[llvm-dev] grouping global variables by alignment: safe to do at LLVM level, or only at Clang level?

[Abe Skolnik via llvm-dev]

>> My question this time is this: is re-ordering globals at the LLVM level both possible [...]

On 07/25/2016 10:53 PM, Mehdi Amini wrote:

> Isn’t something a linker could/should do?

AFAIK & TTBOMK, a compiler should do it [if allowed by the language] and a linker should not 
mess it up.  I don`t know that a linker is allowed to make assumptions about code not 
traversing labels in its data accesses.  After all, a compiler may have assigned labels to e.g. 
the start of a "struct" instance and to a specific part of that same struct.  That doesn`t mean 
the linker is free to move the middle of the struct somewhere else far from the start of the 
struct.

IOW, a linker is required to handle the code no matter what the source language was and what 
its specification says about implementations being allowed to re-order things and whether or 
not the language allows the programmer to _validly_ assume that when two arrays of identical 
element type are created back-to-back, the second array immediately follows the first, thus 
effectively creating a single array with its element count being the sum of the two element 
counts of the as-written arrays.



On 07/25/2016 10:53 PM, Mehdi Amini wrote:

> Such programming language should not use LLVM global variables if they need to provide such guarantee,
>  but probably use instead something more like a global array when lowering to LLVM IR.

Agreed, but I can`t [en]force that.  That would need to be decided by the LLVM steering 
committee or something like that, then documented publicly.

Thanks for your reply.

Regards,

Abe