expand: Fold x - y < 0 to x < y during expansion [PR94802]

My earlier patch to simplify x - y < 0 etc. for signed subtraction
with undefined overflow into x < y in match.pd regressed some tests,
even when it was guarded to be post-IPA, the following patch thus
attempts to optimize that during expansion instead (which is the last
time we can do it, afterwards we lose the information whether it was
x - y < 0 or (int) ((unsigned) x - y) < 0 for which we couldn't
optimize it.

2021-01-05  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/94802
	* expr.h (maybe_optimize_sub_cmp_0): Declare.
	* expr.c: Include tree-pretty-print.h and flags.h.
	(maybe_optimize_sub_cmp_0): New function.
	(do_store_flag): Use it.
	* cfgexpand.c (expand_gimple_cond): Likewise.

	* gcc.target/i386/pr94802.c: New test.
	* gcc.dg/Wstrict-overflow-25.c: Remove xfail.

2 files changed, 60 insertions, 1 deletions

diff --git a/gcc/testsuite/gcc.dg/Wstrict-overflow-25.c b/gcc/testsuite/gcc.dg/Wstrict-overflow-25.c
index 774474d..0091644 100644
--- a/gcc/testsuite/gcc.dg/Wstrict-overflow-25.c
+++ b/gcc/testsuite/gcc.dg/Wstrict-overflow-25.c
@@ -7,5 +7,5 @@
 int
 foo (int x, int y)
 {
-  return x - y < 0; /* { dg-warning "assuming signed overflow does not occur" "correct warning" { xfail *-*-* } } */
+  return x - y < 0; /* { dg-warning "assuming signed overflow does not occur" "correct warning" } */
 }
diff --git a/gcc/testsuite/gcc.target/i386/pr94802.c b/gcc/testsuite/gcc.target/i386/pr94802.c
new file mode 100644
index 0000000..6c1bc66
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr94802.c
@@ -0,0 +1,59 @@
+/* PR tree-optimization/94802 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -masm=att" } */
+/* { dg-final { scan-assembler-not "\ttestl\t" } } */
+/* { dg-final { scan-assembler-times "\tcmpl\t" 8 } } */
+
+void foo (void);
+
+int
+f1 (int a, int b)
+{
+  return (a - b) >= 0;
+}
+
+int
+f2 (int a, int b)
+{
+  return (a - b) > 0;
+}
+
+int
+f3 (int a, int b)
+{
+  return (a - b) <= 0;
+}
+
+int
+f4 (int a, int b)
+{
+  return (a - b) < 0;
+}
+
+void
+f5 (int a, int b)
+{
+  if ((a - b) >= 0)
+    foo ();
+}
+
+void
+f6 (int a, int b)
+{
+  if ((a - b) > 0)
+    foo ();
+}
+
+void
+f7 (int a, int b)
+{
+  if ((a - b) <= 0)
+    foo ();
+}
+
+void
+f8 (int a, int b)
+{
+  if ((a - b) < 0)
+    foo ();
+}