diff --git a/config/c-compiler.m4 b/config/c-compiler.m4
index 5f3e1d1faf930..da40bd6a64755 100644
--- a/config/c-compiler.m4
+++ b/config/c-compiler.m4
@@ -602,6 +602,7 @@ AC_CACHE_CHECK([for _mm512_clmulepi64_epi128], [Ac_cachevar],
     {
       __m128i z;
 
+      x = _mm512_xor_si512(_mm512_zextsi128_si512(_mm_cvtsi32_si128(0)), x);
       y = _mm512_clmulepi64_epi128(x, y, 0);
       z = _mm_ternarylogic_epi64(
                 _mm512_castsi512_si128(y),
diff --git a/configure b/configure
index 4f15347cc9503..3d3d3db97a456 100755
--- a/configure
+++ b/configure
@@ -18227,6 +18227,7 @@ else
     {
       __m128i z;
 
+      x = _mm512_xor_si512(_mm512_zextsi128_si512(_mm_cvtsi32_si128(0)), x);
       y = _mm512_clmulepi64_epi128(x, y, 0);
       z = _mm_ternarylogic_epi64(
                 _mm512_castsi512_si128(y),
diff --git a/meson.build b/meson.build
index 474763ad19f0c..6ffe7b4727556 100644
--- a/meson.build
+++ b/meson.build
@@ -2465,6 +2465,7 @@ int main(void)
 {
      __m128i z;
 
+    x = _mm512_xor_si512(_mm512_zextsi128_si512(_mm_cvtsi32_si128(0)), x);
     y = _mm512_clmulepi64_epi128(x, y, 0);
     z = _mm_ternarylogic_epi64(
             _mm512_castsi512_si128(y),
diff --git a/src/port/pg_crc32c_sse42.c b/src/port/pg_crc32c_sse42.c
index 9af3474a6ca95..1a7172553551a 100644
--- a/src/port/pg_crc32c_sse42.c
+++ b/src/port/pg_crc32c_sse42.c
@@ -123,7 +123,7 @@ pg_comp_crc32c_avx512(pg_crc32c crc, const void *data, size_t len)
 		__m512i		k;
 
 		k = _mm512_broadcast_i32x4(_mm_setr_epi32(0x740eef02, 0, 0x9e4addf8, 0));
-		x0 = _mm512_xor_si512(_mm512_castsi128_si512(_mm_cvtsi32_si128(crc0)), x0);
+		x0 = _mm512_xor_si512(_mm512_zextsi128_si512(_mm_cvtsi32_si128(crc0)), x0);
 		buf += 64;
 
 		/* Main loop. */
diff --git a/src/port/pg_crc32c_sse42_choose.c b/src/port/pg_crc32c_sse42_choose.c
index 802e47788c10c..74d2421ba2be9 100644
--- a/src/port/pg_crc32c_sse42_choose.c
+++ b/src/port/pg_crc32c_sse42_choose.c
@@ -95,9 +95,7 @@ pg_comp_crc32c_choose(pg_crc32c crc, const void *data, size_t len)
 			__cpuidex(exx, 7, 0);
 #endif
 
-#if defined(__clang__) && !defined(__OPTIMIZE__)
-			/* Some versions of clang are broken at -O0 */
-#elif defined(USE_AVX512_CRC32C_WITH_RUNTIME_CHECK)
+#ifdef USE_AVX512_CRC32C_WITH_RUNTIME_CHECK
 			if (exx[2] & (1 << 10) &&	/* VPCLMULQDQ */
 				exx[1] & (1 << 31)) /* AVX512-VL */
 				pg_comp_crc32c = pg_comp_crc32c_avx512;