write to cpu_flags once

To make init cpu flags thread safe, there can only be one write to the variable.

R=richard.winterton@intel.com, harryjin@google.com
BUG=libyuv:508

Review URL: https://codereview.chromium.org/1412793006 .

README.chromium

 Name: libyuv
 URL: http://code.google.com/p/libyuv/
-Version: 1515
+Version: 1516
 License: BSD
 License File: LICENSE
 

include/libyuv/version.h

@@ -11,6 +11,6 @@
 #ifndef INCLUDE_LIBYUV_VERSION_H_  // NOLINT
 #define INCLUDE_LIBYUV_VERSION_H_
 
-#define LIBYUV_VERSION 1515
+#define LIBYUV_VERSION 1516
 
 #endif  // INCLUDE_LIBYUV_VERSION_H_  NOLINT

source/cpu_id.cc

@@ -198,7 +198,7 @@ static LIBYUV_BOOL TestEnv(const char*) {
 LIBYUV_API SAFEBUFFERS
 int InitCpuFlags(void) {
 #if !defined(__pnacl__) && !defined(__CLR_VER) && defined(CPU_X86)
-
+  int cpu_info;
   uint32 cpu_info0[4] = { 0, 0, 0, 0 };
   uint32 cpu_info1[4] = { 0, 0, 0, 0 };
   uint32 cpu_info7[4] = { 0, 0, 0, 0 };
@@ -207,7 +207,7 @@ int InitCpuFlags(void) {
   if (cpu_info0[0] >= 7) {
     CpuId(7, 0, cpu_info7);
   }
-  cpu_info_ = ((cpu_info1[3] & 0x04000000) ? kCpuHasSSE2 : 0) |
+  cpu_info = ((cpu_info1[3] & 0x04000000) ? kCpuHasSSE2 : 0) |
              ((cpu_info1[2] & 0x00000200) ? kCpuHasSSSE3 : 0) |
              ((cpu_info1[2] & 0x00080000) ? kCpuHasSSE41 : 0) |
              ((cpu_info1[2] & 0x00100000) ? kCpuHasSSE42 : 0) |
@@ -219,52 +219,51 @@ int InitCpuFlags(void) {
   // AVX requires CPU has AVX, XSAVE and OSXSave for xgetbv
   if ((cpu_info1[2] & 0x1c000000) == 0x1c000000 &&  // AVX and OSXSave
       !TestEnv("LIBYUV_DISABLE_AVX") && TestOsSaveYmm()) {  // Saves YMM.
-    cpu_info_ |= ((cpu_info7[1] & 0x00000020) ? kCpuHasAVX2 : 0) |
-                 kCpuHasAVX;
+    cpu_info |= ((cpu_info7[1] & 0x00000020) ? kCpuHasAVX2 : 0) | kCpuHasAVX;
   }
 #endif
   // Environment variable overrides for testing.
   if (TestEnv("LIBYUV_DISABLE_X86")) {
-    cpu_info_ &= ~kCpuHasX86;
+    cpu_info &= ~kCpuHasX86;
   }
   if (TestEnv("LIBYUV_DISABLE_SSE2")) {
-    cpu_info_ &= ~kCpuHasSSE2;
+    cpu_info &= ~kCpuHasSSE2;
   }
   if (TestEnv("LIBYUV_DISABLE_SSSE3")) {
-    cpu_info_ &= ~kCpuHasSSSE3;
+    cpu_info &= ~kCpuHasSSSE3;
   }
   if (TestEnv("LIBYUV_DISABLE_SSE41")) {
-    cpu_info_ &= ~kCpuHasSSE41;
+    cpu_info &= ~kCpuHasSSE41;
   }
   if (TestEnv("LIBYUV_DISABLE_SSE42")) {
-    cpu_info_ &= ~kCpuHasSSE42;
+    cpu_info &= ~kCpuHasSSE42;
   }
   if (TestEnv("LIBYUV_DISABLE_AVX2")) {
-    cpu_info_ &= ~kCpuHasAVX2;
+    cpu_info &= ~kCpuHasAVX2;
   }
   if (TestEnv("LIBYUV_DISABLE_ERMS")) {
-    cpu_info_ &= ~kCpuHasERMS;
+    cpu_info &= ~kCpuHasERMS;
   }
   if (TestEnv("LIBYUV_DISABLE_FMA3")) {
-    cpu_info_ &= ~kCpuHasFMA3;
+    cpu_info &= ~kCpuHasFMA3;
   }
 #endif
 #if defined(__mips__) && defined(__linux__)
   // Linux mips parse text file for dsp detect.
-  cpu_info_ = MipsCpuCaps("dsp");  // set kCpuHasMIPS_DSP.
+  cpu_info = MipsCpuCaps("dsp");  // set kCpuHasMIPS_DSP.
 #if defined(__mips_dspr2)
-  cpu_info_ |= kCpuHasMIPS_DSPR2;
+  cpu_info |= kCpuHasMIPS_DSPR2;
 #endif
-  cpu_info_ |= kCpuHasMIPS;
+  cpu_info |= kCpuHasMIPS;
 
   if (getenv("LIBYUV_DISABLE_MIPS")) {
-    cpu_info_ &= ~kCpuHasMIPS;
+    cpu_info &= ~kCpuHasMIPS;
   }
   if (getenv("LIBYUV_DISABLE_MIPS_DSP")) {
-    cpu_info_ &= ~kCpuHasMIPS_DSP;
+    cpu_info &= ~kCpuHasMIPS_DSP;
   }
   if (getenv("LIBYUV_DISABLE_MIPS_DSPR2")) {
-    cpu_info_ &= ~kCpuHasMIPS_DSPR2;
+    cpu_info &= ~kCpuHasMIPS_DSPR2;
   }
 #endif
 #if defined(__arm__) || defined(__aarch64__)
@@ -272,28 +271,30 @@ int InitCpuFlags(void) {
 // __ARM_NEON__ generates code that requires Neon.  NaCL also requires Neon.
 // For Linux, /proc/cpuinfo can be tested but without that assume Neon.
 #if defined(__ARM_NEON__) || defined(__native_client__) || !defined(__linux__)
-  cpu_info_ = kCpuHasNEON;
+  cpu_info = kCpuHasNEON;
 // For aarch64(arm64), /proc/cpuinfo's feature is not complete, e.g. no neon
 // flag in it.
 // So for aarch64, neon enabling is hard coded here.
 #endif
 #if defined(__aarch64__)
-  cpu_info_ = kCpuHasNEON;
+  cpu_info = kCpuHasNEON;
 #else
   // Linux arm parse text file for neon detect.
-  cpu_info_ = ArmCpuCaps("/proc/cpuinfo");
+  cpu_info = ArmCpuCaps("/proc/cpuinfo");
 #endif
-  cpu_info_ |= kCpuHasARM;
+  cpu_info |= kCpuHasARM;
   if (TestEnv("LIBYUV_DISABLE_NEON")) {
-    cpu_info_ &= ~kCpuHasNEON;
+    cpu_info &= ~kCpuHasNEON;
   }
 #endif  // __arm__
   if (TestEnv("LIBYUV_DISABLE_ASM")) {
-    cpu_info_ = 0;
+    cpu_info = 0;
   }
+  cpu_info_ = cpu_info;
   return cpu_info_;
 }
 
+// Note that use of this function is not thread safe.
 LIBYUV_API
 void MaskCpuFlags(int enable_flags) {
   cpu_info_ = InitCpuFlags() & enable_flags;