[PATCH][RFC] Fix PR79432, SSA from the gimplifier and abnormal edges

[Jeff Law]

On 02/10/2017 05:24 AM, Richard Biener wrote:
>
> It turns out the SSA var defs/uses generated by the gimplifier can break
> apart in a way no longer satisfying the dominance requirement of SSA
> uses vs. their defs by means of CFG construction adding abnormal edges
> for stuff like setjmp (but also non-local gotos I guess).
>
> This would be quite costly to overcome in gimplification - one needs
> to check whether a (part?) of an expression to be gimplified may
> produce such edges and disable SSA name generation for them.  With
> the recursive nature of the gimplifier plus the general complexity
> of GENERIC I can't see how to do this.
ISTM there is no apriori way to know if any object is live across a call 
which would create these abnormal edges.  How in the world did this work 
in the past?

Sigh.  Because the problem starts at gimplification, we can't even do 
something like pre-scan the block for problem calls -- we don't have a CFG.

Can this only happen during gimplification of an expression where a 
sub-expression has a problematic call?  How bad do you think things 
would blow up if we scanned for a call and avoided using SSA_NAMEs when 
there's a problematical call within the expression?  Or is there no way 
structurally to do that within the gimplifier?

Are setjmp/longjump the only problems here, or does EH tickle these 
issues as well (if so, then ISTM you'd need a different test).

>
> Thus the following patch "recovers" from the extra abnormal edges
> by effectively treating SSA vars pre into-SSA as "non-SSA" and thus
> doing PHI insertion for them when rewriting the function into SSA.
> Implementation-wise the easiest thing was to re-write the affected
> SSA vars out-of-SSA (replace them by a decl).
If we can't do the right thing from the start, then "recovery" seems to 
be the only option.


>
> The out-of-SSA rewriting is placed in insert_phi_nodes because thats
> the first point in time we have immediate uses present (otherwise
> it could be done at any point after CFG construction).
THe earlier the better.  So as soon as we have a CFG & SSA we have to 
fixup.  So location seems right.

>
> I'm not 100% happy with this but after trying for a day I can't come
> up with a better solution - it has the chance of papering over
> bogus gimplifications but OTOH that we only do this for functions
> calling setjmp or having non-local labels would make those trigger
> SSA verification in the other cases.
>
> Bootstrap and regtest running on x86_64-unknown-linux-gnu.
>
> Any comments / ideas?  Anyone with good arguments that this is
> even conceptually the correct thing to do?
>
> I'm bootstrapping this with the calls_setjmp/has_nonlocal_label
> short-cutted with a 1 || as well as without (to increase testing
> coverage).
>
> Thanks,
> Richard.
>
> 2017-02-10  Richard Biener  <rguenther@suse.de>
>
> 	PR middle-end/79432
> 	* tree-into-ssa.c (insert_phi_nodes): When the function can
> 	have abnormal edges rewrite SSA names with broken use-def
> 	dominance out of SSA and register them for PHI insertion.
>
> 	* gcc.dg/torture/pr79432.c: New testcase.
It's gross.  But unless we have a reasonable way to do this during 
gimplification, I don't see anything better.

jeff