dotnet/runtime

Suboptimal codgen for `Vector128.NarrowWithSaturation`

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#116,526 opened on Jun 11, 2025

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area-CodeGen-coreclrhelp wanted

Description

I expect PackSources0 and PackSources1 to have exactly the same codegen in the following code snippet:

static Vector128<byte> PackSources1(Vector128<ushort> lower, Vector128<ushort> upper)
    => Vector128.NarrowWithSaturation(lower, upper);

static Vector128<byte> PackSources0(Vector128<ushort> lower, Vector128<ushort> upper)
    => Sse2.PackUnsignedSaturate(
        Vector128.Min(lower, Vector128.Create((ushort)255)).AsInt16(),
        Vector128.Min(upper, Vector128.Create((ushort)255)).AsInt16());

// coreclr trunk-20250611+5415b7342d44af9c974905760539f198fad13682

C:PackSources1(System.Runtime.Intrinsics.Vector128`1[ushort],System.Runtime.Intrinsics.Vector128`1[ushort]):System.Runtime.Intrinsics.Vector128`1[byte] (FullOpts):
       vbroadcastss xmm0, dword ptr [reloc @RWD00]
       vpminuw  xmm1, xmm0, xmmword ptr [rsp+0x08]
       vpand    xmm1, xmm1, xmm0
       vpminuw  xmm2, xmm0, xmmword ptr [rsp+0x18]
       vpand    xmm0, xmm2, xmm0
       vpackuswb xmm0, xmm1, xmm0
       vmovups  xmmword ptr [rdi], xmm0
       mov      rax, rdi
       ret      
RWD00  	dd	00FF00FFh		; 2.34184e-38

C:PackSources0(System.Runtime.Intrinsics.Vector128`1[ushort],System.Runtime.Intrinsics.Vector128`1[ushort]):System.Runtime.Intrinsics.Vector128`1[byte] (FullOpts):
       vbroadcastss xmm0, dword ptr [reloc @RWD00]
       vpminuw  xmm1, xmm0, xmmword ptr [rsp+0x08]
       vpminuw  xmm0, xmm0, xmmword ptr [rsp+0x18]
       vpackuswb xmm0, xmm1, xmm0
       vmovups  xmmword ptr [rdi], xmm0
       mov      rax, rdi
       ret      
RWD00  	dd	00FF00FFh		; 2.34184e-38

https://csharp.godbolt.org/z/o39G8GP9T

Related: https://github.com/dotnet/runtime/pull/115525

cc: @tannergooding

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Tech stack
csharp
Domain
performance
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
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over 1 week
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clear
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Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate the JIT's codegen for Vector128.NarrowWithSaturation. Compare the assembly output between PackSources0 (manual implementation) and PackSources1 (intrinsic). The JIT inserts extra vpand instructions in PackSources1 that are not present in the manual version. Examine the JIT's lowering for NarrowWithSaturation in src/coreclr/jit, and review the related PR #115525 for context. The goal is to identify why the intrinsic produces suboptimal code and propose a fix to eliminate the redundant operations.