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LLVM cannot devirtualize with MSVC-style RTTI #49109

Open rnk opened 3 years ago

rnk commented 3 years ago
Bugzilla Link 49765
Version trunk
OS All
CC @aeubanks,@pcc

Extended Description

Consider this simple case when devirtualization should be possible after inlining the constructor: https://gcc.godbolt.org/z/osGe67s9b

struct Foo { Foo() {} virtual ~Foo() {} virtual void f() {} int x; }; int main() { Foo *p = new Foo(); p->f(); delete p; }

When RTTI data is disabled (/GR- / -fno-rtti-data), we cannot devirtualize the call to Foo::f. We can optimize the vptr load, but the vtable alias inhibits full devirtualization. See this instruction: call qword ptr [rip + "??_7Foo@@6B@"+8]

LLVM knows exactly which vtable we are loading from, but because of the MSVC RTTI alias code pattern, it refuses to optimize away the load.

We should either make the optimize more aggressive, or find a better IR pattern for MSVC-style RTTI.

Arthur had a patch to address this, but maybe it isn't sound: https://reviews.llvm.org/D99240

aeubanks commented 3 years ago

Fixed with https://reviews.llvm.org/D101100