Re: [Qemu-devel] [PATCH v5 18/19] tcg-arm: Convert to CONFIG_QEMU_LDST_OPTIMIZATION

[Aurelien Jarno]

On Mon, Apr 22, 2013 at 03:39:42PM +0100, Richard Henderson wrote:
> On 2013-04-22 13:59, Aurelien Jarno wrote:
> >>>+    /* The code buffer is limited to 16MB, with the prologue located
> >>>+       at the end of it.  Therefore we needn't care for any out of
> >>>+       range branches.  */
> >>>+    assert(val - 8 < 0x01fffffd && val - 8 > -0x01fffffd);
> >>>+
> >>>+    tcg_out_b(s, cond, val);
> >While this is currently true, I am not sure we want to get rid of that
> >code, and I hope we'll be able to eventually get rid of the 16MB limit.
> >
> >For me this dramatically reduce the boot time of guests. That said it is
> >not a real benchmark, and it should theoretically reduce the
> >performances in some cases as doing so interleaves code and data.
> >Someone has to spend time doing benchmarks before we can progress on that.
> >
> 
> Ok, sure, but how do we progress in the short term?
> Certainly rejecting this patch and its changes to tcg_out_goto
> is not compatible with approving  19/19, which relies on it.

Oh, I haven't seen realized that the comment about goto not using
multi-insns ops was referring to patch 18.

> IMO, tcg_out_goto should be the simple case of goto within a TB,
> with only the code for INDEX_goto_tb needing to handle the >16MB case.
> 

>From what I have seen in the code, we need to do jumps in the following
conditions:
- jumps within TB
- jumps between slow/fast path in the ld/st code
- jump to the epilogue
- jump between TB

Currently the first three use tcg_goto_out, while only the third one
need to (eventually) jump above the 16MB limit.

The last one should also jump above the 16MB limit, but do not use
tcg_goto_out.

For jumps within TB tcg_out_goto_label can use tcg_out_b directly,
as tcg_out_b_noaddr is already used in the other pass, and the only
benefit of tcg_goto_out over tcg_out_b is the asserts, which can not
really been triggered here.

For jumps from the slow path to the fast path, I think tcg_out_b
can also be used, as anyway tcg_out_b_noaddr is already used from the
fast path to the slow path.

This leaves out tcg_goto_out used only for the jump to the epilogue, and
it can be modified later when someone works on the 16MB limit issue. Of
course that means that patch 19 has to be dropped, but I don't think
it's a big issue.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net