From 1a2e8f4e3077cdafbaf15278bfe393c64263215c Mon Sep 17 00:00:00 2001
From: "Zeyi (Rice) Fan" <zeyi@fb.com>
Date: Fri, 6 Aug 2021 10:38:43 -0700
Subject: fix clang-cl _tzcnt_u64 not defined issue

---
 lib/lz4.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/lib/lz4.c b/lib/lz4.c
index 405e977..5499547 100644
--- a/lib/lz4.c
+++ b/lib/lz4.c
@@ -516,8 +516,14 @@ static unsigned LZ4_NbCommonBytes (reg_t val)
     if (LZ4_isLittleEndian()) {
         if (sizeof(val) == 8) {
 #       if defined(_MSC_VER) && (_MSC_VER >= 1800) && defined(_M_AMD64) && !defined(LZ4_FORCE_SW_BITCOUNT)
+#         if defined(__clang__) && (__clang_major__ < 10)
+            /* Avoid undefined clang-cl intrinics issue.
+             * See https://github.com/lz4/lz4/pull/1017 for details. */
+            return (unsigned)__builtin_ia32_tzcnt_u64(val) >> 3;
+#         else
             /* x64 CPUS without BMI support interpret `TZCNT` as `REP BSF` */
             return (unsigned)_tzcnt_u64(val) >> 3;
+#         endif
 #       elif defined(_MSC_VER) && defined(_WIN64) && !defined(LZ4_FORCE_SW_BITCOUNT)
             unsigned long r = 0;
             _BitScanForward64(&r, (U64)val);
-- 
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