martin krastev
2015-Jan-07 08:03 UTC
[LLVMdev] Inclusion of iostream affects devirtualization in release_35
Hello,
I've stumbled upon the following behavior in branches/release_35 (as of
218689) under ubuntu 14.04 LTS amd64: whenever I include the (system-wise)
gnu iostream header, clang++ stops devirtualizing the following code:
#if BREAKAGE_ENSUES
#include <iostream>
#endif
struct Base {
virtual int foo() const = 0;
};
struct A : Base {
int a;
A(const int a)
: a(a) {
}
int foo() const {
return a;
}
};
struct B : Base {
const Base* b;
B(const Base* const base)
: b(base) {
}
int foo() const {
return b->foo();
}
};
const A a(42);
const B b(&a);
int main(int, char**) {
return b.foo();
}
Is that a known issue, and if not, under what category should I file a
ticket for it?
Best regards,
Martin
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Reid Kleckner
2015-Jan-07 16:38 UTC
[LLVMdev] Inclusion of iostream affects devirtualization in release_35
This isn't really devirtualization so much as global opt plus basic constant propagation. My theory is that iostream injects static initializers for std::cout / std::cerr into your TU. LLVM's global opt pass will fail when those are present. On Wed, Jan 7, 2015 at 12:03 AM, martin krastev <blu.dark at gmail.com> wrote:> Hello, > > I've stumbled upon the following behavior in branches/release_35 (as of > 218689) under ubuntu 14.04 LTS amd64: whenever I include the (system-wise) > gnu iostream header, clang++ stops devirtualizing the following code: > > > #if BREAKAGE_ENSUES > #include <iostream> > #endif > > struct Base { > virtual int foo() const = 0; > }; > > struct A : Base { > int a; > > A(const int a) > : a(a) { > } > > int foo() const { > return a; > } > }; > > struct B : Base { > const Base* b; > > B(const Base* const base) > : b(base) { > } > > int foo() const { > return b->foo(); > } > }; > > const A a(42); > const B b(&a); > > int main(int, char**) { > return b.foo(); > } > > > Is that a known issue, and if not, under what category should I file a > ticket for it? > > Best regards, > Martin > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150107/41a62013/attachment.html>
Nick Lewycky
2015-Jan-07 20:02 UTC
[LLVMdev] Inclusion of iostream affects devirtualization in release_35
On 7 January 2015 at 08:38, Reid Kleckner <rnk at google.com> wrote:> This isn't really devirtualization so much as global opt plus basic > constant propagation. My theory is that iostream injects static > initializers for std::cout / std::cerr into your TU. LLVM's global opt pass > will fail when those are present. >Yikes, that's a limitation right there isn't it. Should we have a PR or is there nothing for the optimizer can do? On Wed, Jan 7, 2015 at 12:03 AM, martin krastev <blu.dark at gmail.com> wrote:> >> Hello, >> >> I've stumbled upon the following behavior in branches/release_35 (as of >> 218689) under ubuntu 14.04 LTS amd64: whenever I include the (system-wise) >> gnu iostream header, clang++ stops devirtualizing the following code: >> >> >> #if BREAKAGE_ENSUES >> #include <iostream> >> #endif >> >> struct Base { >> virtual int foo() const = 0; >> }; >> >> struct A : Base { >> int a; >> >> A(const int a) >> : a(a) { >> } >> >> int foo() const { >> return a; >> } >> }; >> >> struct B : Base { >> const Base* b; >> >> B(const Base* const base) >> : b(base) { >> } >> >> int foo() const { >> return b->foo(); >> } >> }; >> >> const A a(42); >> const B b(&a); >> >> int main(int, char**) { >> return b.foo(); >> } >> >> >> Is that a known issue, and if not, under what category should I file a >> ticket for it? >> >> Best regards, >> Martin >> >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev >> >> > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150107/5619cdd0/attachment.html>