tree-optimization/102880 - improve CD-DCE

The PR shows a missed control-dependent DCE caused by CFG cleanup
merging a forwarder resulting in a partially degenerate PHI node.
With control-dependent DCE we need to mark control dependences
of incoming edges into PHIs as necessary but that is unnecessarily
conservative for the case when two edges have the same value.
There is no easy way to mark only a subset of control dependences
of both edges necessary so the fix is to produce forwarder blocks
where then the control dependence captures the requirements more
precisely.

For gcc.dg/tree-ssa/ssa-dom-thread-7.c the number of edges in the
CFG decrease as we have commonized PHI arguments which in turn
results in different threadings.  The testcase is too complex
and the dump scanning too simple to do anything meaningful here
but to adjust the number of expected threads.

The same CFG massaging could be useful at RTL expansion time to
reduce the number of copies we need to insert on edges.

FAIL: gcc.dg/tree-ssa/ssa-hoist-4.c scan-tree-dump-times optimized "MAX_EXPR" 1

2021-11-12  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102880
	* tree-ssa-dce.c (sort_phi_args): New function.
	(make_forwarders_with_degenerate_phis): Likewise.
	(perform_tree_ssa_dce): Call
	make_forwarders_with_degenerate_phis.

	* gcc.dg/tree-ssa/pr102880.c: New testcase.
	* gcc.dg/tree-ssa/pr69270-3.c: Robustify.
	* gcc.dg/tree-ssa/ssa-dom-thread-7.c: Change the number of
	expected threadings.

3 files changed, 29 insertions, 2 deletions

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr102880.c b/gcc/testsuite/gcc.dg/tree-ssa/pr102880.c
new file mode 100644
index 0000000..0306dee
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr102880.c
@@ -0,0 +1,27 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+void foo(void);
+
+static int b, c, d, e, f, ah;
+static short g, ai, am, aq, as;
+static char an, at, av, ax, ay;
+static char a(char h, char i) { return i == 0 || h && i == 1 ? 0 : h % i; }
+static void ae(int h) {
+  if (a(b, h))
+    foo();
+
+}
+int main() {
+  ae(1);
+  ay = a(0, ay);
+  ax = a(g, aq);
+  at = a(0, as);
+  av = a(c, 1);
+  an = a(am, f);
+  int al = e || ((a(1, ah) && b) & d) == 2;
+  ai = al;
+}
+
+/* We should eliminate the call to foo.  */
+/* { dg-final { scan-tree-dump-not "foo" "optimized" } } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr69270-3.c b/gcc/testsuite/gcc.dg/tree-ssa/pr69270-3.c
index 89735f6..5ffd5f7 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/pr69270-3.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/pr69270-3.c
@@ -3,7 +3,7 @@
 
 /* We're looking for a constant argument a PHI node.  There
    should only be one if we unpropagate correctly.  */
-/* { dg-final { scan-tree-dump-times ", 1" 1 "uncprop1"} } */
+/* { dg-final { scan-tree-dump-times "<1\|, 1" 1 "uncprop1"} } */
 
 typedef long unsigned int size_t;
 typedef union gimple_statement_d *gimple;
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c
index d40a61f..b64e71d 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c
@@ -11,7 +11,7 @@
    to change decisions in switch expansion which in turn can expose new
    jump threading opportunities.  Skip the later tests on aarch64.  */
 /* { dg-final { scan-tree-dump-not "Jumps threaded"  "dom3" { target { ! aarch64*-*-* } } } } */
-/* { dg-final { scan-tree-dump "Jumps threaded: 11"  "thread2" { target { ! aarch64*-*-* } } } } */
+/* { dg-final { scan-tree-dump "Jumps threaded: 7"  "thread2" { target { ! aarch64*-*-* } } } } */
 /* { dg-final { scan-tree-dump "Jumps threaded: 18"  "thread2" { target { aarch64*-*-* } } } } */
 
 enum STATE {