3 ms·

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

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.

jlebar 7y ago
> 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); If malloc is an opaque function, can the compiler nonetheless eliminate the second `b = *a` load as dead because it "knows what malloc does"? Certainly not, right?
- jepler 7y ago
  https://godbolt.org/z/z-92qR https://godbolt.org/z/z-92qR Interestingly, the compiler emits no call to malloc, even. What!?
  - gpderetta 7y ago
    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.
  - steerablesafe 7y ago
    With -fno-builtin. It's still "cheating" as b is local. https://godbolt.org/z/U5VGxt https://godbolt.org/z/U5VGxt Without cheating: https://godbolt.org/z/rzcXg2 https://godbolt.org/z/rzcXg2 It doesn't optimize the two stores to b away.