[Bug c++/94243] New: Missed C++ front-end devirtualizations from Clang testsuite

[Bug c++/94243] New: Missed C++ front-end devirtualizations from Clang testsuite

[hubicka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94243

            Bug ID: 94243
           Summary: Missed C++ front-end devirtualizations from Clang
                    testsuite
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hubicka at gcc dot gnu.org
  Target Milestone: ---

While working on PR93347 I noticed that we do not devirtualize the following
testcases that clang's testsuite tests to be devirtualized:

namespace Test2a {
  struct A {
    virtual ~A() final {}
    virtual int f();
  };

  // CHECK-LABEL: define i32 @_ZN6Test2a1fEPNS_1AE
  int f(A *a) {
    // CHECK: call i32 @_ZN6Test2a1A1fEv
    return a->f();
  }
}

Here I guess the final destructor makes the whole class final?

namespace Test4 {
  struct A {
    virtual void f();
    virtual int operator-();
  };

  struct B final : A {
    virtual void f();
    virtual int operator-();
  };

  // CHECK-LABEL: define void @_ZN5Test41fEPNS_1BE
  void f(B* d) {
    // CHECK: call void @_ZN5Test41B1fEv
    static_cast<A*>(d)->f();
    // CHECK: call i32 @_ZN5Test41BngEv
    -static_cast<A&>(*d);
  }
}

Her I am not sure, I think parameter d may point to instance of struct A,
so is it Clang's bug to devirtualize?

namespace Test5 {
  struct A {
    virtual void f();
    virtual int operator-();
  };

  struct B : A {
    virtual void f();
    virtual int operator-();
  };

  struct C final : B {
  };

  // CHECK-LABEL: define void @_ZN5Test51fEPNS_1CE
  void f(C* d) {
    // FIXME: It should be possible to devirtualize this case, but that is
    // not implemented yet.
    // CHECK: getelementptr
    // CHECK-NEXT: %[[FUNC:.*]] = load
    // CHECK-NEXT: call void %[[FUNC]]
    static_cast<A*>(d)->f();
  }
  // CHECK-LABEL: define void @_ZN5Test53fopEPNS_1CE
  void fop(C* d) {
    // FIXME: It should be possible to devirtualize this case, but that is
    // not implemented yet.
    // CHECK: getelementptr
    // CHECK-NEXT: %[[FUNC:.*]] = load
    // CHECK-NEXT: call i32 %[[FUNC]]
    -static_cast<A&>(*d);
  }
}

this seems similar to me.
namespace Test7 {
  struct foo {
    virtual void g() {}
  };

  struct bar {
    virtual int f() { return 0; }
  };

  struct zed final : public foo, public bar {
    int z;
    virtual int f() {return z;}
  };

  // CHECK-LABEL: define i32 @_ZN5Test71fEPNS_3zedE
  int f(zed *z) {
    // CHECK: alloca
    // CHECK-NEXT: store
    // CHECK-NEXT: load
    // CHECK-NEXT: call i32 @_ZN5Test73zed1fEv
    // CHECK-NEXT: ret
    return static_cast<bar*>(z)->f();
  }
}

namespace Test8 {
  struct A { virtual ~A() {} };
  struct B {
    int b;
    virtual int foo() { return b; }
  };
  struct C final : A, B {  };
  // CHECK-LABEL: define i32 @_ZN5Test84testEPNS_1CE
  int test(C *c) {
    // CHECK: %[[THIS:.*]] = phi
    // CHECK-NEXT: call i32 @_ZN5Test81B3fooEv(%"struct.Test8::B"* %[[THIS]])
    return static_cast<B*>(c)->foo();
  }
}

namespace Test9 {
  struct A {
    int a;
  };
  struct B {
    int b;
  };
  struct C : public B, public A {
  };
  struct RA {
    virtual A *f() {
      return 0;
    }
    virtual A *operator-() {
      return 0;
    }
  };
  struct RC final : public RA {
    virtual C *f() {
      C *x = new C();
      x->a = 1;
      x->b = 2;
      return x;
    }
    virtual C *operator-() {
      C *x = new C();
      x->a = 1;
      x->b = 2;
      return x;
    }
  };
  // CHECK: define {{.*}} @_ZN5Test91fEPNS_2RCE
  A *f(RC *x) {
    // FIXME: It should be possible to devirtualize this case, but that is
    // not implemented yet.
    // CHECK: load
    // CHECK: bitcast
    // CHECK: [[F_PTR_RA:%.+]] = bitcast
    // CHECK: [[VTABLE:%.+]] = load {{.+}} [[F_PTR_RA]]
    // CHECK: [[VFN:%.+]] = getelementptr inbounds {{.+}} [[VTABLE]],
i{{[0-9]+}} 0
    // CHECK-NEXT: %[[FUNC:.*]] = load {{.+}} [[VFN]]
    return static_cast<RA*>(x)->f();
  }
  // CHECK: define {{.*}} @_ZN5Test93fopEPNS_2RCE
  A *fop(RC *x) {
    // FIXME: It should be possible to devirtualize this case, but that is
    // not implemented yet.
    // CHECK: load
    // CHECK: bitcast
    // CHECK: [[F_PTR_RA:%.+]] = bitcast
    // CHECK: [[VTABLE:%.+]] = load {{.+}} [[F_PTR_RA]]
    // CHECK: [[VFN:%.+]] = getelementptr inbounds {{.+}} [[VTABLE]],
i{{[0-9]+}} 1
    // CHECK-NEXT: %[[FUNC:.*]] = load {{.+}} [[VFN]]
    // CHECK-NEXT: = call {{.*}} %[[FUNC]]
    return -static_cast<RA&>(*x);
  }
}

namespace Test10 {
  struct A {
    virtual int f();
  };

  struct B : A {
    int f() final;
  };

  // CHECK-LABEL: define i32 @_ZN6Test101fEPNS_1BE
  int f(B *b) {
    // CHECK: call i32 @_ZN6Test101B1fEv
    return static_cast<A *>(b)->f();
  }
}

[Bug c++/94243] Missed C++ front-end devirtualizations from Clang testsuite

[hubicka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94243

Jan Hubicka <hubicka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at redhat dot com

--- Comment #1 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
Jason,
I wonder if those are all valid transformations?
Honza