[PATCH] D27560: Always run GlobalDCE at O0

[Mehdi AMINI via Phabricator via llvm-commits]

mehdi_amini created this revision.
mehdi_amini added a reviewer: chandlerc.
mehdi_amini added subscribers: llvm-commits, dexonsmith.

Clang can emit unreachable call-graph from valid C++ code. There
is no point keeping this into the binary, and also sometimes the
generated code is broken and does not link, for example:

#include <iostream>
 #include <string>
 namespace {

  	struct VA {
  	};
  	struct A : virtual public VA {
  		A() {
  			static bool b = false;
  			std::string str;
  		}
  	};

}
 int main(int argc, char* argv[]) {

  	A::A a;

}

See: http://lists.llvm.org/pipermail/cfe-dev/2016-October/051152.html


https://reviews.llvm.org/D27560

Files:
  llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
  llvm/test/Other/GlobalDCE_at_O0.ll


Index: llvm/test/Other/GlobalDCE_at_O0.ll
===================================================================
--- /dev/null
+++ llvm/test/Other/GlobalDCE_at_O0.ll
@@ -0,0 +1,7 @@
+; RUN: opt < %s -O0 -S | FileCheck %s
+
+; CHECK-NOT: @test1
+
+define internal i32 @test1() {
+	ret i32 42
+}
Index: llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -405,6 +405,9 @@
     else if (!GlobalExtensions->empty() || !Extensions.empty())
       MPM.add(createBarrierNoopPass());
 
+    // Clang may generate unreachable call-graphs, let's prune it.
+    MPM.add(createGlobalDCEPass());
+
     if (PrepareForThinLTO)
       // Rename anon globals to be able to export them in the summary.
       MPM.add(createNameAnonGlobalPass());


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