Remove a few unsafe bits in BCL

[EgorBo]

A small contribution to #94941

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[EgorBo]

seems to generate almost the same codegen

[stephentoub]

Have you measured any performance impact?

[EgorBo]

It seems to be around noise <ns EgorBot/runtime-utils#239

Codegen diff:

  ; Assembly listing for method System.IO.Hashing.Crc32:UpdateScalarArm32(uint,System.ReadOnlySpan`1[ubyte]):uint 
 G_M20306_IG01:
        push     rbp
        push     rbx
        push     rax
        lea      rbp, [rsp+0x10]
 						;; size=8 bbWeight=1 PerfScore 3.50
 G_M20306_IG02:
        cmp      edx, 4
        jl       SHORT G_M20306_IG04
 						;; size=5 bbWeight=1 PerfScore 1.25
 G_M20306_IG03:
-       mov      rbx, rsi
        mov      eax, edx
        and      eax, -4
-       jg       SHORT G_M20306_IG08
        cmp      eax, edx
-       ja       SHORT G_M20306_IG07
+       ja       SHORT G_M20306_IG08
+       mov      ecx, eax
+       shr      ecx, 2
+       mov      rbx, rsi
+       test     ecx, ecx
+       jg       SHORT G_M20306_IG07
        mov      ecx, eax
        add      rsi, rcx
        sub      edx, eax
-						;; size=21 bbWeight=0.50 PerfScore 1.88
+						;; size=28 bbWeight=0.50 PerfScore 2.38
 G_M20306_IG04:
-       test     edx, edx
+       mov      eax, edx
+       test     eax, eax
        jg       SHORT G_M20306_IG06
        mov      eax, edi
-						;; size=6 bbWeight=1 PerfScore 1.50
+						;; size=8 bbWeight=1 PerfScore 1.75
 G_M20306_IG05:
        add      rsp, 8
        pop      rbx
        pop      rbp
        ret      
 						;; size=7 bbWeight=1 PerfScore 2.25
 G_M20306_IG06:
        movzx    rsi, byte  ptr [rsi]
        mov      rax, 0xD1FFAB1E      ; code for System.Runtime.Intrinsics.Arm.Crc32:ComputeCrc32(uint,ubyte):uint
        call     [rax]System.Runtime.Intrinsics.Arm.Crc32:ComputeCrc32(uint,ubyte):uint
        int3     
 						;; size=17 bbWeight=0.50 PerfScore 2.75
 G_M20306_IG07:
-       mov      rax, 0xD1FFAB1E      ; code for System.ThrowHelper:ThrowArgumentOutOfRangeException()
-       call     [rax]System.ThrowHelper:ThrowArgumentOutOfRangeException()
-       int3     
-						;; size=13 bbWeight=0.50 PerfScore 1.75
-G_M20306_IG08:
        mov      esi, dword ptr [rbx]
        mov      rax, 0xD1FFAB1E      ; code for System.Runtime.Intrinsics.Arm.Crc32:ComputeCrc32(uint,uint):uint
        call     [rax]System.Runtime.Intrinsics.Arm.Crc32:ComputeCrc32(uint,uint):uint
        int3     
 						;; size=15 bbWeight=0.50 PerfScore 2.75
+G_M20306_IG08:
+       mov      rax, 0xD1FFAB1E      ; code for System.ThrowHelper:ThrowArgumentOutOfRangeException()
+       call     [rax]System.ThrowHelper:ThrowArgumentOutOfRangeException()
+       int3     
+						;; size=13 bbWeight=0.50 PerfScore 1.75
 
-; Total bytes of code 92, prolog size 8, PerfScore 17.62, instruction count 33, allocated bytes for code 92 (MethodHash=8ac9b0ad) for method System.IO.Hashing.Crc32:UpdateScalarArm32(uint,System.ReadOnlySpan`1[ubyte]):uint (FullOpts)
+; Total bytes of code 101, prolog size 8, PerfScore 18.38, instruction count 37, allocated bytes for code 101 (MethodHash=8ac9b0ad) for method System.IO.Hashing.Crc32:UpdateScalarArm32(uint,System.ReadOnlySpan`1[ubyte]):uint (FullOpts)
 ; ============================================================

Just a few suboptimal instructions before the main loop, something to improve in JIT I guess

[EgorBo]

PTAL @stephentoub

[stephentoub]

IIRC, we've had issues in the past with XxHash with inlining not inlining things we expected it to inline because it was hitting inlining budgets. It'd be good to confirm this has no effect on the generated code / throughput.

[EgorBo]

Judging by the diffs it indeed creates problems for inliner 😞

[EgorBo]

@EgorBot -arm

using System.IO.Hashing;
using BenchmarkDotNet.Attributes;

public class Benchmarks
{
    [Benchmark]
    [ArgumentsSource(nameof(TestData))]
    public uint Crc32HashToUInt32(byte[] data) => Crc32.HashToUInt32(data);

    public IEnumerable<byte[]> TestData()
    {
        yield return "1"u8.ToArray();
        yield return "44gg"u8.ToArray();
        yield return "egfger"u8.ToArray();
        yield return "egfger45"u8.ToArray();
        yield return "243е3е34пцу4п43п43"u8.ToArray();
        yield return "g[perwkg[perjk[pgkr[ekgkerpgkerpgkperkgp[erk[gkre[kg[rekg[erkgpe[rgk"u8.ToArray();
    }
}

[EgorBo]

@MihuBot

[EgorBo]

@MihuBot