.*optimizing for size" "ch2" } }
diff --git a/gcc/tree-ssa-loop-ch.c b/gcc/tree-ssa-loop-ch.c
index ffb0aa85118..c7d86d751d4 100644
--- a/gcc/tree-ssa-loop-ch.c
+++ b/gcc/tree-ssa-loop-ch.c
@@ -35,30 +35,52 @@ along with GCC; see the file COPYING3.  If not see
 #include "tree-ssa-sccvn.h"
 #include "tree-phinodes.h"
 #include "ssa-iterators.h"
+#include "value-range.h"
+#include "gimple-range.h"
+#include "gimple-range-path.h"
 
 /* Duplicates headers of loops if they are small enough, so that the statements
    in the loop body are always executed when the loop is entered.  This
    increases effectiveness of code motion optimizations, and reduces the need
    for loop preconditioning.  */
 
+/* Return true if the condition on the first iteration of the loop can
+   be statically determined.  */
+
+static bool
+entry_loop_condition_is_static (class loop *l, path_range_query *query)
+{
+  edge e = loop_preheader_edge (l);
+  gcond *last = safe_dyn_cast <gcond *> (last_stmt (e->dest));
+
+  if (!last
+      || !irange::supports_type_p (TREE_TYPE (gimple_cond_lhs (last))))
+    return false;
+
+  int_range<2> r;
+  query->compute_ranges (e);
+  query->range_of_stmt (r, last);
+  return r == int_range<2> (boolean_true_node, boolean_true_node);
+}
+
 /* Check whether we should duplicate HEADER of LOOP.  At most *LIMIT
    instructions should be duplicated, limit is decreased by the actual
    amount.  */
 
 static bool
 should_duplicate_loop_header_p (basic_block header, class loop *loop,
-				int *limit)
+				int *limit, path_range_query *query)
 {
   gimple_stmt_iterator bsi;
 
   gcc_assert (!header->aux);
 
-  /* Loop header copying usually increases size of the code.  This used not to
-     be true, since quite often it is possible to verify that the condition is
-     satisfied in the first iteration and therefore to eliminate it.  Jump
-     threading handles these cases now.  */
+  /* Avoid loop header copying when optimizing for size unless we can
+     determine that the loop condition is static in the first
+     iteration.  */
   if (optimize_loop_for_size_p (loop)
-      && !loop->force_vectorize)
+      && !loop->force_vectorize
+      && !entry_loop_condition_is_static (loop, query))
     {
       if (dump_file && (dump_flags & TDF_DETAILS))
 	fprintf (dump_file,
@@ -267,6 +289,9 @@ class ch_base : public gimple_opt_pass
 
   /* Return true to copy headers of LOOP or false to skip.  */
   virtual bool process_loop_p (class loop *loop) = 0;
+
+  gimple_ranger *m_ranger = NULL;
+  path_range_query *m_query = NULL;
 };
 
 const pass_data pass_data_ch =
@@ -389,7 +414,8 @@ ch_base::copy_headers (function *fun)
 
       exit = NULL;
       n_bbs = 0;
-      while (should_duplicate_loop_header_p (header, loop, &remaining_limit))
+      while (should_duplicate_loop_header_p (header, loop, &remaining_limit,
+					     m_query))
 	{
 	  /* Find a successor of header that is inside a loop; i.e. the new
 	     header after the condition is copied.  */
@@ -526,9 +552,13 @@ pass_ch::execute (function *fun)
   loop_optimizer_init (LOOPS_HAVE_PREHEADERS
 		       | LOOPS_HAVE_SIMPLE_LATCHES
 		       | LOOPS_HAVE_RECORDED_EXITS);
+  m_ranger = new gimple_ranger;
+  m_query = new path_range_query (*m_ranger, /*resolve=*/true);
 
   unsigned int res = copy_headers (fun);
 
+  delete m_query;
+  delete m_ranger;
   loop_optimizer_finalize ();
   return res;
 }
@@ -540,7 +570,12 @@ pass_ch::execute (function *fun)
 unsigned int
 pass_ch_vect::execute (function *fun)
 {
-  return copy_headers (fun);
+  m_ranger = new gimple_ranger;
+  m_query = new path_range_query (*m_ranger, /*resolve=*/true);
+  unsigned int res = copy_headers (fun);
+  delete m_query;
+  delete m_ranger;
+  return res;
 }
 
 /* Apply header copying according to a very simple test of do-while shape.  */
-- 
2.31.1



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