[PATCH, rtl-optimization]: Fix PR77452, ICE: in plus_constant, at explow.c

[Uros Bizjak <ubizjak at gmail dot com>]

Hello!

As shown in the PR [1], combine is able to simplify lowpart
CONST_VECTOR constant pool reference to its inner-mode reference.
However, plus_constant was not able to extract the constant from
narrowed access.

Attached patch teaches plus_constant how to handle this situation.

2016-09-04  Uros Bizjak  <ubizjak@gmail.com>

    PR rtl-optimization/77452
    * explow.c (plus_constant) <case MEM>: Extract scalar constant from
    inner-mode reference to a CONST_VECTOR constant in the constant pool.

2016-09-04  Uros Bizjak  <ubizjak@gmail.com>

testsuite/ChangeLog:

    PR rtl-optimization/77452
    * gcc.target/i386/pr77452.c: New test.

Patch was bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

OK for mainline and gcc-6 branch?

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77452

Uros.

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