[Bug target/56309] -O3 optimizer generates conditional moves instead of compare and branch resulting in almost 2x slower code

["arturomdn at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56309

--- Comment #10 from arturomdn at gmail dot com 2013-02-14 16:43:23 UTC ---
Might be worth mentioning here what I said in the stackoverflow answer, that in
this particular case the entire conditional branch can be avoided because it is
redundant.

This code

    if (tmp >= imax) {
        carry = tmp >> numbits;    // <---- A
        tmp &= imax - 1;                // <---- B
    } else {
        carry = 0;                            // <---- C
    }

can be reduced to

    carry = tmp >> numbits;
    tmp &= imax - 1;

Proof:
    1) numbits is 32
    2) imax is 1ULL << 32 so lower 32 bits are zero
    3) imax - 1 is 0xFFFFFFFF (see 2)
    4) if tmp >= imax then tmp has bits set in upper 32 bits
    5) otherwise if tmp < imax then tmp does not have bits set in upper 32 bits
    6) statement "A" is equivalent to "C" when tmp < imax (because of 4)
    7) statement "B" is a NOP when tmp < imax (because of 3 and 5)