Adding instructions regarding how to check for an unsupported CPU (simdjson#508)

lemire · web-flow · commit 68670301e3b3 · 2020-02-25T11:09:51.000-05:00
* Adding instructions.

* Slighty more documentation.
diff --git a/README.md b/README.md
@@ -90,6 +90,19 @@ On x64 hardware, you should typically build your code by specifying the oldest/l
 
 We also support 64-bit ARM. We assume NEON support. There is no runtime dispatch on ARM.
 
+If you expect your code to run on older processors, you can check that the CPU is supported as follows:
+
+```c++
+if (simdjson::active_implementation->name() == "unsupported") { 
+  printf("unsupported CPU\n"); 
+}
+```
+
+This check is not useful on ARM processors since all 64-bit ARM processors are supported.
+
+It is not necessary to do this check: if you omit it, you will get back the error code `UNSUPPORTED_ARCHITECTURE` when trying to parse documents.
+However, you can call `simdjson::active_implementation->name()` to check which CPU configuration has been detected (e.g., haswell, westmere).
+
 ## Computed GOTOs
 
 For best performance, we use a technique called "computed goto", it is also sometimes described as "Labels as Values".
@@ -172,6 +185,7 @@ for (padded_string json : { string("[1, 2, 3]"), string("true"), string("[ true,
 }
 ```
 
+
 ## Newline-Delimited JSON (ndjson) and  JSON lines 
 
 The simdjson library also support multithreaded JSON streaming through a large file containing many smaller JSON documents in either [ndjson](http://ndjson.org) or [JSON lines](http://jsonlines.org) format. We support files larger than 4GB.
diff --git a/tests/basictests.cpp b/tests/basictests.cpp
@@ -570,6 +570,11 @@ bool skyprophet_test() {
 }
 
 int main() {
+  // this is put here deliberately to check that the documentation is correct (README),
+  // should this fail to compile, you should update the documentation:
+  if (simdjson::active_implementation->name() == "unsupported") { 
+    printf("unsupported CPU\n"); 
+  }
   std::cout << "Running basic tests." << std::endl;
   if(!json_issue467())
     return EXIT_FAILURE;
