[Bug rtl-optimization/122266] [13/14/15 regression] miscompilation of int128 xor-and-shift under -O2 in combine

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122266

--- Comment #12 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-15 branch has been updated by Richard Biener
<rguenth@gcc.gnu.org>:

https://gcc.gnu.org/g:cf7fbeb072142fc38955227b65ea331b1be6a4e7

commit r15-11237-gcf7fbeb072142fc38955227b65ea331b1be6a4e7
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Wed Oct 15 11:21:18 2025 +0100

    PR rtl-optimization/122266: Handle TImode in
reg_num_sign_bit_copies_for_combine

    This patch resolves PR rtl-optimization/122266 by changing the types
    of the last_set_sign_bit_copies and sign_bit_copies fields in combine.cc's
    reg_stat_type struct to be "unsigned short".  This makes both types
    consistent, and fixes the issue that on platforms where char is by
    default signed, combine.cc can overflow when handling TImode values,
    where sign_bit_copies can be 128 bits.

    Conveniently, there are holes (caused by field alignment/padding) in the
    reg_stat_type struct that allows us to upgrade to "unsigned short" without
    increasing the total size of the struct.  This should help reduce problems
    in future handling OImode or XImode values, or possible issues with 256-bit
    and 512-bit vector modes.  Note that it's important to take care when
    reordering the fields of this struct, as the (partial) ordering of fields
    is significant: See the use of offsetof in combine.cc's init_reg_last.

    Before:
    (gdb) ptype /o reg_stat_type
    /* offset      |    size */  type = struct reg_stat_type {
    /*      0      |       8 */    rtx_insn *last_death;
    /*      8      |       8 */    rtx_insn *last_set;
    /*     16      |       8 */    rtx last_set_value;
    /*     24      |       4 */    int last_set_table_tick;
    /*     28      |       4 */    int last_set_label;
    /*     32      |       8 */    unsigned long last_set_nonzero_bits;
    /*     40      |       1 */    char last_set_sign_bit_copies;
    /*     41: 0   |       4 */    machine_mode last_set_mode : 16;
    /*     43      |       1 */    bool last_set_invalid;
    /*     44      |       1 */    unsigned char sign_bit_copies;
    /* XXX  3-byte hole      */
    /*     48      |       8 */    unsigned long nonzero_bits;
    /*     56      |       4 */    int truncation_label;
    /*     60: 0   |       4 */    machine_mode truncated_to_mode : 16;
    /* XXX  2-byte padding   */
                                   /* total size (bytes):   64 */
                                 }

    After:
    /* offset      |    size */  type = struct reg_stat_type {
    /*      0      |       8 */    rtx_insn *last_death;
    /*      8      |       8 */    rtx_insn *last_set;
    /*     16      |       8 */    rtx last_set_value;
    /*     24      |       4 */    int last_set_table_tick;
    /*     28      |       4 */    int last_set_label;
    /*     32      |       8 */    unsigned long last_set_nonzero_bits;
    /*     40      |       2 */    unsigned short last_set_sign_bit_copies;
    /*     42: 0   |       4 */    machine_mode last_set_mode : 16;
    /*     44      |       1 */    bool last_set_invalid;
    /* XXX  1-byte hole      */
    /*     46      |       2 */    unsigned short sign_bit_copies;
    /*     48      |       8 */    unsigned long nonzero_bits;
    /*     56      |       4 */    int truncation_label;
    /*     60: 0   |       4 */    machine_mode truncated_to_mode : 16;
    /* XXX  2-byte padding   */
                                   /* total size (bytes):   64 */
                                 }

    2025-10-15  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            PR rtl-optimization/122266
            * combine.cc (struct reg_stat_type): Change types of
sign_bit_copies
            and last_set_sign_bit_copies to unsigned short, to avoid overflows
            on TImode (and wider) values.

    gcc/testsuite/ChangeLog
            PR rtl-optimization/122266
            * gcc.target/i386/pr122266.c: New test case.

    (cherry picked from commit da293ec6b68fab23c44f6643011cf7ae62d93b16)