Hi!
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.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2025-01-28 Jakub Jelinek <ja...@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.
--- gcc/combine.cc.jj 2025-01-27 16:38:47.000000000 +0100
+++ gcc/combine.cc 2025-01-27 18:55:22.082925556 +0100
@@ -7629,7 +7629,7 @@ make_extraction (machine_mode mode, rtx
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,
--- gcc/testsuite/gcc.c-torture/execute/pr118638.c.jj 2025-01-27
18:56:30.246986900 +0100
+++ gcc/testsuite/gcc.c-torture/execute/pr118638.c 2025-01-27
18:56:13.660215308 +0100
@@ -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 ();
+}
Jakub
