x86: check if we can use an xgetbv inline asm implementation

This only checks whether a call to the instruction can be made,
not that a supported function ABI can be used with asm code
like HAVE_AVX_ASM does.

The former can work while the later fails (which still allows
intrinsics-based AVX code to work).

build.zig

@@ -138,6 +138,7 @@ fn initLibConfig(b: *std.Build, target: std.Build.ResolvedTarget, lib: *Compile)
                 },
             }
             lib.root_module.addCMacro("HAVE_CPUID", "1");
+            lib.root_module.addCMacro("HAVE_XGETBV_ASM", "1");
             lib.root_module.addCMacro("HAVE_MMINTRIN_H", "1");
             lib.root_module.addCMacro("HAVE_EMMINTRIN_H", "1");
             lib.root_module.addCMacro("HAVE_PMMINTRIN_H", "1");

configure.ac

@@ -840,6 +840,21 @@ __asm__ __volatile__ ("xchgl %%ebx, %k1; cpuid; xchgl %%ebx, %k1" :
   ])
 AC_SUBST(HAVE_CPUID_V)
 
+AS_CASE([$host_cpu], [i?86|amd64|x86_64], [
+  AS_IF([test "$enable_asm" != "no"],[
+    AC_MSG_CHECKING(whether XGETBV can be used with inline assembly)
+    AC_LINK_IFELSE([AC_LANG_PROGRAM([[ ]], [[
+unsigned int xcr0;
+__asm__ __volatile__(".byte 0x0f, 0x01, 0xd0" :
+                     "=a" (xcr0) :
+                     "c" ((unsigned int) 0U) : "%edx");
+    ]])],
+    [AC_MSG_RESULT(yes)
+     AC_DEFINE([HAVE_XGETBV_ASM], [1], [XGETBV can be used with inline assembly])],
+    [AC_MSG_RESULT(no)])
+  ])
+])
+
 asm_hide_symbol="unsupported"
 AS_IF([test "$enable_asm" != "no"],[
   AC_MSG_CHECKING(if the .private_extern asm directive is supported)

src/libsodium/sodium/runtime.c

@@ -260,7 +260,7 @@ _sodium_runtime_intel_cpu_features(CPUFeatures * const cpu_features)
             pop ecx
             pop eax
         }
-# elif defined(HAVE_AVX_ASM)
+# elif defined(HAVE_AVX_ASM) || defined(HAVE_XGETBV_ASM)
         __asm__ __volatile__(".byte 0x0f, 0x01, 0xd0" /* XGETBV */
                              : "=a"(xcr0)
                              : "c"((uint32_t) 0U)