combine: Fix up make_extraction [PR118638]

The following testcase is miscompiled at -Os on x86_64-linux.
The problem is during make_compound_operation of
(ashiftrt:SI (ashift:SI (mult:SI (reg:SI 107 [ a_5 ])
            (const_int 3 [0x3]))
        (const_int 31 [0x1f]))
    (const_int 31 [0x1f]))
where it incorrectly returns
(mult:SI (sign_extract:SI (reg:SI 107 [ a_5 ])
        (const_int 2 [0x2])
        (const_int 0 [0]))
    (const_int 3 [0x3]))
which isn't obviously true, the former returns either 0 or -1 depending
on the least significant bit of the multiplication,
the latter returns either 0 or -3 depending on the second least significant
bit of the multiplication argument.

The bug has been introduced in PR96998 r11-4563, which added handling of x
* (2^N) similar to x << N.  In the above case, pos is 0 and len is 1,
sign extracting a single least significant bit of the multiplication.
As 3 is not a power of 2, shift_amt is -1.
But IN_RANGE (-1, 1, 1 - 1) is still true, because the basic requirement of
IN_RANGE that LOWER is not greater than UPPER is violated.
The intention of using 1 as LOWER is to avoid matching multiplication by 1,
that really shouldn't appear in the IL.  But to avoid violating IN_RANGE
requirement, we need to verify that len is at least 2.

I've added this len > 1 check to the inner if rather than outer because I
think for GCC 16 we should add a further optimization.
In the particular case of 1 least significant bit sign extraction from
multiplication by 3, we could actually say it is equivalent to
(sign_extract:SI (reg:SI 107 [ a_5 ])
        (const_int 1 [0x2])
        (const_int 0 [0]))
That is because 3 is an odd number and multiplication by 2 will yield the
least significant bit 0 (we are sign extracting just one) and so the
multiplication doesn't change anything on the outcome.
More generally, even for larger len, multiplication by C which is
(1 << X) + 1 where X is >= len should be optimizable just to extraction
of the multiplicand's least significant len bits.

2025-01-28  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/118638
	* combine.cc (make_extraction): Only optimize (mult x 2^n) if len is
	larger than 1.

	* gcc.c-torture/execute/pr118638.c: New test.

(cherry picked from commit b529a417249335724d1f74bcf3167f6f9a623823)

diff --git a/gcc/combine.cc b/gcc/combine.cc
index 60afe043578de857cf183fc8386588993ed2d775..431a304290163dddeca3f8b8d19e5ce9803ddd55 100644 (file)
--- a/gcc/combine.cc
+++ b/gcc/combine.cc
@@ -7588,7 +7588,7 @@ make_extraction (machine_mode mode, rtx inner, HOST_WIDE_INT pos,
         least significant (LEN - C) bits of X, giving an rtx
         whose mode is MODE, then multiply it by 2^C.  */
       const HOST_WIDE_INT shift_amt = exact_log2 (INTVAL (XEXP (inner, 1)));
-      if (IN_RANGE (shift_amt, 1, len - 1))
+      if (len > 1 && IN_RANGE (shift_amt, 1, len - 1))
        {
          new_rtx = make_extraction (mode, XEXP (inner, 0),
                                     0, 0, len - shift_amt,

diff --git a/gcc/testsuite/gcc.c-torture/execute/pr118638.c b/gcc/testsuite/gcc.c-torture/execute/pr118638.c
new file mode 100644 (file)
index 0000000..5c0dbca
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr118638.c
@@ -0,0 +1,20 @@
+/* PR rtl-optimization/118638 */
+
+__attribute__((noipa)) int
+foo (int x)
+{
+  int a = x != -3, b, c;
+  a *= 3;
+  b = 2 * x - 9;
+  a = a + b;
+  a = ~a;
+  c = a & 1;
+  return -c;
+}
+
+int
+main ()
+{
+  if (foo (0) != -1)
+    __builtin_abort ();
+}