Keep only the simple auxv method for detecting dotprod instructions.
It's available on Linux >= 4.15 in general; on Android, at least Linux 4.14.111 thanks to a late backport. This was backported just before the Android 10 release, so this is leaving out pre-release Android 10 builds as well as earlier Android versions. Part of the rationale for submitting this now is an understanding that most devices with new hardware supporting dotprod instructions either shipped with Android 10 in the first place (Pixel4) or have received an Android 10 update already (LG G8, Samsung Galaxy S10, Note 10). We are probably leaving some devices unsupported, but conversely the signal-handler detection method that this is removing was reported to cause crashes on other devices, so this is a compromise. At least now we won't crash anywhere, at worst some devices won't get the speedup from dotprod instructions. PiperOrigin-RevId: 288340160 Change-Id: I9f1b934f4e3996456af0489b780d567596f6db92
This commit is contained in:
Benoit Jacob
TensorFlower Gardener
parent
87cab44823
commit
c6c81bc173
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@ -13,189 +13,44 @@ See the License for the specific language governing permissions and
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limitations under the License.
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==============================================================================*/
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/* Temporary dotprod-detection until we can rely on proper feature-detection
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such as getauxval on Linux (requires a newer Linux kernel than we can
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currently rely on on Android).
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There are two main ways that this could be implemented: using a signal
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handler or a fork. The current implementation uses a signal handler.
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This is because on current Android, an uncaught signal gives a latency
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of over 100 ms. In order for the fork approach to be worthwhile, it would
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have to save us the hassle of handling signals, and such an approach thus
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has an unavoidable 100ms latency. By contrast, the present signal-handling
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approach has low latency.
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Downsides of the current signal-handling approach include:
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1. Setting and restoring signal handlers is not thread-safe: we can't
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prevent another thread from interfering with us. We at least prevent
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other threads from calling our present code concurrently by using a lock,
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but we can't do anything about other threads using their own code to
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set signal handlers.
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2. Signal handlers are not entirely portable, e.g. b/132973173 showed that
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on Apple platform the EXC_BAD_INSTRUCTION signal is not always caught
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by a SIGILL handler (difference between Release and Debug builds).
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3. The signal handler approach looks confusing in a debugger (has to
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tell the debugger to 'continue' past the signal every time). Fix:
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```
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(gdb) handle SIGILL nostop noprint pass
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```
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Here is what the nicer fork-based alternative would look like.
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Its only downside, as discussed above, is high latency, 100 ms on Android.
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```
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bool try_asm_snippet(bool (*asm_snippet)()) {
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int child_pid = fork();
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if (child_pid == -1) {
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// Fork failed.
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return false;
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}
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if (child_pid == 0) {
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// Child process code path. Pass the raw boolean return value of
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// asm_snippet as exit code (unconventional: 1 means true == success).
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_exit(asm_snippet());
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}
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int child_status;
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waitpid(child_pid, &child_status, 0);
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if (WIFSIGNALED(child_status)) {
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// Child process terminated by signal, meaning the instruction was
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// not supported.
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return false;
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}
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// Return the exit code of the child, which per child code above was
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// the return value of asm_snippet().
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return WEXITSTATUS(child_status);
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}
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```
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*/
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/* Detection of dotprod instructions on ARM.
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* The current Linux-specific code relies on sufficiently new Linux kernels:
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* At least Linux 4.15 in general; on Android, at least Linux 4.14.111 thanks to
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* a late backport. This was backported just before the Android 10 release, so
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* this is leaving out pre-release Android 10 builds as well as earlier Android
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* versions.
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*
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* It is possible to detect instructions in other ways that don't rely on
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* an OS-provided feature identification mechanism:
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*
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* (A) We used to have a SIGILL-handler-based method that worked at least
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* on Linux. Its downsides were (1) crashes on a few devices where
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* signal handler installation didn't work as intended; (2) additional
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* complexity to generalize to other Unix-ish operating systems including
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* iOS; (3) source code complexity and fragility of anything installing
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* and restoring signal handlers; (4) confusing behavior under a debugger.
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*
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* (B) We also experimented with a fork-ing approach where a subprocess
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* tries the instruction. Compared to (A), this is much simpler and more
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* reliable and portable, but also much higher latency on Android where
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* an uncaught signal typically causes a 100 ms latency.
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*
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* Should there be interest in either technique again in the future,
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* code implementing both (A) and (B) can be found in earlier revisions of this
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* file - in actual code for (A) and in a comment for (B).
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*/
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#include "tensorflow/lite/experimental/ruy/detect_arm.h"
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#if defined __aarch64__ && defined __linux__
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#define RUY_IMPLEMENT_DETECT_DOTPROD
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#endif
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#ifdef RUY_IMPLEMENT_DETECT_DOTPROD
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#include <setjmp.h>
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#include <signal.h>
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#include <unistd.h>
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#include <cstdio>
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#include <cstdlib>
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#include <cstring>
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#include <mutex> // NOLINT(build/c++11)
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// Intentionally keep checking for __linux__ here in case we want to
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// extend RUY_IMPLEMENT_DETECT_DOTPROD outside of linux in the future.
