[Bug tree-optimization/116024] [14/15 Regression] unnecessary integer comparison(s) for a simple loop since r14-5628-g53ba8d669550d3

[cvs-commit at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116024

--- Comment #10 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jeff Law <l...@gcc.gnu.org>:

https://gcc.gnu.org/g:65b33d43d29b148e127b1ba997f1bbc2c7028b94

commit r15-4187-g65b33d43d29b148e127b1ba997f1bbc2c7028b94
Author: Artemiy Volkov <artemiy.vol...@synopsys.com>
Date:   Tue Oct 8 17:54:55 2024 -0600

    tree-optimization/116024 - simplify C1-X cmp C2 for unsigned types

    Implement a match.pd transformation inverting the sign of X in
    C1 - X cmp C2, where C1 and C2 are integer constants and X is
    of an unsigned type, by observing that:

    (a) If cmp is == or !=, simply move X and C2 to opposite sides of the
    comparison to arrive at X cmp C1 - C2.

    (b) If cmp is <:
            - C1 - X < C2 means that C1 - X spans the range of 0, 1, ..., C2 -
1;
            - This means that X spans the range of C1 - (C2 - 1),
              C1 - (C2 - 2), ..., C1;
            - Subtracting C1 - (C2 - 1), X - (C1 - (C2 - 1)) is one of 0, 1,
              ..., C1 - (C1 - (C2 - 1));
            - Simplifying the above, X - (C1 - C2 + 1) is one of 0, 1, ...,
             C2 - 1;
            - Summarizing, the expression C1 - X < C2 can be transformed
              into X - (C1 - C2 + 1) < C2.

    (c) Similarly, if cmp is <=:
            - C1 - X <= C2 means that C1 - X is one of 0, 1, ..., C2;
            - It follows that X is one of C1 - C2, C1 - (C2 - 1), ..., C1;
            - Subtracting C1 - C2, X - (C1 - C2) has range 0, 1, ..., C2;
            - Thus, the expression C1 - X <= C2 can be transformed into
              X - (C1 - C2) <= C2.

    (d) The >= and > cases are negations of (b) and (c), respectively.

    This transformation allows to occasionally save load-immediate /
    subtraction instructions, e.g. the following statement:

    300 - (unsigned int)f() < 100;

    now compiles to

    addi    a0,a0,-201
    sltiu   a0,a0,100

    instead of

    li      a5,300
    sub     a0,a5,a0
    sltiu   a0,a0,100

    on 32-bit RISC-V.

    Additional examples can be found in the newly added test file.  This
    patch has been bootstrapped and regtested on aarch64, x86_64, and i386,
    and additionally regtested on riscv32.

    gcc/ChangeLog:

            PR tree-optimization/116024
            * match.pd: New transformation around integer comparison.

    gcc/testsuite/ChangeLog:

            * gcc.dg/tree-ssa/pr116024-1.c: New test.