Suboptimal codgen for `Vector128.NarrowWithSaturation` · dotnet/runtime#116526

(8 留言) (1 反應) (0 負責人)C# (17,886 star) (5,445 fork)batch import

area-CodeGen-coreclrhelp wanted

描述

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

cc: @tannergooding

貢獻者指南

技術棧: csharp
領域: performance
議題類型: performance
難度: 5
預計時間: over 1 week
活動狀態: fresh
清晰度: clear
前置要求: 無
新手友善度: 10
研究方向: 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.