Comment by raverbashing
2 days ago
Nope it is the C committee
> Once you accept that optimizing compilers do, well, optimizations
Why in tarnation it is optimizing out a write to a pointer out before a function that takes said pointer? Imagine it is any other function besides free, see how ridiculous that sounds?
It's been many years since C compilers started making pathological-but-technically-justifiable optimizations that work against the programmer. The problem is the vast sea of "undefined behavior" — if you are not a fully qualified language lawyer versed in every nook and cranny of the C standard, prepare to be surprised.
Many of us who don't like working under such conditions have just moved on to other languages.
I agree that compilers were too aggressive in exploiting UB, but this is not the topic of this thread which has nothing to do with UB. But also the situation with UB is in practice not too bad. While compilers broke some old code which caused frustration, when writing new code most UB can easily be dealt with in practice by following some basic ground rules (e.g. no unsafe casts, being careful with pointer arithmetic) and by activating some compiler flags. It is not anything that should cause much trouble when programming in C.
Because it is a dead store. Removing dead stores does not sound ridiculous to me and neither is it to anybody using an optimizing compiler in the last decades.
Tree shaking is pretty standard. Optimising out the write sounds fine to me - with the exception of a volatile pointer. That, there, is a mistake.
Optimizing out a write to (example) an array on the stack seems fine to me.
Optimizing out a function call to a heap pointer (especially memset) seems wrong to me. You called the function, it should call the function!
But it's again the C language saving time not wearing a seatbelt or checking the tire pressure for saving 10s on a 2h trip
The whole point of the optimizer is that it can detect inefficiencies by treating every statement as some combination of simple, fundamental operations. The compiler is not seeing "call memset() on pointer to heap", it's seeing "write of variable size" just before "deallocation". For some, optimizing that will be a problem, for others, not optimizing it will leave performance on the table.
There are still ways to obtain the desired behavior. Just put a call to a DLL or SO that implements what you need. The compiler cannot inspect the behavior of functions across module boundaries, so it cannot tell whether removing the call preserves semantics or not (for example, it could be that the external function sends the contents of the buffer to a file), so it will not remove it.
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