Use apfloat for full-precision sqrt #3534
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A-shims
Area: This affects the external function shims
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Category: a PR with an enhancement or an issue tracking an accepted enhancement
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Status: blocked on something happening somewhere else
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RalfJung
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May 2, 2024
update host-float comments Turns out most of these do not have guaranteed precision anyway so it's fine to use host floats (see rust-lang/rust#121793 and rust-lang/rust#118217). The exception are sqrt and mul_add, tracked at #3534 and #2995.
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May 4, 2024
update host-float comments Turns out most of these do not have guaranteed precision anyway so it's fine to use host floats (see rust-lang#121793 and rust-lang#118217). The exception are sqrt and mul_add, tracked at rust-lang/miri#3534 and rust-lang/miri#2995.
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With rust-lang/rust#121793 and rust-lang/rust#118217, most float functions are now documented to have varying precision, so using host floats for them in Miri is actually fine. The only exceptions are mul_add (tracked at #2995) and sqrt -- those guarantee an infinite-precision result, so we should ideally use softfloats. (That said, using host floats is not wrong here since the host functions are in fact guaranteeing the infinite-precision result we are supposed to be emulating. Still, it seems cleaner to use softfloats for everything where precision is guaranteed.)
This is blocked on rust-lang/rustc_apfloat#14.
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