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#ifdef __linux__
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#include <sys/auxv.h>
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#include <sys/utsname.h>
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#endif
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#endif
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namespace ruy {
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#ifdef RUY_IMPLEMENT_DETECT_DOTPROD
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namespace {
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// Waits until there are no pending SIGILL's.
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void wait_until_no_pending_sigill() {
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sigset_t pending;
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do {
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sigemptyset(&pending);
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sigpending(&pending);
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} while (sigismember(&pending, SIGILL));
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}
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// long-jump buffer used to continue execution after a caught SIGILL.
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sigjmp_buf& global_sigjmp_buf_just_before_trying_snippet() {
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static sigjmp_buf g;
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return g;
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}
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// SIGILL signal handler. Long-jumps to just before
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// we ran the snippet that we know is the only thing that could have generated
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// the SIGILL.
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void sigill_handler(int) {
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siglongjmp(global_sigjmp_buf_just_before_trying_snippet(), 1);
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}
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// Try an asm snippet. Returns true if it passed i.e. ran without generating
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// a SIGILL and returned true. Returns false if a SIGILL was generated, or
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// if it returned false.
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// Other signals are not handled.
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bool try_asm_snippet(bool (*asm_snippet)()) {
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// This function installs and restores signal handlers. The only way it's ever
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// going to be reentrant is with a big lock around it.
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static std::mutex mutex;
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std::lock_guard<std::mutex> lock(mutex);
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// Install the SIGILL signal handler. Save any existing signal handler for
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// restoring later.
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struct sigaction sigill_action;
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memset(&sigill_action, 0, sizeof(sigill_action));
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sigill_action.sa_handler = sigill_handler;
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sigemptyset(&sigill_action.sa_mask);
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struct sigaction old_action;
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sigaction(SIGILL, &sigill_action, &old_action);
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// Try the snippet.
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bool got_sigill =
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sigsetjmp(global_sigjmp_buf_just_before_trying_snippet(), true);
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bool snippet_retval = false;
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if (!got_sigill) {
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snippet_retval = asm_snippet();
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wait_until_no_pending_sigill();
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}
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// Restore the old signal handler.
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sigaction(SIGILL, &old_action, nullptr);
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return snippet_retval && !got_sigill;
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}
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bool dotprod_asm_snippet() {
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// maratek@ mentioned that for some other ISA extensions (fp16)
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// there have been implementations that failed to generate SIGILL even
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// though they did not correctly implement the instruction. Just in case
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// a similar situation might exist here, we do a simple correctness test.
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int result = 0;
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asm volatile(
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"mov w0, #100\n"
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"dup v0.16b, w0\n"
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"dup v1.4s, w0\n"
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".word 0x6e809401 // udot v1.4s, v0.16b, v0.16b\n"
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"mov %w[result], v1.s[0]\n"
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: [ result ] "=r"(result)
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:
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: "x0", "v0", "v1");
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// Expecting 100 (input accumulator value) + 100 * 100 + ... (repeat 4 times)
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return result == 40100;
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}
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bool DetectDotprodBySigIllMethod() {
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return try_asm_snippet(dotprod_asm_snippet);
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}
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// Intentionally keep checking for __linux__ here in case we want to
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// extend RUY_IMPLEMENT_DETECT_DOTPROD outside of linux in the future.
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#ifdef __linux__
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bool IsLinuxAuxvMethodAvailable() {
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struct utsname utsbuf;
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uname(&utsbuf);
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int major, minor, patch;
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if (3 != sscanf(utsbuf.release, "%d.%d.%d", &major, &minor, &patch)) {
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return false;
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}
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// This is implemented in linux 4.14.111, not in 4.14.105.
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return major > 4 ||
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(major == 4 && (minor > 14 || (minor == 14 && patch >= 111)));
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}
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#if defined __linux__ && defined __aarch64__
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bool DetectDotprodByLinuxAuxvMethod() {
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// This is the value of HWCAP_ASIMDDP in sufficiently recent Linux headers,
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// however we need to support building against older headers for the time
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@ -208,17 +63,11 @@ bool DetectDotprodByLinuxAuxvMethod() {
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} // namespace
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bool DetectDotprod() {
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#ifdef __linux__
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if (IsLinuxAuxvMethodAvailable()) {
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#if defined __linux__ && defined __aarch64__
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return DetectDotprodByLinuxAuxvMethod();
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}
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#endif
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return DetectDotprodBySigIllMethod();
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return false;
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}
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#else // RUY_IMPLEMENT_DETECT_DOTPROD
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bool DetectDotprod() { return false; }
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#endif // RUY_IMPLEMENT_DETECT_DOTPROD
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} // namespace ruy
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