Make exegesis annotator error out when remapping an address (#73)

Currently, there is a failure condition seen in some blocks where the
annotator finds a segfault, tells exegesis to map a page there, and then
the block fails again at the same address. This needs some debugging,
but currently it will be helpful if we can just log a warning message
and skip this basic block.

gematria/datasets/find_accessed_addrs_exegesis.cc

@@ -152,6 +152,14 @@ Expected<AccessedAddrs> ExegesisAnnotator::findAccessedAddrs(
           uintptr_t MapAddress = (CrashInfo.getAddress() & ~0xfff);
           if (MapAddress == 0)
             return make_error<Failure>("Segfault at zero address, cannot map.");
+          // TODO(boomanaiden154): The fault captured below occurs when
+          // exegesis tries to map an address and the mmap fails. When these
+          // errors are handled within exegesis, we should remove this check.
+          for (const auto &MemoryMapping : BenchCode.Key.MemoryMappings) {
+            if (MemoryMapping.Address == MapAddress)
+              return make_error<Failure>(
+                  "Segfault at an address already mapped.");
+          }
           MemMap.Address = MapAddress;
           MemMap.MemoryValueName = "memdef1";
           BenchCode.Key.MemoryMappings.push_back(MemMap);

gematria/datasets/find_accessed_addrs_exegesis_test.cc

@@ -121,5 +121,17 @@ TEST_F(FindAccessedAddrsExegesisTest, ExegesisZeroAddressError) {
   ASSERT_FALSE(static_cast<bool>(AddrsOrErr));
 }
 
+TEST_F(FindAccessedAddrsExegesisTest, ExegesisMultipleSameAddressError) {
+  // Try and load memory from an address above the current user space address
+  // space ceiling (assuming five level page tables are not enabled) as the
+  // script will currently try and annotate this, but exegesis will fail
+  // to map the address when it attempts to.
+  auto AddrsOrErr = FindAccessedAddrsExegesis(R"asm(
+    movabsq $0x0000800000000000, %rax
+    movq (%rax), %rax
+  )asm");
+  ASSERT_FALSE(static_cast<bool>(AddrsOrErr));
+}
+
 }  // namespace
 }  // namespace gematria