Hi!

The AVX512{F,VL} vector comparisons that set %kN registers are represented
in RTL as comparisons with vector mode operands and scalar integral result,
where at runtime the scalar integer is filled with a bitmask.
Unfortunately, simplify_relational_operation would fold e.g.
(eq:SI (reg:V32HI x) (reg:V32HI x))
into (const_int 1) rather than (const_int -1) that is expected (all elements
equal).  simplify_const_relational_operation is documented to always return
just const0_rtx or const_true_rtx and simplify_relational_operation is
expected to fix this up, for vector comparisons with vector result it
duplicates the 0 or -1 into all elements, etc., so this patch adds handling
for this case there too.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2019-12-11  Jakub Jelinek  <ja...@redhat.com>

        PR target/92908
        * simplify-rtx.c (simplify_relational_operation): For vector cmp_mode
        and scalar mode, if simplify_relational_operation returned
        const_true_rtx, return a scalar bitmask of all ones.
        (simplify_const_relational_operation): Change VOID_mode in function
        comment to VOIDmode.

        * gcc.target/i386/avx512bw-pr92908.c: New test.

--- gcc/simplify-rtx.c.jj       2019-11-19 22:27:02.000058742 +0100
+++ gcc/simplify-rtx.c  2019-12-11 13:31:57.197809704 +0100
@@ -5037,6 +5037,23 @@ simplify_relational_operation (enum rtx_
          return NULL_RTX;
 #endif
        }
+      if (VECTOR_MODE_P (cmp_mode)
+         && SCALAR_INT_MODE_P (mode)
+         && tem == const_true_rtx)
+       {
+         /* Vector comparisons that expect a scalar integral
+            bitmask.  For const0_rtx the result is already correct,
+            for const_true_rtx we need all bits set.  */
+         int n_elts;
+         scalar_int_mode smode = as_a <scalar_int_mode> (mode);
+         gcc_assert (GET_MODE_NUNITS (cmp_mode).is_constant (&n_elts)
+                     && GET_MODE_PRECISION (smode) <= n_elts);
+         if (GET_MODE_PRECISION (smode) == n_elts)
+           return constm1_rtx;
+         if (n_elts < HOST_BITS_PER_WIDE_INT)
+           return GEN_INT ((HOST_WIDE_INT_1U << n_elts) - 1);
+         return NULL_RTX;
+       }
 
       return tem;
     }
@@ -5383,7 +5400,7 @@ comparison_result (enum rtx_code code, i
 }
 
 /* Check if the given comparison (done in the given MODE) is actually
-   a tautology or a contradiction.  If the mode is VOID_mode, the
+   a tautology or a contradiction.  If the mode is VOIDmode, the
    comparison is done in "infinite precision".  If no simplification
    is possible, this function returns zero.  Otherwise, it returns
    either const_true_rtx or const0_rtx.  */
--- gcc/testsuite/gcc.target/i386/avx512bw-pr92908.c.jj 2019-12-11 
14:24:12.083418762 +0100
+++ gcc/testsuite/gcc.target/i386/avx512bw-pr92908.c    2019-12-11 
14:23:56.071665326 +0100
@@ -0,0 +1,21 @@
+/* PR target/92908 */
+/* { dg-do run } */
+/* { dg-options "-Og -fno-tree-fre -mavx512bw" } */
+/* { dg-require-effective-target avx512bw } */
+
+#define AVX512BW
+#include "avx512f-helper.h"
+
+typedef unsigned short V __attribute__ ((__vector_size__ (64)));
+
+V v;
+
+void
+TEST (void)
+{
+  int i;
+  v = (V) v == v;
+  for (i = 0; i < 32; i++)
+    if (v[i] != 0xffff)
+      abort ();
+}

        Jakub

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