Fix rounding mode check in SSE4.1 round functions

[eduardosm]

Now it masks out the correct bit and adds some explanatory comments. Also extends the tests.

[RalfJung]

Is the SSE status register different from the float status register?

For the float status register, we just assume it is always in NearestTiesToEven mode... but that's the rounding mode for float-to-float operations, right? Is the integer rounding mode even controlled in a status register for regular floats?

So for the SSE status register, I assume we'd just say that it is always assumed to have a particular rounding mode. But I don't know which rounding mode e.g. LLVM will assume here.

[eduardosm]

Yes, it is the same register that specifies rounding mode for arithmetic operations, so I guess we can assume to be round-to-nearest (we already do that for float-to-int conversions).

It looks like LLVM does not const-fold it, even when the rounding mode does not depend on the status register.
https://godbolt.org/z/zdh3McGcb

[RalfJung]

Wait, don't the normal float-to-int casts do a round-to-zero?

[eduardosm]

SSE has two version: one that rounds to zero (_mm_cvttss_si32) and one that uses the rounding mode of the status register (_mm_cvtss_si32). We assume round-to-nearest for the later.

miri/src/shims/x86/sse.rs

Lines 168 to 176 in c8d4e83

	let rnd = match unprefixed_name {
	// "current SSE rounding mode", assume nearest
	// https://www.felixcloutier.com/x86/cvtss2si
	"cvtss2si" | "cvtss2si64" => rustc_apfloat::Round::NearestTiesToEven,
	// always truncate
	// https://www.felixcloutier.com/x86/cvttss2si
	"cvttss2si" | "cvttss2si64" => rustc_apfloat::Round::TowardZero,
	_ => unreachable!(),
	};

[RalfJung]

SSE has two version: one that rounds to zero (_mm_cvttss_si32) and one that uses the rounding mode of the status register (_mm_cvtss_si32). We assume round-to-nearest for the later.

Ah okay, let's also do that here then.

But independently of that -- f32 as i32 casts also always round to zero, so I supposed they also ignore the status register? IOW, that status register really only affects SSE operations, not scalar float operations?

[eduardosm]

Yes, Rust's built-in cast will use the same instruction as _mm_cvttss_si32.

[RalfJung]

Suggested change

	// Miri (or Rust), we assume to be at its default mode (round-to-nearest).
	// Miri (or Rust), we assume it to be at its default mode (round-to-nearest).

[RalfJung]

r=me with the last typo fixed.
@bors delegate+

[bors]

✌️ @eduardosm, you can now approve this pull request!

If @RalfJung told you to "r=me" after making some further change, please make that change, then do @bors r=@RalfJung

[eduardosm]

@bors r=@RalfJung

[bors]

📌 Commit 2a88ae4 has been approved by RalfJung

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2a88ae4 with merge aaaac66...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing aaaac66 to master...