[PATCH] ARMv8 IfConversion must skip narrow instructions that a) define CPSR and b) wouldn't affect CPSR in an IT block

[Artyom Skrobov]

Hello,

As discussed earlier (the bug report is at
http://llvm.org/bugs/show_bug.cgi?id=18370  and the follow-up discussion is
at
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140120/202386.
html), many narrow Thumb instructions change their behaviour depending on
whether they're in an IT block. These instructions only affect CPSR if
they're *not* in an IT block.

Therefore, when IfConversion applies the new ARMv8 restrictions, it must not
embed such an instruction into an IT block *whenever CPSR is live after the
instruction*.

Note that this doesn't fix the original reported bug, which is about
emitting a wrong assembly mnemonic in the other case, when an instruction is
safe to be embedded into an IT block.

OK to commit this?
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