From c0e500ad996dd7174f6f37481f2da48dc6d98f56 Mon Sep 17 00:00:00 2001
From: Anton Seoane Ampudia <aseoane@openjdk.org>
Date: Fri, 27 Mar 2026 09:44:37 +0000
Subject: [PATCH] 8379794: C2: UBSAN runtime error: shift exponent 64 is too
 large for 64-bit type 'long unsigned int'

Reviewed-by: rcastanedalo, qamai, dlong
---
 src/hotspot/cpu/x86/x86.ad | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/hotspot/cpu/x86/x86.ad b/src/hotspot/cpu/x86/x86.ad
index d7014141234..eaa88d900c7 100644
--- a/src/hotspot/cpu/x86/x86.ad
+++ b/src/hotspot/cpu/x86/x86.ad
@@ -23972,8 +23972,12 @@ instruct vmask_gen_imm(kReg dst, immL len, rRegL temp) %{
   format %{ "vector_mask_gen $len \t! vector mask generator" %}
   effect(TEMP temp);
   ins_encode %{
-    __ mov64($temp$$Register, (0xFFFFFFFFFFFFFFFFUL >> (64 -$len$$constant)));
-    __ kmovql($dst$$KRegister, $temp$$Register);
+    if ($len$$constant > 0) {
+      __ mov64($temp$$Register, right_n_bits($len$$constant));
+      __ kmovql($dst$$KRegister, $temp$$Register);
+    } else {
+      __ kxorql($dst$$KRegister, $dst$$KRegister, $dst$$KRegister);
+    }
   %}
   ins_pipe( pipe_slow );
 %}