On Sun, Oct 19, 2014 at 8:13 PM, Uros Bizjak <ubiz...@gmail.com> wrote:
> 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 <ubiz...@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?
Ok.
Thanks,
Richard.
> [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.
