Re: [PATCH] tree-optimization: [PR102622]: wrong code due to signed one bit integer and "a?-1:0"

[Richard Biener]

On October 10, 2021 7:42:19 AM GMT+02:00, apinski--- via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>From: Andrew Pinski <apinski@marvell.com>
>
>So it turns out this is kinda of a latent bug but not really latent.
>In GCC 9 and 10, phi-opt would transform a?-1:0 (even for signed 1-bit integer)
>to -(type)a but the type is an one bit integer which means the negation is
>undefined. GCC 11 fixed the problem by checking for a?pow2cst:0 transformation
>before a?-1:0 transformation.
>
>When I added the transformations to match.pd, I had swapped the order not paying
>attention and I didn't expect anything of it. Because there was no testcase failing
>due to this.
>Anyways this fixes the problem on the trunk by swapping the order in match.pd and
>adding a comment of why the order is this way.
>
>I will try to come up with a patch for GCC 9 and 10 series later on which fixes
>the problem there too.
>
>Note I didn't include the original testcase which requires the vectorizer and AVX-512f
>as I can't figure out the right dg options to restrict it to avx-512f but I did come up
>with a testcase which shows the problem and even more shows the problem with the 9/10
>series as mentioned.
>
>OK? Bootstrapped and tested on x86_64-linux-gnu.

OK. 

Richard. 


>	PR tree-optimization/102622
>
>gcc/ChangeLog:
>
>	* match.pd: Swap the order of a?pow2cst:0 and a?-1:0 transformations.
>	Swap the order of a?0:pow2cst and a?0:-1 transformations.
>
>gcc/testsuite/ChangeLog:
>
>	* gcc.c-torture/execute/bitfld-10.c: New test.
>---
> gcc/match.pd                                  | 26 ++++++++++++-------
> .../gcc.c-torture/execute/bitfld-10.c         | 24 +++++++++++++++++
> 2 files changed, 41 insertions(+), 9 deletions(-)
> create mode 100644 gcc/testsuite/gcc.c-torture/execute/bitfld-10.c
>
>diff --git a/gcc/match.pd b/gcc/match.pd
>index 9d7c1ac637f..c153e9a6e98 100644
>--- a/gcc/match.pd
>+++ b/gcc/match.pd
>@@ -3949,15 +3949,16 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>     /* a ? 1 : 0 -> a if 0 and 1 are integral types. */
>     (if (integer_onep (@1))
>      (convert (convert:boolean_type_node @0)))
>-    /* a ? -1 : 0 -> -a. */
>-    (if (INTEGRAL_TYPE_P (type) && integer_all_onesp (@1))
>-     (negate (convert (convert:boolean_type_node @0))))
>     /* a ? powerof2cst : 0 -> a << (log2(powerof2cst)) */
>     (if (INTEGRAL_TYPE_P (type) && integer_pow2p (@1))
>      (with {
>        tree shift = build_int_cst (integer_type_node, tree_log2 (@1));
>       }
>-      (lshift (convert (convert:boolean_type_node @0)) { shift; })))))
>+      (lshift (convert (convert:boolean_type_node @0)) { shift; })))
>+    /* a ? -1 : 0 -> -a.  No need to check the TYPE_PRECISION not being 1
>+       here as the powerof2cst case above will handle that case correctly.  */
>+    (if (INTEGRAL_TYPE_P (type) && integer_all_onesp (@1))
>+     (negate (convert (convert:boolean_type_node @0))))))
>   (if (integer_zerop (@1))
>    (with {
>       tree booltrue = constant_boolean_node (true, boolean_type_node);
>@@ -3966,16 +3967,23 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>      /* a ? 0 : 1 -> !a. */
>      (if (integer_onep (@2))
>       (convert (bit_xor (convert:boolean_type_node @0) { booltrue; } )))
>-     /* a ? -1 : 0 -> -(!a). */
>-     (if (INTEGRAL_TYPE_P (type) && integer_all_onesp (@2))
>-      (negate (convert (bit_xor (convert:boolean_type_node @0) { booltrue; } ))))
>      /* a ? powerof2cst : 0 -> (!a) << (log2(powerof2cst)) */
>-     (if (INTEGRAL_TYPE_P (type) &&  integer_pow2p (@2))
>+     (if (INTEGRAL_TYPE_P (type) &&  integer_pow2p (@2)
>+         && TYPE_PRECISION (type) != 1)
>       (with {
> 	tree shift = build_int_cst (integer_type_node, tree_log2 (@2));
>        }
>        (lshift (convert (bit_xor (convert:boolean_type_node @0) { booltrue; } ))
>-        { shift; }))))))))
>+        { shift; })))
>+     /* a ? -1 : 0 -> -(!a).  No need to check the TYPE_PRECISION not being 1
>+       here as the powerof2cst case above will handle that case correctly.  */
>+     (if (INTEGRAL_TYPE_P (type) && integer_all_onesp (@2))
>+      (negate (convert (bit_xor (convert:boolean_type_node @0) { booltrue; } ))))
>+    )
>+   )
>+  )
>+ )
>+)
> #endif
> 
> /* Simplification moved from fold_cond_expr_with_comparison.  It may also
>diff --git a/gcc/testsuite/gcc.c-torture/execute/bitfld-10.c b/gcc/testsuite/gcc.c-torture/execute/bitfld-10.c
>new file mode 100644
>index 00000000000..bdbf5733ce7
>--- /dev/null
>+++ b/gcc/testsuite/gcc.c-torture/execute/bitfld-10.c
>@@ -0,0 +1,24 @@
>+/* PR tree-optimization/102622 */
>+/* Wrong code introduced due to phi-opt
>+   introducing undefined signed interger overflow
>+   with one bit signed integer negation. */
>+
>+struct f{signed t:1;};
>+int g(struct f *a, int t) __attribute__((noipa));
>+int g(struct f *a, int t)
>+{
>+    if (t)
>+      a->t = -1;
>+    else
>+      a->t = 0;
>+    int t1 = a->t;
>+    if (t1) return 1;
>+    return t1;
>+}
>+
>+int main(void)
>+{
>+    struct f a;
>+    if (!g(&a, 1))  __builtin_abort();
>+    return 0;
>+}