diff --git a/ext/digest/blake3/extconf.rb b/ext/digest/blake3/extconf.rb
index c470659..b02bb07 100644
--- a/ext/digest/blake3/extconf.rb
+++ b/ext/digest/blake3/extconf.rb
@@ -29,7 +29,9 @@ def blake3_disable(macro)
 end
 
 # Probe used to confirm the compiler both accepts +flag+ and can compile the
-# intrinsics the backend relies on.
+# intrinsics the backend relies on.  It compiles the backend source itself:
+# a small snippet misses assemblers that reject what the compiler emits (e.g.
+# binutils 2.30 on RHEL 8 rejects AVX-512 code from gcc 8.5).
 def blake3_have_isa?(name, flag, snippet)
   checking_for("#{name} intrinsics" + (flag ? " (#{flag})" : "")) do
     try_compile(snippet, flag)
@@ -40,23 +42,15 @@ def blake3_have_isa?(name, flag, snippet)
 when /\A(x86_64|amd64|x64)\z/i
   # Try to detect which SIMD features this x86 machine and compiler has
   x86_backends = [
-    ["blake3_sse2",   "SSE2",    ["-msse2", "-arch:SSE2"],
-     "#include <immintrin.h>\nint main(void){ volatile __m128i x = _mm_setzero_si128(); (void)x; return 0; }\n",
-     "BLAKE3_NO_SSE2"],
-    ["blake3_sse41",  "SSE4.1",  ["-msse4.1", "-arch:AVX"],
-     "#include <immintrin.h>\nint main(void){ volatile __m128i x = _mm_setzero_si128(); return _mm_testz_si128(x, x); }\n",
-     "BLAKE3_NO_SSE41"],
-    ["blake3_avx2",   "AVX2",    ["-mavx2", "-arch:AVX2"],
-     "#include <immintrin.h>\nint main(void){ volatile __m256i x = _mm256_setzero_si256(); (void)x; return 0; }\n",
-     "BLAKE3_NO_AVX2"],
-    ["blake3_avx512", "AVX-512", ["-mavx512f -mavx512vl", "-arch:AVX512"],
-     "#include <immintrin.h>\nint main(void){ volatile __m512i x = _mm512_setzero_si512(); (void)x; return 0; }\n",
-     "BLAKE3_NO_AVX512"],
+    ["blake3_sse2",   "SSE2",    ["-msse2", "-arch:SSE2"],                 "BLAKE3_NO_SSE2"],
+    ["blake3_sse41",  "SSE4.1",  ["-msse4.1", "-arch:AVX"],                "BLAKE3_NO_SSE41"],
+    ["blake3_avx2",   "AVX2",    ["-mavx2", "-arch:AVX2"],                 "BLAKE3_NO_AVX2"],
+    ["blake3_avx512", "AVX-512", ["-mavx512f -mavx512vl", "-arch:AVX512"], "BLAKE3_NO_AVX512"],
   ]
 
-  x86_backends.each do |obj, name, flags, snippet, no_macro|
+  x86_backends.each do |obj, name, flags, no_macro|
     [nil, *flags].any? do |flag|
-      if blake3_have_isa?(name, flag, snippet)
+      if blake3_have_isa?(name, flag, %{#include "#{$srcdir}/#{obj}.c"\n})
         objs << obj
         simd_cflags[obj] = flag
         true