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

Ralf Jung via llvm-dev llvm-dev at lists.llvm.org
Mon Feb 25 06:58:32 PST 2019

Hi Bruce,

On 25.02.19 13:10, Bruce Hoult wrote:

LLVM has no idea whether the address computed by GEP is actually within a legal object. The "inbounds" keyword is just you, the programmer, promising LLVM that you know it's ok and that you don't care what happens if it is actually out of bounds.

https://llvm.org/docs/GetElementPtr.html#what-happens-if-an-array-index-is-out-of-bounds

The LangRef says I get a poison value when I am violating the bounds. What I am asking is what exactly this means when the offset is 0 -- what are the conditions under which an offset-by-0 is "out of bounds" and hence yields poison? Of course LLVM cannot always statically determine this, but it relies on (dynamically, on the "LLVM abstract machine") such things not happening, and I am asking what exactly these dynamic conditions are.

Kind regards, Ralf

On Sun, Feb 24, 2019 at 9:05 AM Ralf Jung via llvm-dev <llvm-dev at lists.llvm.org> wrote:

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 inbounds_ _with offset 0 on such a pointer a NOP, or does it result in poison? And if_ _that makes a difference, how does that square with the fact that, e.g., the_ _integer 0x4000 could well be inside such an allocation, but doing_ _getelementptr inbounds` with offset 0 on that would fall under the first question above? Kind regards, Ralf

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