Fixed Jpeg IDCT missing scalar implementation

[br3aker]

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Description

Fixes #1917.

Fix is simple: I've removed Debug check for Vector4.IsHardwareAccelerated as Vector4 code can be compiled to a scalar implementation. That check was deprecated long ago - when I've merged de-zigzag and single transpose call so now every IDCT implementation must do a single transpose call.

FDCT method does have a scalar implementation right now because current encoding pipeline does not have a merged zig-zag/transpose code. I'm yet to implement it in #1778 (coding those sse/avx zigzag tables is a horror). Because of two transpose calls scalar code can do DCT stuff a little trickier than simd without transpose calls at all but with a tranposed block input we must transpose it second time durint IDCT call.

[br3aker]

As @JimBobSquarePants mentioned it's a problem that tests missed this bug, test suite clearly has a 'no-simd' path for IDCT method:

ImageSharp/tests/ImageSharp.Tests/Formats/Jpg/DCTTests.cs

Lines 149 to 157 in fe044d1

	// 4 paths:
	// 1. AllowAll - call avx/fma implementation
	// 2. DisableFMA - call avx without fma implementation
	// 3. DisableAvx - call sse Vector4 implementation
	// 4. DisableHWIntrinsic - call scalar fallback implementation
	FeatureTestRunner.RunWithHwIntrinsicsFeature(
	RunTest,
	seed,
	HwIntrinsics.AllowAll | HwIntrinsics.DisableFMA | HwIntrinsics.DisableAVX | HwIntrinsics.DisableHWIntrinsic);

[codecov]

Codecov Report

Merging #1922 (d61b845) into master (fe044d1) will increase coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1922   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         961     961           
  Lines       51034   51034           
  Branches     6324    6324           
======================================
+ Hits        44793   44863   +70     
+ Misses       5202    5133   -69     
+ Partials     1039    1038    -1     

Flag	Coverage Δ
unittests	87% <ø> (+<1%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...rp/Formats/Jpeg/Components/FastFloatingPointDCT.cs	100% <ø> (+36%)	⬆️

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[JimBobSquarePants]

@br3aker Ah... I see where the issue lies. HwIntrinsics.DisableHWIntrinsic doesn't disable System.Numerics so our tests will never hit that DebugGuard. We need HwIntrinsics.DisableSIMD when depending on Vector.IsHardwareAccelerated.

ImageSharp/tests/ImageSharp.Tests/TestUtilities/FeatureTesting/FeatureTestRunner.cs

Lines 379 to 394 in fe044d1

	/// <summary>
	/// See <see href="https://github.com/dotnet/runtime/blob/50ac454d8d8a1915188b2a4bb3fff3b81bf6c0cf/src/coreclr/src/jit/jitconfigvalues.h#L224"/>
	/// <remarks>
	/// <see cref="DisableSIMD"/> ends up impacting all SIMD support(including System.Numerics)
	/// but not things like <see cref="DisableBMI1"/>, <see cref="DisableBMI2"/>, and <see cref="DisableLZCNT"/>.
	/// </remarks>
	/// </summary>
	[Flags]
	#pragma warning disable RCS1135 // Declare enum member with zero value (when enum has FlagsAttribute).
	public enum HwIntrinsics
	#pragma warning restore RCS1135 // Declare enum member with zero value (when enum has FlagsAttribute).
	{
	// Use flags so we can pass multiple values without using params.
	// Don't base on 0 or use inverse for All as that doesn't translate to string values.
	DisableSIMD = 1 << 0,
	DisableHWIntrinsic = 1 << 1,

[br3aker]

Ups!
Makes sense it didn't work, thanks for the explanation!

Edit:
And now we also got a better coverage, nice.

[JimBobSquarePants]

Awesome. Glad the fix was simple.