Teach GlobalDCE how to remove empty global_ctor entries.

This moves most of GlobalOpt's constructor optimization
code out of GlobalOpt into Transforms/Utils/CDtorUtils.{h,cpp}. The
public interface is a single function OptimizeGlobalCtorsList() that
takes a predicate returning which constructors to remove.

GlobalOpt calls this with a function that statically evaluates all
constructors, just like it did before. This part of the change is
behavior-preserving.

Also add a call to this from GlobalDCE with a filter that removes global
constructors that contain a "ret" instruction and nothing else – this
fixes PR19590.

llvm-svn: 207856

1 files changed, 45 insertions, 0 deletions

diff --git a/llvm/test/Transforms/GlobalDCE/global_ctors_integration.ll b/llvm/test/Transforms/GlobalDCE/global_ctors_integration.ll
new file mode 100644
index 00000000000..5e6cc79f103
--- /dev/null
+++ b/llvm/test/Transforms/GlobalDCE/global_ctors_integration.ll
@@ -0,0 +1,45 @@
+; RUN: opt -S -O2 < %s | FileCheck %s
+
+; This test checks that -O2 is able to delete constructors that become empty
+; only after some optimization passes have run, even if the pass structure
+; changes.
+; CHECK-NOT: @_GLOBAL__I_a
+
+%class.Foo = type { i32 }
+
+@foo = global %class.Foo zeroinitializer, align 4
+@_ZN3Bar18LINKER_INITIALIZEDE = external constant i32
+@llvm.global_ctors = appending global [1 x { i32, void ()* }] [{ i32, void ()* } { i32 65535, void ()* @_GLOBAL__I_a }]
+
+define internal void @__cxx_global_var_init() section "__TEXT,__StaticInit,regular,pure_instructions" {
+  %1 = load i32* @_ZN3Bar18LINKER_INITIALIZEDE, align 4
+  call void @_ZN3FooC1E17LinkerInitialized(%class.Foo* @foo, i32 %1)
+  ret void
+}
+
+; Function Attrs: ssp uwtable
+define linkonce_odr void @_ZN3FooC1E17LinkerInitialized(%class.Foo* %this, i32) unnamed_addr #0 align 2 {
+  %2 = alloca %class.Foo*, align 8
+  %3 = alloca i32, align 4
+  store %class.Foo* %this, %class.Foo** %2, align 8
+  store i32 %0, i32* %3, align 4
+  %4 = load %class.Foo** %2
+  %5 = load i32* %3, align 4
+  call void @_ZN3FooC2E17LinkerInitialized(%class.Foo* %4, i32 %5)
+  ret void
+}
+
+; Function Attrs: nounwind ssp uwtable
+define linkonce_odr void @_ZN3FooC2E17LinkerInitialized(%class.Foo* %this, i32) unnamed_addr #1 align 2 {
+  %2 = alloca %class.Foo*, align 8
+  %3 = alloca i32, align 4
+  store %class.Foo* %this, %class.Foo** %2, align 8
+  store i32 %0, i32* %3, align 4
+  %4 = load %class.Foo** %2
+  ret void
+}
+
+define internal void @_GLOBAL__I_a() section "__TEXT,__StaticInit,regular,pure_instructions" {
+  call void @__cxx_global_var_init()
+  ret void
+}