Doing aggressive loop removal at early CD-DCE might break OpenACC
transformation. To the issue, this version 3 gave a somewhat simpler solution
different from previous version 2. It uses a flag to postpone loop removal to
late CD-DCE pass.
Feng
---------
diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 9f0f889..f18abdc 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,14 @@
+2019-05-23 Feng Xue <f...@os.amperecomputing.com>
+
+ PR tree-optimization/89713
+ * doc/invoke.texi (-ffinite-loop): Document new option.
+ * common.opt (-ffinite-loop): New option.
+ * tree-ssa-dce.c (loop_has_true_exits): New function.
+ (find_obviously_necessary_stmts): Use flag to control
+ removal of a loop with assumed finiteness.
+ * tree-ssa-loop.c (tree_ssa_loop_done): Update
+ flag_finite_loop.
+
2019-05-22 David Malcolm <dmalc...@redhat.com>
PR c++/90462
diff --git a/gcc/common.opt b/gcc/common.opt
index d342c4f..e98a34d 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1437,6 +1437,10 @@ ffinite-math-only
Common Report Var(flag_finite_math_only) Optimization SetByCombined
Assume no NaNs or infinities are generated.
+ffinite-loop
+Common Report Var(flag_finite_loop) Optimization
+Assume loops are finite if can not be analytically determined.
+
ffixed-
Common Joined RejectNegative Var(common_deferred_options) Defer
-ffixed-<register> Mark <register> as being unavailable to the compiler.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 6c89843..caa0852 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -412,6 +412,7 @@ Objective-C and Objective-C++ Dialects}.
-fdevirtualize-at-ltrans -fdse @gol
-fearly-inlining -fipa-sra -fexpensive-optimizations -ffat-lto-objects @gol
-ffast-math -ffinite-math-only -ffloat-store -fexcess-precision=@var{style}
@gol
+-ffinite-loop @gol
-fforward-propagate -ffp-contract=@var{style} -ffunction-sections @gol
-fgcse -fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity @gol
-fgcse-sm -fhoist-adjacent-loads -fif-conversion @gol
@@ -9501,6 +9502,20 @@ that may set @code{errno} but are otherwise free of side
effects. This flag is
enabled by default at @option{-O2} and higher if @option{-Os} is not also
specified.
+@item -ffinite-loop
+@opindex ffinite-loop
+@opindex fno-finite-loop
+Allow the compiler to assume that if finiteness of a loop can not be
+analytically determined, the loop must be finite. With the assumption, some
+aggressive transformation could be possible, such as removal of this kind
+of empty loop by dead code elimination (DCE).
+
+This option is not turned on by any @option{-O} option since it might result
+in incorrect behaviour for programs that contain seemly finite, but actually
+infinite loop.
+
+The default is @option{-fno-finite-loop}.
+
@item -ftree-dominator-opts
@opindex ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy
diff --git a/gcc/testsuite/g++.dg/tree-ssa/empty-loop.C
b/gcc/testsuite/g++.dg/tree-ssa/empty-loop.C
new file mode 100644
index 0000000..f6564a2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/empty-loop.C
@@ -0,0 +1,33 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-cddce3 -ffinite-loop" } */
+
+#include <string>
+#include <vector>
+#include <list>
+#include <set>
+#include <map>
+
+using namespace std;
+
+int foo (vector<string> &v, list<string> &l, set<string> &s, map<int, string>
&m)
+{
+ for (vector<string>::iterator it = v.begin (); it != v.end (); ++it)
+ it->length();
+
+ for (list<string>::iterator it = l.begin (); it != l.end (); ++it)
+ it->length();
+
+ for (map<int, string>::iterator it = m.begin (); it != m.end (); ++it)
+ it->first + it->second.length();
+
+ for (set<string>::iterator it0 = s.begin (); it0 != s.end(); ++it0)
+ for (vector<string>::reverse_iterator it1 = v.rbegin(); it1 != v.rend();
++it1)
+ {
+ it0->length();
+ it1->length();
+ }
+
+ return 0;
+}
