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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/59747] [4.9 Regression] wrong code at -Os and above on x86_64-linux-gnu in 64-bit mode
Date: Fri, 10 Jan 2014 13:14:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59747

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
The actual REGNO != REGNO transformation is correct, the problem is that the
second extension is to a wider mode and while handling that we change the
destination mode on the def_insn to an even wider mode.  The code just assumes
that the def insn sets the right REGNO rather than some other.  So, I think it
shouldn't be hard to detect that case.  Then we either need to ensure the
corresponding copy insn from the first extension will be adjusted to the wider
mode, or for this case swap back the registers (revert to the original REGNO
for the def insn and put the swap insn the other way around), or do it the
other way around always (then no special casing would be needed; Jeff, what was
the reason why you haven't handled
(set (reg1) (expression))
(set (reg2) (any_extend (reg1))
as
(set (reg1) (any_extend (expression)))
(set (reg2) (reg1))
but instead
(set (reg2) (any_extend (expression)))
(set (reg1) (reg2))
?).