[llvm] r212337 - GlobalDCE: Delete available_externally initializers if it allows removing the value the initializer is referring to.
Pete Cooper
pete.cooper at gmail.com
Mon Jul 7 11:20:30 PDT 2014
Hey Ben
This is crashing our test suite. Mind taking a look? This may be related
to what Richard has just pointed out too.
Attached a reduced test which crashes the following "opt dce.ll -globaldce
-o file.bc”
Cheers,
Pete
On Fri, Jul 4, 2014 at 1:36 PM, 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())) {
> + 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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