Re: [PATCH] Redesign jump threading profile updates

[Jeff Law <law@redhat.com>]

On 10/01/14 14:04, Teresa Johnson wrote:
> The block frequencies are very small in this case leading to rounding
> errors when computing the edge frequency. The rounding error was then
> propagated into the recomputed probabilities, leading to insanities in
> the outgoing edge probabilities on the jump threading path. Eventually
> during rtl expansion these insanities somehow caused an ICE.
>
> (Before this patch the probabilities weren't even being updated,
> leading to insanities in other cases where they needed to be updated.)
>
> Specifically, in this case we had a block with frequency = 1. It had
> two outgoing edges each with probability 50%. Because the
> EDGE_FREQUENCY computation uses a rounding divide, the outgoing edge
> frequencies were both computed as 1. Later use of these block and edge
> frequencies to recompute the probability lead to both outgoing edges
> getting 100%.
>
> To address this, in the routine freqs_to_counts_path can simply scale
> up the frequencies when recording them in the count field. I added a
> scale by REG_BR_PROB_BASE. The largest count possible would therefore
> be REG_BR_PROB_BASE * BB_FREQ_MAX which is 10000 * 10000 = 100mil
> which safely fits in gcov_type.
>
> Here is the patch I am testing. Confirmed it fixes the reported issue.
> Currently bootstrapping and testing on x86_64-unknown-linux-gnu. Ok
> for trunk if it passes?
>
> (The whitespace is getting messed up when I copy the patch in here -
> the indentations do line up in the patch.)
>
> Thanks,
> Teresa
>
> 2014-10-01  Teresa Johnson  <tejohnson@google.com>
>
>          * tree-ssa-threadupdate.c (freqs_to_counts_path): Scale frequencies
>          up when synthesizing counts to avoid rounding errors.
OK.
jeff