Add optimizer hints

Summary:
Some Redex optimizations in the pipe which move/delete/inline static methods. Those optimizations may fubar what's going on in `SysUtil.LollipopSysdeps`, so prepping a hint annotation to guide Redex not to disturb these methods.

I expect the redex hints for fbandroid-at-large to be more precise in their language. However, since `//libraries/soloader` can not depend on code in `//java`, I'm just going to create a catch-all annotation here which will disable the relevant optimizations in redex for our fbandroid apps.

(FWIW, redex will not operate on hardcoded annotations; a mapping layer will say "this set of annotations means this, this set means that". So DoNotOptimize will be listed as suppressing inlining/movement/deletion etc.)

@public Add a hint (which may or may not be observed) to any optimizers in our tool chain that we don't want optimizations being applied to the annotated elements. Necessary to prevent inlining/movement of some methods on SysUtil$LollipopSysdeps.

Reviewed By: dariorussi

Differential Revision: D2989738

fbshipit-source-id: 4d4c5b8d91b146e7ac3700c2edfe1d008c3a08e1

java/com/facebook/soloader/DoNotOptimize.java

@@ -0,0 +1,19 @@
+// Copyright 2004-present Facebook. All Rights Reserved.
+
+package com.facebook.soloader;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import static java.lang.annotation.RetentionPolicy.CLASS;
+
+/**
+ * A hint (which may or may not be observed) to any optimizers in our tool
+ * chain that we don't want optimizations being applied to the annotated
+ * elements.
+ */
+@Target({ ElementType.TYPE, ElementType.FIELD, ElementType.METHOD, ElementType.CONSTRUCTOR })
+@Retention(CLASS)
+public @interface DoNotOptimize {
+}

java/com/facebook/soloader/SysUtil.java

@@ -121,10 +121,12 @@ public static void dumbDeleteRecursive(File file) throws IOException {
    * downlevel.
    */
   private static final class LollipopSysdeps {
+    @DoNotOptimize
     public static String[] getSupportedAbis() {
       return Build.SUPPORTED_32_BIT_ABIS; // We ain't doing no newfangled 64-bit
     }
 
+    @DoNotOptimize
     public static void fallocateIfSupported(FileDescriptor fd, long length) throws IOException {
       try {
         Os.posix_fallocate(fd, 0, length);