[Bug rtl-optimization/115565] [11/12/13/14/15 Regression] CSE: Comparison incorrectly evaluated as constant causing optimization to produce wrong code

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115565

--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Maciej W. Rozycki
<macro@gcc.gnu.org>:

https://gcc.gnu.org/g:8d8f804b18e4a38671957b3e4c239ef625506317

commit r12-10633-g8d8f804b18e4a38671957b3e4c239ef625506317
Author: Maciej W. Rozycki <macro@orcam.me.uk>
Date:   Sat Jun 29 23:26:55 2024 +0100

    [PR115565] cse: Don't use a valid regno for non-register in comparison_qty

    Use INT_MIN rather than -1 in `comparison_qty' where a comparison is not
    with a register, because the value of -1 is actually a valid reference
    to register 0 in the case where it has not been assigned a quantity.

    Using -1 makes `REG_QTY (REGNO (folded_arg1)) == ent->comparison_qty'
    comparison in `fold_rtx' to incorrectly trigger in rare circumstances
    and return true for a memory reference, making CSE consider a comparison
    operation to evaluate to a constant expression and consequently make the
    resulting code incorrectly execute or fail to execute conditional
    blocks.

    This has caused a miscompilation of rwlock.c from LinuxThreads for the
    `alpha-linux-gnu' target, where `rwlock->__rw_writer != thread_self ()'
    expression (where `thread_self' returns the thread pointer via a PALcode
    call) has been decided to be always true (with `ent->comparison_qty'
    using -1 for a reference to to `rwlock->__rw_writer', while register 0
    holding the thread pointer retrieved by `thread_self') and code for the
    false case has been optimized away where it mustn't have, causing
    program lockups.

    The issue has been observed as a regression from commit 08a692679fb8
    ("Undefined cse.c behaviour causes 3.4 regression on HPUX"),
    <https://gcc.gnu.org/ml/gcc-patches/2004-10/msg02027.html>, and up to
    commit 932ad4d9b550 ("Make CSE path following use the CFG"),
    <https://gcc.gnu.org/ml/gcc-patches/2006-12/msg00431.html>, where CSE
    has been restructured sufficiently for the issue not to trigger with the
    original reproducer anymore.  However the original bug remains and can
    trigger, because `comparison_qty' will still be assigned -1 for a memory
    reference and the `reg_qty' member of a `cse_reg_info_table' entry will
    still be assigned -1 for register 0 where the entry has not been
    assigned a quantity, e.g. at initialization.

    Use INT_MIN then as noted above, so that the value remains negative, for
    consistency with the REGNO_QTY_VALID_P macro (even though not used on
    `comparison_qty'), and then so that it should not ever match a valid
    negated register number, fixing the regression with commit 08a692679fb8.

            gcc/
            PR rtl-optimization/115565
            * cse.cc (record_jump_cond): Use INT_MIN rather than -1 for
            `comparison_qty' if !REG_P.

    (cherry picked from commit 69bc5fb97dc3fada81869e00fa65d39f7def6acf)