Optimize suffix building in FileTest::ReadNext().

Updating HPKE to draft-irtf-cfrg-hpke-07 added a number of tests to
crypto/hpke/hpke_test_vectors.txt. The time to run HPKE crypto tests
increased from 0.09 to 0.56 seconds (averaged over 6 runs). The runtime
for the whole crypto_test suite increased from 86.44 to 88.19 (measured
once).

Profiling revealed an excessive amount of CPU time (~50% for HPKE tests)
spent on std::map lookups in ReadNext(). I found a slow loop that
computes the suffix for a repeated attribute by hammering a std::map
with incremental guesses.

This CL adds a std::map<std::string, size_t> for counting repeated
attributes, eliminating the need for the loop. This reduces the runtime
for HPKE tests from 0.56 to 0.12 seconds (averaged over 6 runs). For the
whole crypto_test suite, runtime is reduced from 88.19 to 86.71
seconds (measured once).

Change-Id: Ie87f4a7f3edc95d434226d2959dcf09974f0656f
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/44905
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/test/file_test.cc

@@ -205,11 +205,10 @@ FileTest::ReadResult FileTest::ReadNext() {
 
       // Duplicate keys are rewritten to have “/2”, “/3”, … suffixes.
       std::string mapped_key = key;
-      for (unsigned i = 2; attributes_.count(mapped_key) != 0; i++) {
-        char suffix[32];
-        snprintf(suffix, sizeof(suffix), "/%u", i);
-        suffix[sizeof(suffix)-1] = 0;
-        mapped_key = key + suffix;
+      // If absent, the value will be zero-initialized.
+      const size_t num_occurrences = ++attribute_count_[key];
+      if (num_occurrences > 1) {
+        mapped_key += "/" + std::to_string(num_occurrences);
       }
 
       unused_attributes_.insert(mapped_key);
@@ -317,6 +316,7 @@ void FileTest::ClearTest() {
   start_line_ = 0;
   type_.clear();
   parameter_.clear();
+  attribute_count_.clear();
   attributes_.clear();
   unused_attributes_.clear();
   unused_instructions_.clear();

crypto/test/file_test.h

@@ -221,6 +221,9 @@ class FileTest {
   std::string type_;
   // parameter_ is the value of the first attribute.
   std::string parameter_;
+  // attribute_count_ maps unsuffixed attribute names to the number of times
+  // they have occurred so far.
+  std::map<std::string, size_t> attribute_count_;
   // attributes_ contains all attributes in the test, including the first.
   std::map<std::string, std::string> attributes_;
   // instructions_ contains all instructions in scope for the test.