[llvm] r212337 - GlobalDCE: Delete available_externally initializers if it allows removing the value the initializer is referring to.

[Richard Smith]

On 4 Jul 2014 05:52, "Benjamin Kramer" <benny.kra at googlemail.com> wrote:
>
> Author: d0k
> Date: Fri Jul  4 07:36:05 2014
> New Revision: 212337
>
> URL: http://llvm.org/viewvc/llvm-project?rev=212337&view=rev
> Log:
> GlobalDCE: Delete available_externally initializers if it allows removing
the value the initializer is referring to.
>
> This is useful for functions that are not actually available externally
but
> referenced by a vtable of some kind. Clang emits functions like this for
the MS
> ABI.
>
> PR20182.
>
> Modified:
>     llvm/trunk/lib/Transforms/IPO/GlobalDCE.cpp
>     llvm/trunk/test/Transforms/GlobalDCE/externally_available.ll
>
> Modified: llvm/trunk/lib/Transforms/IPO/GlobalDCE.cpp
> URL:
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/IPO/GlobalDCE.cpp?rev=212337&r1=212336&r2=212337&view=diff
>
==============================================================================
> --- llvm/trunk/lib/Transforms/IPO/GlobalDCE.cpp (original)
> +++ llvm/trunk/lib/Transforms/IPO/GlobalDCE.cpp Fri Jul  4 07:36:05 2014
> @@ -44,14 +44,19 @@ namespace {
>      bool runOnModule(Module &M) override;
>
>    private:
> -    SmallPtrSet<GlobalValue*, 32> AliveGlobals;
> +    SmallPtrSet<Constant *, 32> AliveGlobals;
>      SmallPtrSet<Constant *, 8> SeenConstants;
> +    SmallPtrSet<GlobalVariable *, 8> DiscardableGlobalInitializers;
>
>      /// GlobalIsNeeded - mark the specific global value as needed, and
>      /// recursively mark anything that it uses as also needed.
>      void GlobalIsNeeded(GlobalValue *GV);
>      void MarkUsedGlobalsAsNeeded(Constant *C);
>
> +    /// \brief Checks if C is alive or is a ConstantExpr that refers to
an alive
> +    /// value.
> +    bool ContainsUsedGlobal(Constant *C);
> +
>      bool RemoveUnusedGlobalValue(GlobalValue &GV);
>    };
>  }
> @@ -162,6 +167,19 @@ bool GlobalDCE::runOnModule(Module &M) {
>        I->setAliasee(nullptr);
>      }
>
> +  // Look for available externally constants that we can turn into normal
> +  // externals by deleting their initalizers. This allows us to remove
other
> +  // globals that are referenced by the initializer.
> +  if (!DiscardableGlobalInitializers.empty()) {
> +    for (GlobalVariable *GV : DiscardableGlobalInitializers) {
> +      if (!ContainsUsedGlobal(GV->getInitializer())) {

Shouldn't we remove it if it contains any unused global, rather than if it
does not contain any used global?

> +        GV->setInitializer(nullptr);
> +        GV->setLinkage(GlobalValue::ExternalLinkage);
> +        Changed = true;
> +      }
> +    }
> +  }
> +
>    if (!DeadFunctions.empty()) {
>      // Now that all interferences have been dropped, delete the actual
objects
>      // themselves.
> @@ -209,8 +227,12 @@ void GlobalDCE::GlobalIsNeeded(GlobalVal
>    if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {
>      // If this is a global variable, we must make sure to add any global
values
>      // referenced by the initializer to the alive set.
> -    if (GV->hasInitializer())
> -      MarkUsedGlobalsAsNeeded(GV->getInitializer());
> +    if (GV->hasInitializer()) {
> +      if (GV->hasAvailableExternallyLinkage())
> +        DiscardableGlobalInitializers.insert(GV);
> +      else
> +        MarkUsedGlobalsAsNeeded(GV->getInitializer());
> +    }
>    } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(G)) {
>      // The target of a global alias is needed.
>      MarkUsedGlobalsAsNeeded(GA->getAliasee());
> @@ -248,6 +270,21 @@ void GlobalDCE::MarkUsedGlobalsAsNeeded(
>    }
>  }
>
> +bool GlobalDCE::ContainsUsedGlobal(Constant *C) {
> +  // C contains a used global If C is alive or we visited it while
marking
> +  // values alive.
> +  if (AliveGlobals.count(C) || SeenConstants.count(C))
> +    return true;
> +
> +  // Now check all operands of a ConstantExpr.
> +  for (User::op_iterator I = C->op_begin(), E = C->op_end(); I != E;
++I) {
> +    Constant *Op = dyn_cast<Constant>(*I);
> +    if (Op && ContainsUsedGlobal(Op))
> +      return true;
> +  }
> +  return false;
> +}
> +
>  // RemoveUnusedGlobalValue - Loop over all of the uses of the specified
>  // GlobalValue, looking for the constant pointer ref that may be
pointing to it.
>  // If found, check to see if the constant pointer ref is safe to
destroy, and if
>
> Modified: llvm/trunk/test/Transforms/GlobalDCE/externally_available.ll
> URL:
http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/GlobalDCE/externally_available.ll?rev=212337&r1=212336&r2=212337&view=diff
>
==============================================================================
> --- llvm/trunk/test/Transforms/GlobalDCE/externally_available.ll
(original)
> +++ llvm/trunk/test/Transforms/GlobalDCE/externally_available.ll Fri Jul
 4 07:36:05 2014
> @@ -1,10 +1,43 @@
> -; RUN: opt < %s -globaldce -S | not grep test_
> +; RUN: opt < %s -globaldce -S | FileCheck %s
>
>  ; test_function should not be emitted to the .s file.
> +; CHECK-NOT: @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
>  @test_global = available_externally global i32 4
>
> +; CHECK: @x = external constant void ()*
> + at x = available_externally constant void()* @f
> +; CHECK: @y = external constant i32
> + at y = available_externally constant i32 ptrtoint (void()* @g to i32)
> +; @h is still alive, so don't remove the initializer too eagerly.
> +; CHECK: @z = available_externally constant i8 ptrtoint (void (i8)* @h
to i8)
> + at z = available_externally constant i8 ptrtoint (void(i8)* @h to i8)
> +
> +; CHECK-NOT: @f
> +define linkonce_odr void @f() {
> +  ret void
> +}
> +
> +; CHECK-NOT: @g
> +define linkonce_odr void @g() {
> +  ret void
> +}
> +
> +; CHECK: define linkonce_odr void @h
> +define linkonce_odr void @h(i8) {
> +  ret void
> +}
> +
> +define i32 @main() {
> +  %f = load void()** @x
> +  call void %f()
> +  %g = load i32* @y
> +  %h = load i8* @z
> +  call void @h(i8 %h)
> +  ret i32 %g
> +}
>
>
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