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Subject: Re: [PATCH] Fix phiopt ICE in Factor conversion in COND_EXPR (PR tree-optimization/66949)
To: Marek Polacek <polacek@redhat.com>, GCC Patches <gcc-patches@gcc.gnu.org>
References: <20151208162133.GQ3175@redhat.com>
Cc: Kugan <kugan.vivekanandarajah@linaro.org>
From: Jeff Law <law@redhat.com>
Message-ID: <5669FC58.3070901@redhat.com>
Date: Thu, 10 Dec 2015 22:27:00 -0000
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On 12/08/2015 09:21 AM, Marek Polacek wrote:
> The following is a conservative fix for this PR.  This is an ICE transpiring
> in the new "Factor conversion in COND_EXPR" optimization added in r225722.
>
> Before this optimization kicks in, we have
>    <bb 2>:
>    ...
>    p1_32 = (short unsigned int) _20;
>
>    <bb 3>:
>    ...
>    iftmp.0_18 = (short unsigned int) _20;
>
>    <bb 4>:
>    ...
>    # iftmp.0_19 = PHI <iftmp.0_18(3), p1_32(2)>
>
> after factor_out_conditional_conversion does its work, we end up with those two
> def stmts removed and instead of the PHI we'll have
>
>    # _35 = PHI <_20(3), _20(2)>
>    iftmp.0_19 = (short unsigned int) _35;
>
> That itself looks like a fine optimization, but after factor_out_conditional_conversion
> there's
>   320               phis = phi_nodes (bb2);
>   321               phi = single_non_singleton_phi_for_edges (phis, e1, e2);
>   322               gcc_assert (phi);
> and phis look like
>    b.2_38 = PHI <b.2_9(3), b.2_9(2)>
>    _35 = PHI <_20(3), _20(2)>
> so single_non_singleton_phi_for_edges returns NULL and the subsequent assert triggers.
Cute.
>
> With this patch we won't ICE (and PRE should clean this up anyway), but I don't know,
> maybe I should try harder to optimize even this problematical case (not sure how hard
> it would be...)?
So if this kind of situation is created by the first phiopt, then DOM 
should fix it.  As you note PRE will catch it if the second phiopt pass 
creates this degenerate PHI.  If created by the last phiopt pass, then 
hopefully coalescing would save us.

Jeff