ART: Enable ISA features run-time detection for ARM64

On a target run-time detected ISA features can be more accurate than
instruction set features based on a build-time information such as an
instruction set variant or C++ defines. Build-time based features can
be too generic and do not include all features a target CPU supports.

This CL enables instruction feature run-time detection in the JIT/AOT
compilers:

- The value "runtime" to the option "--instruction-set-features" to try
to detect CPU features at run time. If a target does not support run-time
detection it has the same effect as the value "default".
- Runtime uses "--instruction-set-features=runtime" if run-time detection is
supported.

The CL also cleans up how an instruction set feature string is processed
by InstructionSetFeatures::AddFeaturesFromString. It used to make redundant
uses of Trim in subclasses. The calls are replaced with DCHECKs
verifying that feature names are already trimmed.

Test: m test-art-target-gtest
Test: m test-art-host-gtest
Test: art/test.py --target --optimizing --interpreter --jit
Test: art/test.py --host --optimizing --interpreter --jit
Test: Pixel 3 UI booted

Change-Id: I223d5bc968d589dba5c09f6b03ee8c25987610b0

1 files changed, 3 insertions, 0 deletions

diff --git a/compiler/jit/jit_compiler.cc b/compiler/jit/jit_compiler.cc
index 0d35fecb7b..4d7ae9bd1b 100644
--- a/compiler/jit/jit_compiler.cc
+++ b/compiler/jit/jit_compiler.cc
@@ -99,7 +99,10 @@ void JitCompiler::ParseCompilerOptions() {
       }
     }
   }
+
   if (instruction_set_features == nullptr) {
+    // '--instruction-set-features/--instruction-set-variant' were not used.
+    // Use build-time defined features.
     instruction_set_features = InstructionSetFeatures::FromCppDefines();
   }
   compiler_options_->instruction_set_features_ = std::move(instruction_set_features);