[swift-dev] Weird dyn_cast code gen / performance problem
mgottesman at apple.com
Mon Jan 1 11:53:36 CST 2018
Do you have the llvm-ir handy?
> On Jan 1, 2018, at 11:30 AM, David Zarzycki via swift-dev <swift-dev at swift.org> wrote:
> I noticed recently that the code gen of CanType::getClassOrBoundGenericClass() could be better and along the way I found a clang/LLVM bug. Where exactly, I do not know, although my bet is the LLVM optimizer.
> When more than one dyn_cast() happens in a row, LLVM/clang emits redundant and pointless nullptr checks. Both Apple clang-900.0.39.2 and clang/llvm top-of-tree generate essentially the same code:
> <+35>: movb 0x8(%rbx), %cl ; getKind()
> <+38>: testq %rbx, %rbx ; XXX - nullptr check after deref is pointless
> <+41>: je 0x1377df6 ; <+54>
> <+43>: cmpb $0x12, %cl ; isa<ClassType>()
> <+46>: jne 0x1377df6 ; <+54>
> <+48>: addq $0x10, %rbx ; (void*)this + offsetof(ClassType, TheDecl)
> <+52>: jmp 0x1377e06 ; <+70>
> <+54>: xorl %eax, %eax ; the default return value (nullptr)
> <+56>: testq %rbx, %rbx ; XXX - another pointless nullptr check?
> <+59>: je 0x1377e09 ; <+73>
> <+61>: cmpb $0x29, %cl ; isa<BoundGenericClassType>()
> <+64>: jne 0x1377e09 ; <+73>
> <+66>: addq $0x18, %rbx ; (void*)this + offsetof(BoundGenericClassType, TheDecl)
> <+70>: movq (%rbx), %rax ; load the decl pointer
> <+73>: popq %rbx
> <+74>: retq
> I’ve tried adding different “nonnull” spellings in various parts of both Swift and LLVM’s casting machinery, but with no luck. The only thing that seems to work is to create a free function that takes a non-null “const TypeBase *” parameter and then have CanType::getClassOrBoundGenericClass() call that.
> FWIW – I *suspect* this is because LLVM’s casting machinery internally converts traditional pointers into C++ references before ultimately calling classof(&Val).
> Before I file a bug against clang/llvm, might I be missing something? Can anybody think of a good workaround?
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