Fix a getauxval comment and expand the Krait idiv workaround (#206)

* Fix getauxval comment (API 18 not 20)

getauxval is available in Android starting with API 18, not 20.

The comment about __ANDROID_API__ appears to have been copied from the
NDK's cpufeatures, which always uses dlopen/dlsym and doesn't assume it
can directly call getauxval, even if __ANDROID_API__ is new enough.
With this project, though, when __ANDROID_API__ is 18 or up, the
CMakeLists.txt file would detect that getauxval is available and define
HAVE_STRONG_GETAUXVAL.

* Broaden Qualcomm Krait idiv workaround

Some Qualcomm Krait CPUs have IDIV support but the kernel doesn't
report it. Previously, this code looked for two CPUs:
 - 0x510006F2 (0x51/'Q', variant 0, part 0x06f, rev 2)
 - 0x510006F3 (0x51/'Q', variant 0, part 0x06f, rev 3)

This check misses my 2013 Nexus 7 device, which has this CPU ID:
 - 0x511006f0 (0x51/'Q', variant 1, part 0x06f, rev 0)

My Nexus 7 device doesn't report idiv through AT_HWCAP or through
/proc/cpuinfo (AT_HWCAP is 0x1b0d7).

Expand the check to anything with:
 - implementer 0x51
 - architecture 7
 - part 0x4d or 0x6f

Part 0x4d appears to be a dual-core Krait (e.g. see
https://crbug.com/341598#c43).

This new matching behavior is a subset of what the upstream kernel
does (patch[1] contributed by CodeAurora), and also closely matches the
behavior of pytorch/cpuinfo.

[1] 120ecfafab

src/hwcaps.c

@@ -68,13 +68,7 @@ static unsigned long GetElfHwcapFromGetauxval(uint32_t hwcap_type) {
 #elif defined(HAVE_DLFCN_H)
 // On Android we probe the system's C library for a 'getauxval' function and
 // call it if it exits, or return 0 for failure. This function is available
-// since API level 20.
-//
-// This code does *NOT* check for '__ANDROID_API__ >= 20' to support the edge
-// case where some NDK developers use headers for a platform that is newer than
-// the one really targetted by their application. This is typically done to use
-// newer native APIs only when running on more recent Android versions, and
-// requires careful symbol management.
+// since API level 18.
 //
 // Note that getauxval() can't really be re-implemented here, because its
 // implementation does not parse /proc/self/auxv. Instead it depends on values

src/impl_arm_linux_or_android.c

@@ -152,13 +152,14 @@ static void FixErrors(ArmInfo* const info,
       // https://crbug.com/341598.
       info->features.neon = false;
       break;
-    case 0x510006F2:
-    case 0x510006F3:
-      // The Nexus 4 (Qualcomm Krait) kernel configuration forgets to report
-      // IDIV support.
-      info->features.idiva = true;
-      info->features.idivt = true;
-      break;
+  }
+
+  // Some Qualcomm Krait kernels forget to report IDIV support.
+  // https://github.com/torvalds/linux/commit/120ecfafabec382c4feb79ff159ef42a39b6d33b
+  if (info->implementer == 0x51 && info->architecture == 7 &&
+      (info->part == 0x4d || info->part == 0x6f)) {
+    info->features.idiva = true;
+    info->features.idivt = true;
   }
 
   // Propagate cpu features.

test/cpuinfo_arm_test.cc

@@ -327,6 +327,24 @@ CPU revision	: 3)");
   EXPECT_EQ(GetArmCpuId(&info), 0x510006f3);
 }
 
+// The 2013 Nexus 7 (Qualcomm Krait) kernel configuration forgets to report IDIV
+// support.
+TEST(CpuinfoArmTest, Nexus7_2013_0x511006f0) {
+  ResetHwcaps();
+  auto& fs = GetEmptyFilesystem();
+  fs.CreateFile("/proc/cpuinfo",
+                R"(CPU implementer  : 0x51
+CPU architecture: 7
+CPU variant : 0x1
+CPU part  : 0x06f
+CPU revision  : 0)");
+  const auto info = GetArmInfo();
+  EXPECT_TRUE(info.features.idiva);
+  EXPECT_TRUE(info.features.idivt);
+
+  EXPECT_EQ(GetArmCpuId(&info), 0x511006f0);
+}
+
 // The emulator-specific Android 4.2 kernel fails to report support for the
 // 32-bit ARM IDIV instruction. Technically, this is a feature of the virtual
 // CPU implemented by the emulator.