Re: [Qemu-devel] [PATCH] tcg/README: Expand advice on number of TCG ops per target insn

[Peter Maydell]

Ping?

On 22 June 2011 15:40, Peter Maydell <address@hidden> wrote:
> Expand the note on the number of TCG ops generated per target insn,
> to be clearer about the range of applicability of the 20 op rule
> of thumb. Also add a note about the hard MAX_OP_PER_INSTR limit.
>
> Signed-off-by: Peter Maydell <address@hidden>
> ---
> The expansion of the first bullet point is based on remarks by
> Aurelien in this email:
> http://lists.gnu.org/archive/html/qemu-devel/2011-06/msg00395.html
>
>  tcg/README |   10 +++++++++-
>  1 files changed, 9 insertions(+), 1 deletions(-)
>
> diff --git a/tcg/README b/tcg/README
> index 6600122..cfdfd96 100644
> --- a/tcg/README
> +++ b/tcg/README
> @@ -504,7 +504,15 @@ register.
>  - Don't hesitate to use helpers for complicated or seldom used target
>   instructions. There is little performance advantage in using TCG to
>   implement target instructions taking more than about twenty TCG
> -  instructions.
> +  instructions. Note that this rule of thumb is more applicable to
> +  helpers doing complex logic or arithmetic, where the C compiler has
> +  scope to do a good job of optimisation; it is less relevant where
> +  the instruction is mostly doing loads and stores, and in those cases
> +  inline TCG may still be faster for longer sequences.
> +
> +- The hard limit on the number of TCG instructions you can generate
> +  per target instruction is set by MAX_OP_PER_INSTR in exec-all.h --
> +  you cannot exceed this without risking a buffer overrun.
>
>  - Use the 'discard' instruction if you know that TCG won't be able to
>   prove that a given global is "dead" at a given program point. The
> --
> 1.7.1
>
>
>



-- 
12345678901234567890123456789012345678901234567890123456789012345678901234567890
         1         2         3         4         5         6         7         8