In the new tcmalloc (and, I think, hoard?) the fastest pools are essentially slabs with bump allocation, so the fastest (and by far, the most common) calls are a grand total of 15 or so instructions, without many cache misses (size class lookups tend to stay in the cache). Call overhead can be a substantial chunk of that.
But that’s not the whole story. Malloc perf is about what happens on free and what happens when the program accesses that memory that malloc gave it.
When you factor that all in, it doesn’t matter how many instructions the malloc has. It matters whether those instructions form a bad dependency chain, if they miss cache, whether the memory we return is in the “best” place, and how much work happens in free (using the same metrics - dependency chain length and misses, not total number of instructions or whether there’s a call).
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.
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.
int *a = ...;
int b = *a;
malloc(42);
b = *a;
printf(b);
Interestingly, the compiler emits no call to malloc, even. What!?
Without cheating:
It doesn't optimize the two stores to b away.
(parent: read username :))
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.