[PATCH] [msan] Run more optimization after MemorySanitizer pass

[Evgeniy Stepanov]

  Here, the best I can say without writing a page-long essay with examples. Sorry, it's not much.

  MSan instrumentation is driven by the original code. We essentially take every incoming instruction and emit another instruction (or ten) next to it, operating on the shadow values (but sometimes on the real values, too). Two programs in one, essentially. There can be any kinds of redundancies in the second one, so we just run whatever is normally run at -O2, and then exclude some passes that do not help much with benchmarks.

Hi kcc, chandlerc,

http://llvm-reviews.chandlerc.com/D189

CHANGE SINCE LAST DIFF
  http://llvm-reviews.chandlerc.com/D189?vs=800&id=830#toc

Files:
  lib/CodeGen/BackendUtil.cpp

Index: lib/CodeGen/BackendUtil.cpp
===================================================================
--- lib/CodeGen/BackendUtil.cpp
+++ lib/CodeGen/BackendUtil.cpp
@@ -184,6 +184,18 @@
   const CodeGenOptions &CGOpts = BuilderWrapper.getCGOpts();
   PM.add(createMemorySanitizerPass(CGOpts.SanitizeMemoryTrackOrigins,
                                    CGOpts.SanitizerBlacklistFile));
+
+  // MemorySanitizer inserts complex instrumentation that mostly follows
+  // the logic of the original code, but operates on "shadow" values.
+  // It can benefit from re-running some general purpose optimization passes.
+  if (Builder.OptLevel > 0) {
+    PM.add(createEarlyCSEPass());
+    PM.add(createReassociatePass());
+    PM.add(createLICMPass());
+    PM.add(createGVNPass());
+    PM.add(createInstructionCombiningPass());
+    PM.add(createDeadStoreEliminationPass());
+  }
 }
 
 static void addThreadSanitizerPass(const PassManagerBuilder &Builder,
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