+/* { dg-final { scan-tree-dump-not "if" "cddce3"} } */
+
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/dce-2.c
b/gcc/testsuite/gcc.dg/tree-ssa/dce-2.c
new file mode 100644
index 0000000..d9b9fd2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/dce-2.c
@@ -0,0 +1,37 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-cddce3 -ffinite-loop" } */
+
+typedef struct list {
+ char pad[15];
+ struct list *next;
+} list;
+
+int data;
+
+list *head, *tail;
+
+int __attribute__((pure)) pfn (int);
+
+int foo (unsigned u, int s)
+{
+ unsigned i;
+ list *p;
+ int j;
+
+ for (i = 0; i < u; i += 2)
+ ;
+
+ for (p = head; p; p = p->next)
+ ;
+
+ for (j = data; j & s; j = pfn (j + 3))
+ ;
+
+ for (p = head; p != tail; p = p->next)
+ for (j = data + 1; j > s; j = pfn (j + 2))
+ ;
+
+ return 0;
+}
+/* { dg-final { scan-tree-dump-not "if" "cddce3"} } */
+
diff --git a/gcc/tree-ssa-dce.c b/gcc/tree-ssa-dce.c
index 2478219..25f722f 100644
--- a/gcc/tree-ssa-dce.c
+++ b/gcc/tree-ssa-dce.c
@@ -357,6 +357,28 @@ mark_control_dependent_edges_necessary (basic_block bb,
bool ignore_self)
}
+/* Check whether a loop has any non-EH exit. */
+
+static bool
+loop_has_true_exits (const struct loop *loop)
+{
+ vec<edge> exits = get_loop_exit_edges (loop);
+ bool found = false;
+ edge e;
+ unsigned i;
+
+ FOR_EACH_VEC_ELT (exits, i, e)
+ if (!(e->flags & EDGE_EH))
+ {
+ found = true;
+ break;
+ }
+
+ exits.release ();
+ return found;
+}
+
+
/* Find obviously necessary statements. These are things like most function
calls, and stores to file level variables.
@@ -419,6 +441,25 @@ find_obviously_necessary_stmts (bool aggressive)
FOR_EACH_LOOP (loop, 0)
if (!finite_loop_p (loop))
{
+ /* If done at early CD-DCE pass, aggressive loop removal using
+ assumed finiteness could break OpenACC transformation. It might
+ incorrectly change or even remove IFN_GOACC_LOOP calls, which
+ represent parameters (i.e. step, bound) of an abstract OpenACC
+ partitioned loop, therefore are required by OpenACC loop
+ offload lowering. Here we need a way to postpone aggressive
+ loop removal to late pass.
+
+ When flag_finite_loop is 1, it means the optimization is
+ enabled, but time is not ready. Once ready, flag_finite_loop
+ will be updated with larger integer somewhere, and loop
+ removal is truely allowed. */
+ if (flag_finite_loop > 1 && loop_has_true_exits (loop))
+ {
+ if (dump_file)
+ fprintf (dump_file, "assume loop %i to be finite\n",
loop->num);
+ continue;
+ }
+
if (dump_file)
fprintf (dump_file, "cannot prove finiteness of loop %i\n",
loop->num);
mark_control_dependent_edges_necessary (loop->latch, false);
diff --git a/gcc/tree-ssa-loop.c b/gcc/tree-ssa-loop.c
index 1ac6cee..4175f9e 100644
--- a/gcc/tree-ssa-loop.c
+++ b/gcc/tree-ssa-loop.c
@@ -527,6 +527,11 @@ make_pass_iv_optimize (gcc::context *ctxt)
static unsigned int
tree_ssa_loop_done (void)
{
+ /* After all loop optimizations are done, increase flag_finite_loop to
+ truely allow aggressive loop removal in CD-DCE pass. */
+ if (flag_finite_loop)
+ flag_finite_loop++;
+
free_numbers_of_iterations_estimates (cfun);
scev_finalize ();
loop_optimizer_finalize ();
diff --git a/libgomp/testsuite/libgomp.oacc-c-c++-common/pr84955-1.c
b/libgomp/testsuite/libgomp.oacc-c-c++-common/pr84955-1.c
new file mode 100644
index 0000000..65a34d8
--- /dev/null
+++ b/libgomp/testsuite/libgomp.oacc-c-c++-common/pr84955-1.c
@@ -0,0 +1,31 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-cddce3 -ffinite-loop" } */
+
+int
+f1 (void)
+{
+ int i, j;
+
+#pragma acc parallel loop tile(2,3)
+ for (i = 1; i < 10; i++)
+ for (j = 1; j < 10; j++)
+ for (;;)
+ ;
+
+ return i + j;
+}
+
+int
+f2 (void)
+{
+ int i, j, k;
+
+#pragma acc parallel loop tile(2,3)
+ for (i = 1; i < 10; i++)
+ for (j = 1; j < 10; j++)
+ for (k = 1; k < 10; k++)
+ ;
+
+ return i + j;
+}
+/* { dg-final { scan-tree-dump-not "if" "cddce3"} } */
