Implement BinaryPrimitives.ReverseEndianness for arm64 using rev

[EgorBo]

@Gnbrkm41noticed it's not currently optimized to rev.

staticuintTest1(uinta)=>BinaryPrimitives.ReverseEndianness(a);staticulongTest2(ulonga)=>BinaryPrimitives.ReverseEndianness(a);

Old codegen:

; Assembly listing for method Test1(int):intG_M29289_IG01: A9BF7BFD stp fp, lr,[sp,#-16]! 910003FD mov fp,spG_M29289_IG02: 12009C01 and w1, w0, #0xff00ff13812021ror w1, w1, #8 12089C00 and w0, w0, #0xff00ff0013806000ror w0, w0, #24 0B000020 add w0, w1, w0G_M29289_IG03: A8C17BFD ldp fp, lr,[sp],#16 D65F03C0 ret lr; Assembly listing for method Test2(long):longG_M43753_IG01: A9BF7BFD stp fp, lr,[sp,#-16]! 910003FD mov fp,spG_M43753_IG02: 2A0003E1 mov w1, w0 12009C22 and w2, w1, #0xff00ff13822042ror w2, w2, #8 12089C21 and w1, w1, #0xff00ff0013816021ror w1, w1, #24 0B010041 add w1, w2, w1 2A0103E1 mov w1, w1 D3607C21 lsl x1, x1, #32 D360FC00 lsr x0, x0, #32 12009C02 and w2, w0, #0xff00ff13822042ror w2, w2, #8 12089C00 and w0, w0, #0xff00ff0013806000ror w0, w0, #24 0B000040 add w0, w2, w0 2A0003E0 mov w0, w0 8B010000 add x0, x0, x1G_M43753_IG03: A8C17BFD ldp fp, lr,[sp],#16 D65F03C0 ret lr

New codegen:

; Assembly listing for method Test1(int):intG_M29289_IG01: A9BF7BFD stp fp, lr,[sp,#-16]! 910003FD mov fp,spG_M29289_IG02: 5AC00800 rev w0, w0G_M29289_IG03: A8C17BFD ldp fp, lr,[sp],#16 D65F03C0 ret lr; Assembly listing for method Test2(long):longG_M43753_IG01: A9BF7BFD stp fp, lr,[sp,#-16]! 910003FD mov fp,spG_M43753_IG02: DAC00C00 rev x0, x0G_M43753_IG03: A8C17BFD ldp fp, lr,[sp],#16 D65F03C0 ret lr

int16 is not implemented yet, perhaps should be implemented in C# instead? To call the int-overload and apply a shift.

Tests already exist: https://github.com/dotnet/runtime/blob/master/src/coreclr/tests/src/JIT/Intrinsics/BinaryPrimitivesReverseEndianness.cs

[tannergooding]

int16 is not implemented yet, perhaps should be implemented in C# instead? To call the int-overload and apply a shift.

Would REV + shift or just a REV16 be better?

@TamarChristinaArm, could you advise? Is there also a good location to find this information or cycle approximations in general?

[EgorBo]

int16 is not implemented yet, perhaps should be implemented in C# instead? To call the int-overload and apply a shift.

Would REV + shift or just a REV16 be better?

@TamarChristinaArm, could you advise? Is there also a good location to find this information or cycle approximations in general?

Was confused by the clang's codegen: https://godbolt.org/z/w_8LUC

[EgorBo]

New codegen for ushort overload:

staticushortFoo1(ushorta)=>BinaryPrimitives.ReverseEndianness(a);

; Assembly listing for method Program:Foo1(ushort):ushort; Emitting BLENDED_CODE for generic ARM64 CPU - Unix; optimized code; fp based frame; partially interruptible; Final local variable assignments;; V00 arg0 [V00,T00] ( 3, 3 ) ushort -> x0 ;# V01 OutArgs [V01 ] ( 1, 1 ) lclBlk ( 0) [sp+0x00] "OutgoingArgSpace";; Lcl frame size = 0G_M15320_IG01: A9BF7BFD stp fp, lr,[sp,#-16]! 910003FD mov fp,sp ;; bbWeight=1 PerfScore 1.50G_M15320_IG02: 53003C00 uxth w0, w0 5AC00400 rev16 w0, w0 53003C00 uxth w0, w0 ;; bbWeight=1 PerfScore 1.50G_M15320_IG03: A8C17BFD ldp fp, lr,[sp],#16 D65F03C0 ret lr ;; bbWeight=1 PerfScore 2.00; Total bytes of code 28, prolog size 8, PerfScore 7.80, (MethodHash=a63ec427) for method Program:Foo1(ushort):ushort; ============================================================

[EgorBo]

Here is what Mono-LLVM-JIT (LLVM 6.0) emits on my Cortex A57 machine:

staticushortFoo1(ushorta)=>BinaryPrimitives.ReverseEndianness(a);

define monocc i16 @"Program:Foo1 (uint16)"(i16%arg_a) #0 {
BB0:
%rev = calli16@llvm.bswap.i16(i16%arg_a)
reti16%rev
}

0:	5ac00808 rev	w8, w04:	53107d00 lsr	w0, w8, #168:	d65f03c0 ret

[TamarChristinaArm]

int16 is not implemented yet, perhaps should be implemented in C# instead? To call the int-overload and apply a shift.

Would REV + shift or just a REV16 be better?

The REV16 is better, REV and REV16 have the same latency so adding a shift makes that sequence slower.

@TamarChristinaArm, could you advise? Is there also a good location to find this information or cycle approximations in general?

You can find these in the optimization guides, the one for Cortex-A55 is https://developer.arm.com/docs/epm128372/30/arm-cortex-a55-software-optimization-guide

[EgorBo]

I guess it's ready for review then

[tannergooding]

CC. @CarolEidt, @echesakovMSFT, @dotnet/jit-contrib

[tannergooding]

LGTM

[echesakov]

LGTM

[EgorBo]

Don't know how to restart the "in progress" leg 😕

[sandreenko]

AzDO shows that it has passed: https://dev.azure.com/dnceng/public/_build/results?buildId=605791&view=results

[EgorBo]

@BruceForstall can this be merged?

[echesakov]

It looks that we never gonna see runtime (Installer Build and Test Windows_NT_arm Debug) leg completed :)