From 83025327576ba5f95ae979b688107c59a227bb50 Mon Sep 17 00:00:00 2001
From: xtqqczze <45661989+xtqqczze@users.noreply.github.com>
Date: Mon, 14 Oct 2024 17:54:43 +0100
Subject: [PATCH] Avoid `ExtractMostSignificantBits` in AVX512 path

---
 .../System/Buffers/Text/Base64Helper/Base64DecoderHelper.cs    | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Helper/Base64DecoderHelper.cs b/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Helper/Base64DecoderHelper.cs
index 97765e340acff..4ea898af62273 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Helper/Base64DecoderHelper.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Base64Helper/Base64DecoderHelper.cs
@@ -663,6 +663,7 @@ private static unsafe void Avx512Decode<TBase64Decoder, T>(TBase64Decoder decode
             // The JIT won't hoist these "constants", so help it
             Vector512<sbyte> vbmiLookup0 = Vector512.Create(decoder.VbmiLookup0).AsSByte();
             Vector512<sbyte> vbmiLookup1 = Vector512.Create(decoder.VbmiLookup1).AsSByte();
+            Vector512<sbyte> errorMask = Vector512.Create(sbyte.MinValue);
             Vector512<byte> vbmiPackedLanesControl = Vector512.Create(
                 0x06000102, 0x090a0405, 0x0c0d0e08, 0x16101112,
                 0x191a1415, 0x1c1d1e18, 0x26202122, 0x292a2425,
@@ -687,7 +688,7 @@ private static unsafe void Avx512Decode<TBase64Decoder, T>(TBase64Decoder decode
                 // origIndex      = [...|00dddddd|00cccccc|00bbbbbb|00aaaaaa]
                 Vector512<sbyte> origIndex = Avx512Vbmi.PermuteVar64x8x2(vbmiLookup0, str, vbmiLookup1);
                 Vector512<sbyte> errorVec = (origIndex.AsInt32() | str.AsInt32()).AsSByte();
-                if (errorVec.ExtractMostSignificantBits() != 0)
+                if ((errorVec & errorMask) != Vector512<sbyte>.Zero)
                 {
                     break;
                 }