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[PATCH, rtl-optimization]: Remove const_alias_set


Hello!

The fix that fixed scheduler issues with AND addresses (the fix
prevented early exit for MEM_READONLY_P addresses when AND alignment
addresses were involved) caused some fall-out for libgo testsuite.
These tests triggered an assert in mems_in_disjoint_alias_sets_p,
which checks for zero alias set when flag_strict_aliasing is false. We
have had some off-list discussion with Ian Lance Taylor about this
issue.

The problem was, that Go dynamically switches off flag_strict_aliasing
after compilation started and when "unsafe" package is imported
(similar to when__attribute__ ((optimize ("-fno-strict-aliasing"))) is
used in c). To mitigate this issue, the Go frontend called
varasm_init_once again to recalculated (= cleared) const_alias_set in
this case.

As observed in [1], the fix for canon_true_depence [2] that introduced
quick exit for a MEM_READONLY_P operands made const_alias_set
redundant, it is no longer user for anything.

The patch that fixed scheduling of AND operands removed early
MEM_READONLY_P exit for memory operands with AND realignment, so
operands could reach more complex code later in the function that was
able to determine dependence of memory operands. This code includes
the call to mems_in_disjoint_alias_sets_p, and the assert triggered
again for some MEM_READONLY_P operands that have had non-zero alias
set, set from the value, cached in const_alias_set from before
flag_strict_aliasing flag was cleared.

The proposed solution is to remove const_alias_set altogether. The
MEM_READONLY_P successfully supersedes const_alias_set functionality,
and this is also confirmed by the removal of the second
varasm_init_once call in the Go frontend. In an off-list discussion,
Ian agrees that attached patch should also fix the problem.

2014-10-19  Uros Bizjak  <ubizjak@gmail.com>

    * varasm.c (const_alias_set): Remove.
    (init_varasm_once): Remove initialization of const_alias_set.
    (build_constant_desc): Do not set alias set to const_alias_set.

The patch was tested on alpha-linux-gnu [3], alphaev68-linux-gnu and
x86_64-linux-gnu {,-m32} for all default languages plus Go and
obj-c++.

The patch fixes all mentioned libgo failures on alpha.

OK for mainline?

[1] https://gcc.gnu.org/ml/gcc-patches/2013-07/msg01033.html
[2] https://gcc.gnu.org/ml/gcc-patches/2010-07/msg01758.html
[3] https://gcc.gnu.org/ml/gcc-testresults/2014-10/msg02041.html

Uros.

Attachment: p.diff.txt
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