[Bug tree-optimization/100112] missed optimization for dead code elimination at -O3, -Os (vs. -O1, -O2)

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100112

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
This circles around redundant store elimination which is tricky with TBAA since
even a "value-redundant store" can change the dynamic type and thus we need to
preserve it.  See also PR101641 for a pending wrong-code issue here.

I _think_ we have an almost exact duplicate but let me take it, this is
related to last_vuse - we're doing the lookup w/ VN_NOWALK and thus don't
see the hashtable entry with the "optimized" VUSE from last_vuse handling.

IIRC VN_NOWALK is mainly a compile-time optimization, at elimination time
we even use VN_WALKREWRITE.  It originally changed with
g:649caaad399d6f4865a4d0015a1ac76c3cce7eb0 as a wrong-code fix though.