[release/9.0] JIT: Cast UInt64 to Single directly during const folding

[github-actions]

Backport of #106419 to release/9.0

/cc @amanasifkhalid

Customer Impact

• Customer reported
• Found internally

#106338 exposed different cast behavior between ulong and float on ARM64 (and x64 with AVX-512) in Debug and Release builds. In the unoptimized case, we emit code to do the cast, and ARM64 supports casting directly from 64-bit integers to float without an intermediate cast to double. When optimizing, we evaluate the cast at compile-time, but the JIT's constant folding logic does the cast in two steps (i.e. ulong -> double -> float), which as demonstrated by the above issue, can result in subtle rounding differences. Our toolchain's compilers (MSVC/Clang/GCC/etc.) all emit the correct instruction sequence for ulong -> float casts, so the simplest fix for Debug/Release diffs is to adjust the constant-folding logic to cast directly to float.

While this fixes Debug/Release diffs, it's worth noting that the JIT still performs two-step casts on some platforms (x86, x64 without AVX-512, among others). This has to do partly with the fact that Roslyn emits a two-opcode IL sequence for ulong -> float casts, and RyuJIT imports this IL as a ulong -> double -> float cast; later on, RyuJIT may or may not decide to morph this into a ulong -> float cast, depending on the target architecture. We plan to fix this for all platforms such that an intermediate cast is never introduced (and if we import a two-step cast, we don't incorrectly morph it into one cast), but this work would likely be too disruptive to include in .NET 9 at this point. We're soliciting discussion on a solution for this in #106646.

Regression

• Yes
• No

Testing

This issue was exposed when one of our fuzz testers generated a case that exercises subtle floating-point semantics. My fix includes a regression test to check the casting behavior we currently expect across our supported platforms. Once #106646 is fixed, we can expect the casting behavior this test demonstrates to be unified across platforms.

Risk

Low. This fix is quite surgical in removing Debug/Release diffs on platforms RyuJIT currently supports ulong -> float casts for.

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[amanasifkhalid]

@tannergooding PTAL, thank you!

[jeffschwMSFT]

approved. we can merge when ready

[amanasifkhalid]

I opened #106731 to track the build failures on arm64, but Build Analysis is stuck, despite not listing any unknown issues (link).

[amanasifkhalid]

/ba-g Build Analysis is red, despite all issues being known

[amanasifkhalid]

@jeffschwMSFT @carlossanlop could you merge this when you have a moment? I'm not authorized to merge to release/9.0. Thanks!

[jeffschwMSFT]

@amanasifkhalid can you update the branch as it is out of date?

[amanasifkhalid]

@jeffschwMSFT @carlossanlop does this PR need anything else, or can it be merged?

[jeffschwMSFT]

looks like it still needs a branch update. with that it can be merged

[amanasifkhalid]

looks like it still needs a branch update. with that it can be merged

Done