The compiler is able to merge LTU comparisons with PLUS or MINUS pattern to
form addition with carry (ADC) and subtraction with borrow (SBB) instructions:

    op = op + carry        [ADC $0, op]
    op = op - carry        [SBB $0, op]

The patch introduces reversed ADC and SBB insn patterns:

    op = op + !carry    [SBB $-1, op]
    op = op - !carry    [ADC $-1, op]

allowing the compiler to also merge GEU comparisons.

2021-04-30  Uroš Bizjak  <ubizjak@gmail.com>

gcc/
    PR target/98060
    * config/i386/i386.md (*add<mode>3_carry_0r): New insn pattern.
    (*addsi3_carry_zext_0r): Ditto.
    (*sub<mode>3_carry_0): Ditto.
    (*subsi3_carry_zext_0r): Ditto.
    * config/i386/predicates.md (ix86_carry_flag_unset_operator):
    New predicate.
    * config/i386/i386.c (ix86_rtx_costs) <case PLUS, case MINUS>:
    Also consider ix86_carry_flag_unset_operator to calculate
    the cost of adc/sbb insn.

gcc/testsuite/

    PR target/98060
    * gcc.target/i386/pr98060.c: New test.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Pushed to master.

Uros.