[LLVMdev] constant pool in large functions

[Ralph Corderoy]

Hi,

Evan wrote:
> On Jan 5, 2007, at 9:15 AM, Lauro Ramos Venancio wrote:
> > The LLVM prints constant pool at the beginning of functions.
> >
> > In large functions loads can be far from constant pool. GCC
> > resolves this situations by spreading constants throughtout
> > functions. A C test and the code generated by LLVM and GCC are
> > attached.
>
> This is a very tricky problem to resolve. Basically you need a pass
> just before code emission to find the proper locations for each
> constant pool. That means you need:
> 
> 1. Exact size of each instruction. This can be added with a custom
> instrinfo that specify instruction length for each target instruction.
> 2. Probably some kind of constantpool pseudo instruction.
> 
> The pass can then insert new constantpool instructions at the right
> locations to solve this problem.

A bodgy alternative to get you over the hurdle is to construct constants
with instructions rather than load them from memory, e.g.

    mov r9, #&3f8000           # r9 = 0x3f8000
    sub r9, r9, #&400000       # r9 = 0xffff8000 (WDTCTRL)

comp.sys.arm may be able to point you to ARM's recommended way of coming
up with the needed instructions for a 32-bit constant.

Cheers,


Ralph.