[PATCH] D21041: [GVN] PRE can't hoist loads across calls in general.

[Eli Friedman via llvm-commits]

On Mon, Jun 6, 2016 at 4:38 PM, Daniel Berlin <dberlin at dberlin.org> wrote:

>
>
> On Mon, Jun 6, 2016 at 4:06 PM, Eli Friedman <eli.friedman at gmail.com>
> wrote:
>
>> eli.friedman added a comment.
>>
>> It probably doesn't make sense to turn every `call` instruction into an
>> `invoke` or equivalent; that would cause the size of the IR to explode,
>
>
> I'm going to disagree.  GCC has EH/abnormal "fake" edges to handle these
> cases, and works much better than LLVM in this regard, and has no such
> explosion problem on normal C++ code. In fact, it's going to be faster.
>
> If we have a modeling problem, we should fix the modeling problem.
> You can abstract this to the block level.  The block has an EH successor,
>   That just means in some fashion, the block may go to this EH edge.
> It does not require you terminate the block at every mayThrow instruction.
>

Oh... so instead of precisely tracking the CFG inside the block, you just
approximate and say "this block as a whole can terminate early"?  That
makes sense, especially for stuff like loop optimization.

There are some places that want more precise tracking, though; I ended up
rewriting my patch for DSE to precisely track unwind edges.  Maybe that
isn't common enough to be worth worrying about.

-Eli
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