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Walter from https://forum.dlang.org/post/vv6s8p$lfh$1@digitalmars.com:

D relies on the null dereference not being "optimized away". Maybe there's a switch for that on ldc, there should be if there isn't one.

That needs to be in the spec, hence this pull.

This is going to need @kinke and @ibuclaw also, as they support targets that the codegen doesn't currently offer read barriers that would require them.

It's probably no big deal to "bounds check" pointer and class deferences in safe code only. Pointer slicing and indexing is already disallowed, and array slicing/indexing should rightly assume the underlying structure of the array isn't corrupted.

Timon & Derek Fawcus on the forum found this for LLVM:

If "null-pointer-is-valid" is set to "true", then null address in address-space 0 is considered to be a valid address for memory loads and stores. Any analysis or optimization should not treat dereferencing a pointer to null as undefined behavior in this function

From under https://releases.llvm.org/10.0.0/docs/LangRef.html#function-attributes.

@ibuclaw: Do you explicitly opt out of these null-deref optimizations for GDC? Apparently not just for @safe functions. I'm wondering if I should really only restrict it to @safe functions.

@ibuclaw: Do you explicitly opt out of these null-deref optimizations for GDC? Apparently not just for @safe functions. I'm wondering if I should really only restrict it to @safe functions.

I don't think GCC implements null dereference optimizations.

Off the top of my head, infinite loop optimizations are opt-in. As it's part of C++ language to assume all loops are finite.

That is to say, if any nul pointer opts do exist its probably opt-in as well.

Oh I see, it's only clang that 'optimizes' the artificial tiny testcase, not gcc.