[llvm-dev] getelementptr inbounds with offset 0 (original) (raw)

Ralf Jung via llvm-dev llvm-dev at lists.llvm.org
Sun Feb 24 09:04:56 PST 2019

Hi all,

What exactly are the rules for getelementptr inbounds with offset 0?

In Rust, we are relying on the fact that if we use, for example, inttoptr to turn 4 into a pointer, we can then do getelementptr inbounds with offset 0 on that without LLVM deducing that there actually is any dereferencable memory at location 4. The argument is that we can think of there being a zero-sized allocation. Is that a reasonable assumption? Can something like this be documented in the LangRef?

Relatedly, how does the situation change if the pointer is not created "out of thin air" from a fixed integer, but is actually a dangling pointer obtained previously from malloc (or alloca or whatever)? Is getelementptr inboundswith offset 0 on such a pointer a NOP, or does it result inpoison? And if that makes a difference, how does that square with the fact that, e.g., the integer 0x4000could well be inside such an allocation, but doinggetelementptr inbounds` with offset 0 on that would fall under the first question above?

Kind regards, Ralf

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