[gcc r17-3401] aarch64: Swap the zeroness flags when swapping vec_perm operands [PR126597]

https://gcc.gnu.org/g:75b259d068e1db2c8ac2246e3118dd5d24724674

commit r17-3401-g75b259d068e1db2c8ac2246e3118dd5d24724674
Author: Kyrylo Tkachov <[email protected]>
Date:   Tue Aug 18 14:19:57 2026 +0200

    aarch64: Swap the zeroness flags when swapping vec_perm operands [PR126597]
    
    aarch64_expand_vec_perm_const_1 normalizes a permutation whose first index
    selects the second operand by rotating the indices and swapping op0 and op1.
    It left zero_op0_p and zero_op1_p pointing at the old operands, so the later
    recognizers that consult them, aarch64_evpc_and and aarch64_evpc_tbl, read
    the wrong vector.
    
    Swap the two flags together with the operands.
    
    For
    
            typedef int v4si __attribute__ ((vector_size (16)));
            v4si f (v4si x)
            {
              const v4si m = { 4, 1, 2, 3 };
              return __builtin_shuffle (x, (v4si) { 0, 0, 0, 0 }, m);
            }
    
    at -O0 the AND was applied to the all-zero operand, so the function returned
    {0,0,0,0} instead of {0,x1,x2,x3}:
    
            sub     sp, sp, #32
            str     q0, [sp]
            adrp    x0, .LC0
            add     x0, x0, :lo12:.LC0
            ldr     q31, [x0]
            str     q31, [sp, 16]
            movi    v31.4s, 0
            fmov    s31, s31
            mov     v0.16b, v31.16b
            add     sp, sp, 32
            ret
    
    With the fix the AND is applied to the incoming vector:
    
            sub     sp, sp, #32
            str     q0, [sp]
            adrp    x0, .LC0
            add     x0, x0, :lo12:.LC0
            ldr     q31, [x0]
            str     q31, [sp, 16]
            ldr     q30, [sp]
            adrp    x0, .LC1
            add     x0, x0, :lo12:.LC1
            ldr     q31, [x0]
            and     v31.16b, v30.16b, v31.16b
            mov     v0.16b, v31.16b
            add     sp, sp, 32
            ret
    
    Bootstrapped and tested on aarch64-none-linux-gnu.
    
    gcc/ChangeLog:
    
            PR target/126597
            * config/aarch64/aarch64.cc (aarch64_expand_vec_perm_const_1): Swap
            zero_op0_p and zero_op1_p along with the operands.
    
    gcc/testsuite/ChangeLog:
    
            PR target/126597
            * gcc.target/aarch64/pr126597.c: New test.
    
    Signed-off-by: Kyrylo Tkachov <[email protected]>

Diff:
---
 gcc/config/aarch64/aarch64.cc               |  1 +
 gcc/testsuite/gcc.target/aarch64/pr126597.c | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
index fdffb13ad222..8a788a5c7e41 100644
--- a/gcc/config/aarch64/aarch64.cc
+++ b/gcc/config/aarch64/aarch64.cc
@@ -28772,6 +28772,7 @@ aarch64_expand_vec_perm_const_1 (struct expand_vec_perm_d *d)
     {
       d->perm.rotate_inputs (1);
       std::swap (d->op0, d->op1);
+      std::swap (d->zero_op0_p, d->zero_op1_p);
     }
 
   if (((d->vec_flags == VEC_ADVSIMD && TARGET_SIMD)
diff --git a/gcc/testsuite/gcc.target/aarch64/pr126597.c b/gcc/testsuite/gcc.target/aarch64/pr126597.c
new file mode 100644
index 000000000000..922d673fa6a7
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr126597.c
@@ -0,0 +1,21 @@
+/* { dg-do run } */
+/* { dg-options "-O0" } */
+
+typedef int v4si __attribute__ ((vector_size (16)));
+
+__attribute__((noipa)) v4si
+f (v4si x)
+{
+  const v4si mask = { 4, 1, 2, 3 };
+  return __builtin_shuffle (x, (v4si) { 0, 0, 0, 0 }, mask);
+}
+
+int
+main (void)
+{
+  v4si x = { 1, 2, 3, 4 };
+  v4si r = f (x);
+  if (r[0] != 0 || r[1] != 2 || r[2] != 3 || r[3] != 4)
+    __builtin_abort ();
+  return 0;
+}