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Work around a valgrind false-positive in CPUID.

Valgrind versions prior to 3.17.0 assume that an uninitialized value
 in ECX causes the whole output of CPUID to be uninitialized, even
 though ECX is only "read" by CPUID for certain values of EAX.
Work around that by guaranteeing that ECX is initialized.

--- a/celt/x86/x86cpu.c

+++ b/celt/x86/x86cpu.c

@@ -68,7 +68,8 @@

         "=r" (CPUInfo[1]),

         "=c" (CPUInfo[2]),

         "=d" (CPUInfo[3]) :

-        "0" (InfoType)

+        /* We clear ECX to avoid a valgrind false-positive prior to v3.17.0. */

+        "0" (InfoType), "2" (0)

     );

 #else

     __asm__ __volatile__ (

@@ -77,12 +78,15 @@

         "=b" (CPUInfo[1]),

         "=c" (CPUInfo[2]),

         "=d" (CPUInfo[3]) :

-        "0" (InfoType)

+        /* We clear ECX to avoid a valgrind false-positive prior to v3.17.0. */

+        "0" (InfoType), "2" (0)

     );

 #endif

 #elif defined(CPU_INFO_BY_C)

-    if !(__get_cpuid(InfoType, &(CPUInfo[0]), &(CPUInfo[1]), &(CPUInfo[2]), &(CPUInfo[3]))) {

-        /* Our function cannot fail, but __get_cpuid can.

+    /* We use __get_cpuid_count to clear ECX to avoid a valgrind false-positive

+        prior to v3.17.0.*/

+    if (!__get_cpuid_count(InfoType, 0, &(CPUInfo[0]), &(CPUInfo[1]), &(CPUInfo[2]), &(CPUInfo[3]))) {

+        /* Our function cannot fail, but __get_cpuid{_count} can.

            Returning all zeroes will effectively disable all SIMD, which is

             what we want on CPUs that don't support CPUID. */

         CPUInfo[3] = CPUInfo[2] = CPUInfo[1] = CPUInfo[0] = 0;