[aarch64] [missed-opt] Irrelevant subtraction pessimizes llvm.uadd/usub.with.overflow

[purplesyringa]

Godbolt

#include <stdint.h>

uint64_t f(uint64_t x, uint64_t y) {
    uint64_t sum;
    if (__builtin_uaddl_overflow(x, y, &sum)) {
        return sum - 100;
    } else {
        return 0;
    }
}

uint64_t g(uint64_t x, uint64_t y) {
    uint64_t sum;
    if (__builtin_uaddl_overflow(x, y, &sum)) {
        return sum - 100;
    } else {
        return sum;
    }
}

f(unsigned long, unsigned long):
        adds    x8, x0, x1
        sub     x8, x8, #100
        cmn     x0, x1
        csel    x0, x8, xzr, hs
        ret

g(unsigned long, unsigned long):
        adds    x8, x0, x1
        sub     x8, x8, #100
        adds    x9, x0, x1
        csel    x0, x8, x9, hs
        ret

In f, note adds computing the sum and having its output flag is ignored and recomputed with cmn. In g, note adds being run twice.

This problem affects __builtin_uaddl_overflow and __builtin_usubl_overflow equally.

Replacing - 100 with + 100 miraculously fixes the lowering, but obviously changes semantics.

[llvmbot]

@llvm/issue-subscribers-backend-aarch64

Author: Alisa Sireneva (purplesyringa)

[Godbolt](https://godbolt.org/z/948EWPoqM)

#include <stdint.h>

uint64_t f(uint64_t x, uint64_t y) {
    uint64_t sum;
    if (__builtin_uaddl_overflow(x, y, &sum)) {
        return sum - 100;
    } else {
        return 0;
    }
}

uint64_t g(uint64_t x, uint64_t y) {
    uint64_t sum;
    if (__builtin_uaddl_overflow(x, y, &sum)) {
        return sum - 100;
    } else {
        return sum;
    }
}

f(unsigned long, unsigned long):
        adds    x8, x0, x1
        sub     x8, x8, #<!-- -->100
        cmn     x0, x1
        csel    x0, x8, xzr, hs
        ret

g(unsigned long, unsigned long):
        adds    x8, x0, x1
        sub     x8, x8, #<!-- -->100
        adds    x9, x0, x1
        csel    x0, x8, x9, hs
        ret

In f, note adds computing the sum and having its output flag is ignored and recomputed with cmn. In g, note adds being run twice.

This problem affects __builtin_uaddl_overflow and __builtin_usubl_overflow equally.

Replacing - 100 with + 100 miraculously fixes the lowering, but obviously changes semantics.

[EugeneZelenko]

@purplesyringa: If you plan to file more issues in future, it'll be good idea to request triage access (examples), so you'll able to label issues at creation.

[purplesyringa]

Thanks, I wasn't aware of this option. Wouldn't want to burden you more 😅

[EugeneZelenko]

Thanks, I wasn't aware of this option. Wouldn't want to burden you more 😅

Adding label(s) during issues creation also reduces amount on notification emails for subscribers :-)