Re: [PATCH] libgccjit: Fix float playback for cross-compilation

[David Malcolm <dmalcolm@redhat.com>]

On Thu, 2024-01-11 at 18:42 -0500, Antoni Boucher wrote:
> Hi.
> This patch fixes the bug 113343.
> I'm wondering if there's a better solution than using mpfr.
> The only other solution I found is real_from_string, but that seems
> overkill to convert the number to a string.
> I could not find a better way to create a real value from a host
> double.

I took a look, and I don't see a better way; it seems weird to go
through a string stage.  Ideally there would be a
real_from_host_double, but I don't see one.

Is there a cross-platform way to directly access the representation of
a host double?

> If there's no solution, do we lose some precision by using mpfr?
> Running Rust's core library tests, there was a difference of one
> decimal, so I'm wondering if there's some lost precision, or if it's
> just because those tests don't work on m68k which was my test target.

Sorry, can you clarify what you mean by "a difference of one decimal"
above?

> Also, I'm not sure how to write a test this fix. Any ideas?

I think we don't need cross-compilation-specific tests, we should just
use and/or extend the existing coverage for
gcc_jit_context_new_rvalue_from_double e.g. in test-constants.c and
test-types.c

We probably should have test coverage for "awkward" values; we already
have coverage for DBL_MIN and DBL_MAX, but we don't yet have test
coverage for:
* quiet/signaling NaN
* +ve/-ve inf
* -ve zero

Thanks
Dave