LTO I will grant you. I don't think you need to meaningfully inline the body of delete to get sized-delete to work effectively. You do want to statically link your sized operator delete body though, obviously. As of the last time I tried this, trying to inline the actual body was difficult to make effective, though.
(parent: read username :))
Hi, Andrew. :)
Agree, I shouldn't have been loose wrt the distinction between inlining and LTO. I think I also ran that experiment with inlining sized-deletes and came to the same (surprising, to me) conclusion.
Re:Benchmarking C++ Allocators
by pizlonator on Monday April 27, 2020 @06:08PM
(#23000683)
Let’s be clear: even without LTO, the compiler knows what malloc and free do at a semantic level.
I buy the sized delete win, I’ve seen that too, in some niche cases. But that’s a far cry from the claim that not inlining malloc is bananas. If that’s all you got then this sounds like a stunning retreat from the thesis of Jeff’s post. You’re just saying that you’ve got a free that benefits from size specialization, which has nothing to do with whether malloc got inlined.
> even without LTO, the compiler knows what malloc and free do at a semantic level.
int *a = ...;
int b = *a;
malloc(42);
b = *a;
printf(b);
https://godbolt.org/z/z-92qR
Interestingly, the compiler emits no call to malloc, even. What!?
Re:Benchmarking C++ Allocators
by steerablesafe on Tuesday April 28, 2020 @05:01AM
(#23005107)
With -fno-builtin. It's still "cheating" as b is local.
Without cheating:
It doesn't optimize the two stores to b away.
The compiler knows the semantics of malloc. Malloc as it appear in the source code doesn't even need to map 1:1 to calls to the library function, the compiler can remove and coalesce calls at will.
This is only thinking at the hardware layer. There are also effects at the software layer.
Function calls not known to LTO prevent all sorts of compiler optimizations as well. In addition, as Jeff says on this thread, not being able to inline free means you get no benefit from sized-delete, which is a substantial improvement on some workloads. (I'd cite the exact number, but I'm not sure Google ever released it.)
Source: I literally worked on this.