[PATCH] D33535: [PM] Teach the PGO instrumentation pasess to run GlobalDCE before instrumenting code.

[Chandler Carruth via Phabricator via llvm-commits]

chandlerc created this revision.
Herald added a subscriber: mcrosier.

This is important in the new pass manager. The old pass manager's
inliner has a small DCE routine embedded within it. The new pass manager
relies on the actual GlobalDCE pass for this.

Without this patch, instrumentation profiling with the new PM results in
massive code bloat in the object files because the instrumentation
itself ends up preventing DCE from working to remove the code.

We should probably change the instrumentation (and/or DCE) so that we
can eliminate dead code even if instrumented, but we shouldn't even
spend the time generating instrumentation for that code so this still
seems like a good patch.


https://reviews.llvm.org/D33535

Files:
  lib/Passes/PassBuilder.cpp
  test/Transforms/GlobalDCE/externally_available.ll


Index: test/Transforms/GlobalDCE/externally_available.ll
===================================================================
--- test/Transforms/GlobalDCE/externally_available.ll
+++ test/Transforms/GlobalDCE/externally_available.ll
@@ -1,12 +1,21 @@
 ; RUN: opt < %s -globaldce -S | FileCheck %s
 
+; test_global should not be emitted to the .s file.
+; CHECK-NOT: @test_global =
+ at test_global = available_externally global i32 4
+
+; test_global2 is a normal global using an available externally function.
+; CHECK: @test_global2 =
+ at test_global2 = global i32 ()* @test_function2
+
 ; test_function should not be emitted to the .s file.
-; CHECK-NOT: test_function
+; CHECK-NOT: define {{.*}} @test_function()
 define available_externally i32 @test_function() {
   ret i32 4
 }
 
-; test_global should not be emitted to the .s file.
-; CHECK-NOT: test_global
- at test_global = available_externally global i32 4
-
+; test_function2 isn't actually dead even though its available externaly.
+; CHECK: define available_externally i32 @test_function2()
+define available_externally i32 @test_function2() {
+  ret i32 4
+}
Index: lib/Passes/PassBuilder.cpp
===================================================================
--- lib/Passes/PassBuilder.cpp
+++ lib/Passes/PassBuilder.cpp
@@ -437,6 +437,11 @@
     MPM.addPass(createModuleToPostOrderCGSCCPassAdaptor(std::move(CGPipeline)));
   }
 
+  // Delete anything that is now dead to make sure that we don't instrument
+  // dead code. Instrumentation can end up keeping dead code around and
+  // dramatically increase code size.
+  MPM.addPass(GlobalDCEPass());
+
   if (RunProfileGen) {
     MPM.addPass(PGOInstrumentationGen());
 


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