[LLVMdev] A new mechanism to compiler kernel modules using llvm: Defer type evaluation in clang?

[Joshua Cranmer 🐧]

On 4/29/2013 7:32 AM, Jovi Zhang wrote:
> The case I really want to solve is some kernel change cause ko must 
> need to recompile, like data structure field offset changed in new 
> kernel, this will make offset mismatch between new kernel and old ko. 
> This is the reason why I want to delay type structure evaluation into 
> install time in Clang, In theory, ko compiled now could work in 10 
> years later whatever how kernel changes. 

This kind of fix is not possible with LLVM IR right now and generally 
not possible with C code ever unless you make a lot of simplifying 
assumptions. Note that, for example, offsetof and sizeof are resolved to 
a constant by Clang before they are ever lowered to LLVM IR in the first 
place. Also, the way that structs are referenced in LLVM IR do not 
satisfy binary compatibility: a struct is lowered to a sequence of its 
constituent members accessed by their index and not be a given name. 
This means that inserting a member into the struct invalidates the IR.

-- 
Joshua Cranmer
Thunderbird and DXR developer
Source code archæologist