Fix AVX detection

nikic · nikic · commit 4e30ab33d842 · 2020-11-27T14:49:31.000+01:00
Our CPU detection code currently only checks whether hardware support for AVX exists. However, we also need to check for operating system support for XSAVE, as well as whether XCR0 has the SSE and AVX bits set. If this is not the case, unset the AVX and AVX2 bits in the cpuinfo structure. Hopefully this resolves our issues with CPU support detection. Closes GH-6460.
diff --git a/Zend/zend_cpuinfo.c b/Zend/zend_cpuinfo.c
@@ -73,6 +73,38 @@ static void __zend_cpuid(uint32_t func, uint32_t subfunc, zend_cpu_info *cpuinfo
 }
 #endif
 
+/* Function based on compiler-rt implementation. */
+static unsigned get_xcr0_eax() {
+#if defined(__GNUC__) || defined(__clang__)
+	// Check xgetbv; this uses a .byte sequence instead of the instruction
+	// directly because older assemblers do not include support for xgetbv and
+	// there is no easy way to conditionally compile based on the assembler used.
+	unsigned eax, edx;
+	__asm__(".byte 0x0f, 0x01, 0xd0" : "=a"(eax), "=d"(edx) : "c"(0));
+	return eax;
+#elif defined(ZEND_WIN32) && defined(_XCR_XFEATURE_ENABLED_MASK)
+	return _xgetbv(_XCR_XFEATURE_ENABLED_MASK);
+#else
+	return 0;
+#endif
+}
+
+static zend_bool is_avx_supported() {
+	if (!(cpuinfo.ecx & ZEND_CPU_FEATURE_AVX)) {
+		/* No support for AVX */
+		return 0;
+	}
+	if (!(cpuinfo.ecx & ZEND_CPU_FEATURE_OSXSAVE)) {
+		/* The operating system does not support XSAVE. */
+		return 0;
+	}
+	if ((get_xcr0_eax() & 0x6) != 0x6) {
+		/* XCR0 SSE and AVX bits must be set. */
+		return 0;
+	}
+	return 1;
+}
+
 void zend_cpu_startup(void)
 {
 	if (!cpuinfo.initialized) {
@@ -95,6 +127,11 @@ void zend_cpu_startup(void)
 		} else {
 			cpuinfo.ebx = 0;
 		}
+
+		if (!is_avx_supported()) {
+			cpuinfo.edx &= ~ZEND_CPU_FEATURE_AVX;
+			cpuinfo.ebx &= ~(ZEND_CPU_FEATURE_AVX2 & ~ZEND_CPU_EBX_MASK);
+		}
 	}
 }
 
