Re: Where does the time go?

[Steven Bosscher <stevenb.gcc@gmail.com>]

On Thu, May 20, 2010 at 10:54 PM, Duncan Sands <baldrick@free.fr> wrote:
> I noticed while working on the dragonegg plugin that replacing gimple -> RTL
> with gimple -> LLVM IR significantly reduced the amount of memory used by
> the compiler at -O0.  I didn't investigate where the memory was going, but
> it seems likely that RTL either contains a whole lot more information than
> the LLVM IR, or doesn't represent it in a very memory efficient way.

The latter. LLVM IR contains a bit more information (or at least,
contains it in a more natural way) but the problem with RTL is, I
think, the tree-like representation. If you have an instruction like
(set (a) (b+c)) you could have, at the simples, three integers (insn
uid, basic block, instruction code) and three pointers for operands.
In total, on a 64 bits host: 3*4+3*8 = 36 bytes.

An RTL instruction of that form, assuming all operands are registers,
is 6*sizeof(struct rtx_def) = 6*48 = 288 bytes, give or take a few.
Those 6 rtx'en are for:

1. insn
2. set
3. set_dest operand
4. set_source: a plus
5. source operand 1
6. source operand 2

All in all, perhaps not the most efficient representation for memory
foot print, and the pointer chasing probably doesn't help (cache!).
But changing it is a lot more difficult than the GIMPLE tuples
project. I don't think it can be done.

Ciao!
Steven