Re: [tree-ssa] More aggressive dead code elimination

[Steven Bosscher <s dot bosscher at student dot tudelft dot nl>]

On Wednesday 07 January 2004 23:28, law@redhat.com wrote:

>  >This patch re-introduces the classic SSA DCE algorithm as described
>  >in Morgan's book.  The current implementation takes the conservative
>  >approach and marks all conditional branches inherently necessary.
>  >This prevents us from removing some of them at the tree level.  With
>  >this patch we do eliminate unnecessary branches, e.g. stuff like,
>
> We decided a few months ago against using this approach to DCE due to
> performance issues.

Yes.  We had slow iterators, and dominance info was slow.  Both are
now a lot faster.

> The code we have now misses two cases that the approach cited in Morgan
> and other sources handles:
>
>   1. Useless loop removal.  Frankly, I don't care about this.

We never kill useless loops, not even on RTL.  And they appear to be
not very uncommon especially with C++ code, and with the tree loop
optimizers we're planning, they may become more common.

>   2. If the last DCE pass kills a conditional it does not arrange to
>      kill the statements which feed the conditional.  This can be
>      easily solved by re-running the DCE pass in that case.  These are
>      extremely rare.

Running the existing DCE twice is just as expensive as running the more
aggressive version just once, so this argument doesn't make sense to me.

Gr.
Steven