[Bug gas/24444] assembling big file with a lot of .loc views takes forever

[cvs-commit at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=24444

--- Comment #4 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot 
gnu.org> ---
The master branch has been updated by Alan Modra <address@hidden>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=38c3873e5d53902cf9cc73a4a5a05adf371296a6

commit 38c3873e5d53902cf9cc73a4a5a05adf371296a6
Author: Alexandre Oliva <address@hidden>
Date:   Sat Apr 13 05:55:34 2019 -0300

    Speed up locview resolution with relaxable frags

    Targets such as xtensa incur a much higher overhead to resolve
    location view numbers than e.g. x86, because the expressions used to
    compute view numbers cannot be resolved soon enough.

    Each view number is computed by incrementing the previous view, if
    they are both at the same address, or by resetting it to zero
    otherwise.  If PV is the previous view number, PL is its location, and
    NL is the location of the next view, its number is computed by
    evaluating NV = !(NL > PL) * (PV + 1).

    set_or_check_view uses resolve_expression to decide whether portions
    of this expression can be simplified to constants.  The (NL > PL)
    subexpression is one that can often be resolved to a constant,
    breaking chains of view number computations at instructions of nonzero
    length, but not after alignment that might be unnecessary.

    Alas, when nearly every frag ends with a relaxable instruction,
    frag_offset_fixed_p will correctly fail to determine a known offset
    between two unresolved addresses in neighboring frags, so the
    unresolved symbolic operation will be constructed and used in the
    computation of most view numbers.  This results in very deep
    expressions.

    As view numbers get referenced in location view lists, each operand in
    the list goes through symbol_clone_if_forward_ref, which recurses on
    every subexpression.  If each view number were to be referenced, this
    would exhibit O(n^2) behavior, where n is the depth of the view number
    expressions, i.e., the length of view number sequences without an
    early resolution that cuts the expression short.

    This patch enables address compares used by view numbering to be
    resolved even when exact offsets are not known, using new logic to
    determine when the location either remained the same or changed for
    sure, even with the possibility of relaxation.  This enables most view
    number expressions to be resolved with a small, reasonable depth.

        PR gas/24444
        * frags.c (frag_gtoffset_p): New.
        * frags.h (frag_gtoffset_p): Declare it.
        * expr.c (resolve_expression): Use it.

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