[llvm-dev] Why are GEPs type based?

[Nuno Lopes via llvm-dev]

You are right that it’s mostly a convenience for the front-ends. So they don’t have to deal with boring things like padding and sizing things.

Otherwise it adds no semantic value. Object aliasing is not field sensitive in LLVM, so it doesn’t matter. Though someone may want to add support for that in the future for languages where it’s ok to do so.

FWIW, Alive2’s GEP instruction works over bytes only (pairs of constant * %reg). Though I’m not sure I would advocate to change LLVM’s representation.

 

Nuno

 

 

From: Nikita Popov
Sent: 13 July 2020 21:08
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [llvm-dev] Why are GEPs type based?

 

Hi,

 

I've been wondering why LLVMs GEP instructions are based on types, rather than encoding the raw address calculation as a base pointer plus some scaled offsets (still in the form of a GEP, to retain provenance).

 

The type information does not seem particularly useful (shouldn't be used as an optimization base, because struct layouts lie), but increases the non-canonical IR space (there are many ways to encode the same GEP) and increases compile-time (optimizations need to constantly decompose GEPs, e.g. to get constant offsets).

 

What am I missing here?

 

Nikita,

Regards

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