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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103351] [12 Regression] '-fcompare-debug'
 failure (length) at -O2 since r12-5301-g045206450386bcd7
Date: Mon, 22 Nov 2021 10:20:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103351

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
So the issue is that iterative_hash_expr of &__str._M_local_buf is not the
same for -g vs. -g0 which results in different ordering of the vector of
PHI arguments in sorted form.  First there's missing ordering we can use
based on ->dest_idx which should stabilize the sort somewhat.  But then
it might still be that we end up with different basic-block numbers when
there are two clusters of same PHI args and thus two forwarders to be
generated.

In the end this is because we hash in DECL_UID which are not the same here
(2872 vs 2873) which is OK.  So for the latter issue not exposed in this
testcase we should order clusters after the first ->dest_idx for example.
Though I'm not sure it is an isssue (but it could also improve stability
of testcases when reducing).  I'll try to fixup piecewise.=