Add a method to force the CPU flags

Adds a method that forces the CPU flags. Useful when using libyuv inside
a sandboxed process which may not have access to the file system.

Bug: libyuv:787
Change-Id: I01f71e39a7301085d9de388eba930b4cac0fd7be
Reviewed-on: https://chromium-review.googlesource.com/972338
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Frank Barchard <fbarchard@chromium.org>

include/libyuv/cpu_id.h

@@ -80,6 +80,31 @@ int ArmCpuCaps(const char* cpuinfo_name);
 LIBYUV_API
 int MaskCpuFlags(int enable_flags);
 
+// Sets the CPU flags to |cpu_flags|, bypassing the detection code. |cpu_flags|
+// should be a valid combination of the kCpuHas constants above and include
+// kCpuInitialized. Use this method when running in a sandboxed process where
+// the detection code might fail (as it might access /proc/cpuinfo). In such
+// cases the cpu_info can be obtained from a non sandboxed process by calling
+// InitCpuFlags() and passed to the sandboxed process (via command line
+// parameters, IPC...) which can then call this method to initialize the CPU
+// flags.
+// Notes:
+// - when specifying 0 for |cpu_flags|, the auto initialization is enabled
+//   again.
+// - enabling CPU features that are not supported by the CPU will result in
+//   undefined behavior.
+// TODO(fbarchard): consider writing a helper function that translates from
+// other library CPU info to libyuv CPU info and add a .md doc that explains
+// CPU detection.
+static __inline void SetCpuFlags(int cpu_flags) {
+  LIBYUV_API extern int cpu_info_;
+#ifdef __ATOMIC_RELAXED
+  __atomic_store_n(&cpu_info_, cpu_flags, __ATOMIC_RELAXED);
+#else
+  cpu_info_ = cpu_flags;
+#endif
+}
+
 // Low level cpuid for X86. Returns zeros on other CPUs.
 // eax is the info type that you want.
 // ecx is typically the cpu number, and should normally be zero.

source/cpu_id.cc

@@ -261,11 +261,7 @@ static SAFEBUFFERS int GetCpuFlags(void) {
 LIBYUV_API
 int MaskCpuFlags(int enable_flags) {
   int cpu_info = GetCpuFlags() & enable_flags;
-#ifdef __ATOMIC_RELAXED
-  __atomic_store_n(&cpu_info_, cpu_info, __ATOMIC_RELAXED);
-#else
-  cpu_info_ = cpu_info;
-#endif
+  SetCpuFlags(cpu_info);
   return cpu_info;
 }
 

unit_test/cpu_test.cc

@@ -158,4 +158,27 @@ TEST_F(LibYUVBaseTest, TestLinuxNeon) {
 #endif
 }
 
+TEST_F(LibYUVBaseTest, TestSetCpuFlags) {
+  // Reset any masked flags that may have been set so auto init is enabled.
+  MaskCpuFlags(0);
+
+  int original_cpu_flags = TestCpuFlag(-1);
+
+  // Test setting different CPU configurations.
+  int cpu_flags = kCpuHasARM | kCpuHasNEON | kCpuInitialized;
+  SetCpuFlags(cpu_flags);
+  EXPECT_EQ(cpu_flags, TestCpuFlag(-1));
+
+  cpu_flags = kCpuHasX86 | kCpuInitialized;
+  SetCpuFlags(cpu_flags);
+  EXPECT_EQ(cpu_flags, TestCpuFlag(-1));
+
+  // Test that setting 0 turns auto-init back on.
+  SetCpuFlags(0);
+  EXPECT_EQ(original_cpu_flags, TestCpuFlag(-1));
+
+  // Restore the CPU flag mask.
+  MaskCpuFlags(benchmark_cpu_info_);
+}
+
 }  // namespace libyuv