Re: [PATCH] range-op: Implement floating point division fold_range [PR107569]

[Iain Buclaw <ibuclaw@gdcproject.org>]

Excerpts from Jakub Jelinek via Gcc-patches's message of November 11, 2022 10:09 am:
> Hi!
> 
> Here is the floating point division fold_range implementation,
> as I wrote in the last mail, we could outline some of the common parts
> into static methods with descriptive names and share them between
> foperator_div and foperator_mult.
> 
> Bootstrapped/regtested on top of the earlier version of the multiplication
> fold_range on x86_64-linux and i686-linux, regressions are
> +FAIL: gcc.dg/pr95115.c execution test
> +FAIL: libphobos.phobos/std/math/hardware.d execution test
> +FAIL: libphobos.phobos_shared/std/math/hardware.d execution test

I've had some time to look at the Phobos failures, and seems to me that
it's a poorly written test.

    pragma(inline, false) static void blockopt(ref real x) {}
    real a = 3.5;
    // Set all the flags to zero
    resetIeeeFlags();
    assert(!ieeeFlags.divByZero);
    blockopt(a); // avoid constant propagation by the optimizer
    // Perform a division by zero.
    a /= 0.0L;
    assert(a == real.infinity);
    assert(ieeeFlags.divByZero);
    blockopt(a); // avoid constant propagation by the optimizer


1. Since this patch, that `a /= 0.0L` operation no longer appears in the
final assembly - so no divide-by-zero flags are raised.

2. Whoever introduced blockopt() perhaps did not understand that
`a /= 0.0L` is not safe from constant propagation just because it is
surrounded by some uninlinable call.

I'll fix the test in upstream, it should really be something like:

    pragma(inline, false)
    static real forceDiv(real x, real y) { return x / y; }
    a = forceDiv(a, 0.0L);
    assert(a == real.infinity);
    assert(ieeeFlags.divByZero);


Regards,
Iain.