[LLVMdev] Argument Lowering Redux

[David Chisnall]

On 4 Aug 2014, at 05:24, Vadim Chugunov <vadimcn at gmail.com> wrote:

> Hi,
> Having just found an ABI conformance bug in a compiler front-end, I am curious: is the state of target-independent argument lowering in LLVM still the same as when this thread was taking place?: http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-February/thread.html#59387 

This comes up periodically.  The consensus last time was that LLVM should have some sort of ABIBuilder class that would take C types (as most ABIs are defined in terms of C types) and allow building function definitions and calls from them.  Inside the function, you'd get an alloca containing each C parameter.  Unfortunately, no one has had the combination of time, inclination, and expertise to do it.

Each back end would then be responsible for maintaining its corresponding ABIBuilder, to ensure that it expected in the IR is what the ABIBuilder produces.

The other problem is that the current approach in LLVM IR complicates optimisations.  It's bad enough that clang and the x86 back end both have to somehow know that an i64 return value is returned in two integer registers and so is the correct thing to use (on some platforms) for a struct of two i32s.  It's even worse that an optimisation has to know that the i64 that a function is returning might actually be a structure.

It is probably the ugliest bit of LLVM IR currently, that we have front ends that all either consume C or know how to map their values to C types, and have back ends that must implement ABIs defined in specifications in terms of C types, but we insist on mapping this knowledge to something different in the middle.

David