Add known instruction set variants to logging strings

If a user had a wrong variant, they would get a message like
`Unexpected CPU variant for X86 using defaults: x86` but they
wouldn't know which ones we accept, unless they take a look at
the code.

This change comes after a developer's suggestion when encountering
said warning string.

Test: Presubmit tests
Change-Id: I3a7b9408cf55114229843f6ff6013999350d7185

4 files changed, 25 insertions, 2 deletions

diff --git a/runtime/arch/arm64/instruction_set_features_arm64.cc b/runtime/arch/arm64/instruction_set_features_arm64.cc
index cef35fc355..e65514e709 100644
--- a/runtime/arch/arm64/instruction_set_features_arm64.cc
+++ b/runtime/arch/arm64/instruction_set_features_arm64.cc
@@ -147,7 +147,9 @@ Arm64FeaturesUniquePtr Arm64InstructionSetFeatures::FromVariant(
     };
     if (!FindVariantInArray(arm64_known_variants, arraysize(arm64_known_variants), variant)) {
       std::ostringstream os;
-      os << "Unexpected CPU variant for Arm64: " << variant;
+      os << "Unexpected CPU variant for Arm64: " << variant << ".\n";
+      os << "Known variants: ";
+      CommaSeparateVariants(os, arm64_known_variants, arraysize(arm64_known_variants));
       *error_msg = os.str();
       return nullptr;
     }
diff --git a/runtime/arch/instruction_set_features.cc b/runtime/arch/instruction_set_features.cc
index ec1e340245..5ae920bef1 100644
--- a/runtime/arch/instruction_set_features.cc
+++ b/runtime/arch/instruction_set_features.cc
@@ -267,6 +267,16 @@ bool InstructionSetFeatures::FindVariantInArray(const char* const variants[], si
   return std::find(begin, end, variant) != end;
 }
 
+void InstructionSetFeatures::CommaSeparateVariants(std::ostream& os,
+                                                   const char* const variants[],
+                                                   size_t num_variants) {
+  std::string separator = "";
+  for (size_t i = 0u; i < num_variants; ++i) {
+    os << separator << variants[i];
+    separator = ", ";
+  }
+}
+
 std::unique_ptr<const InstructionSetFeatures> InstructionSetFeatures::AddRuntimeDetectedFeatures(
     const InstructionSetFeatures *features ATTRIBUTE_UNUSED) const {
   UNIMPLEMENTED(FATAL) << kRuntimeISA;
diff --git a/runtime/arch/instruction_set_features.h b/runtime/arch/instruction_set_features.h
index b80d36f153..4fd96566ed 100644
--- a/runtime/arch/instruction_set_features.h
+++ b/runtime/arch/instruction_set_features.h
@@ -130,6 +130,11 @@ class InstructionSetFeatures {
   static bool FindVariantInArray(const char* const variants[], size_t num_variants,
                                  const std::string& variant);
 
+  // Comma separates the variants and adds them to the `os` buffer.
+  static void CommaSeparateVariants(std::ostream& os,
+                                    const char* const variants[],
+                                    size_t num_variants);
+
   // Add architecture specific features in sub-classes.
   virtual std::unique_ptr<const InstructionSetFeatures>
       AddFeaturesFromSplitString(const std::vector<std::string>& features,
diff --git a/runtime/arch/x86/instruction_set_features_x86.cc b/runtime/arch/x86/instruction_set_features_x86.cc
index d2dbe1dde3..11cfbc9469 100644
--- a/runtime/arch/x86/instruction_set_features_x86.cc
+++ b/runtime/arch/x86/instruction_set_features_x86.cc
@@ -133,7 +133,13 @@ X86FeaturesUniquePtr X86InstructionSetFeatures::FromVariant(
   bool known_variant = FindVariantInArray(x86_known_variants, arraysize(x86_known_variants),
                                           variant);
   if (!known_variant && variant != "default") {
-    LOG(WARNING) << "Unexpected CPU variant for X86 using defaults: " << variant;
+    std::ostringstream os;
+    os << "Unexpected CPU variant for x86: " << variant << ".\n";
+    os << "Known variants: ";
+    CommaSeparateVariants(os, x86_known_variants, arraysize(x86_known_variants));
+    // `default` is a valid variant too.
+    os << ", default";
+    LOG(WARNING) << os.str();
   }
 
   return Create(x86_64, has_SSSE3, has_SSE4_1, has_SSE4_2, has_AVX, has_AVX2, has_POPCNT);