That’s not specific to Zig — local heap allocators generally don’t zero deallocated memory — that’s a significant, unnecessary performance hit.
If you need data to be isolated when memory is corrupt, you need it to be isolated always.
If you need data to be isolated when memory is corrupt, you need it to be isolated always.
If we use a modern Xeon chip as an example, an AVX2 store has a throughput of 2 instructions / cycle. Doing that 256 times for 8 KB totals 128 cycles, plus a few extra cycles to account for the latency of issuing the first instruction and the last store to the L1 cache. With a 2 GHz clock frequency, it still takes less than 70 nanoseconds. For comparison, an integer divide has a worst-case latency of 90ish cycles, or 45ish nanoseconds.
Zeroing memory on deallocation can be important for sensitive data. Otherwise, it makes more sense to zero on allocation if you know that it's needed because the allocated structure will be used without initilazation and the memory isn't zero by guarantee (most OSes guarantee newly allocated memory will be zero, and have a process to zero pages in the background when possible)
If you're exposing an HTTP API to clients, it's likely that any response's contents will contain sensitive client-specific data. If memory corruption bugs are more likely than bottlenecking on zeroing out your request/response buffer, then zeroing the request/response buffer is a good idea, until proven otherwise by benchmarks or profiling.
If it was freed after already being removed from the L1 cache, then you also need to evict other L1 cache contents and wait for it to be read into L1 so you can write to it.
128 cycles is a generous estimate, and ignores the costs to the rest of the program.
But yes, it can be an over-optimization.
Memcpy, bzero and friends are insanely fast. Practically free when those bytes are in the cpu’s cache already.
(But it also has a 16KB page size.)
(By the way, x86 does have a dedicated instruction--rep stosb--but compilers differ as to how often they use it, for the reason cited above.)
Arm64 provides `dc zva` for this.
There are various _s variants of memset, etc that require the compiler to perform the operations even if it “proves” the data cannot be read.
And finally modern hardware has mechanisms to say “this is now zero” and not actually zero the memory and instead just tell the MMU that the region is now zero (which removes the cpu time and cache impact of accessing the memory directly).
On macOS and iOS I believe all memory is now zero’d on free and I think malloc ostensibly therefore guarantees zero’d memory (the problem I think is whether calloc tries to rely on that behavior, because then calloc can produce non-zero memory courtesy of a buffer overrun/UaF after free has ostensibly zero’d memory)
In Linux you mean.