match.pd: Avoid ICE with shifts [PR97979]

My recent wide_int_binop changes caused ICE on this testcase.
The problem is that for shift where amount has MSB set now fails to optimize
into a constant (IMHO we should treat out of bounds shifts the same later),
but there is a precedent for that already - e.g. division by zero fails
to optimize into a constant too.  I think it is better if path isolation
checks for these UBs and does something the user chooses (__builtin_trap vs.
__builtin_unreachable, and either a deferred warning about the UB or
nothing).
This patch just doesn't optimize if int_const_binop failed.

2020-11-26  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/97979
	* match.pd ((X {&,^,|} C2) << C1 into (X << C1) {&,^,|} (C2 << C1)):
	Only optimize if int_const_binop returned non-NULL.

	* gcc.dg/pr97979.c: New test.
	* gcc.c-torture/compile/pr97979.c: New test.

diff --git a/gcc/match.pd b/gcc/match.pd
index 4d290ad..f8b6515 100644 (file)
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -3119,7 +3119,8 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
    (shift (convert?:s (bit_op:s @0 INTEGER_CST@2)) INTEGER_CST@1)
    (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
     (with { tree mask = int_const_binop (shift, fold_convert (type, @2), @1); }
-     (bit_op (shift (convert @0) @1) { mask; }))))))
+     (if (mask)
+      (bit_op (shift (convert @0) @1) { mask; })))))))
 
 /* ~(~X >> Y) -> X >> Y (for arithmetic shift).  */
 (simplify

diff --git a/gcc/testsuite/gcc.c-torture/compile/pr97979.c b/gcc/testsuite/gcc.c-torture/compile/pr97979.c
new file mode 100644 (file)
index 0000000..f4f88a4
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/compile/pr97979.c
@@ -0,0 +1,7 @@
+/* PR tree-optimization/97979 */
+
+int
+foo (int x)
+{
+  return (x & 0x123) << -3;
+}

diff --git a/gcc/testsuite/gcc.dg/pr97979.c b/gcc/testsuite/gcc.dg/pr97979.c
new file mode 100644 (file)
index 0000000..44aaff2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr97979.c
@@ -0,0 +1,13 @@
+/* PR tree-optimization/97979 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fno-tree-ccp" } */
+
+short a = 0;
+int b = 0;
+
+void
+foo (void)
+{
+  unsigned short d = b;
+  a = d >> -2U;        /* { dg-warning "right shift count >= width of type" } */
+}