Convert.Try{From/To}HexString #86556

hrrrrustic · 2023-05-21T16:34:13Z

src/libraries/System.Private.CoreLib/src/System/Convert.cs

gfoidl · 2023-05-21T19:17:56Z

src/libraries/System.Private.CoreLib/src/System/Convert.cs

+        /// <returns>true if the conversion was successful; otherwise, false.</returns>
+        public static bool TryFromHexString(ReadOnlySpan<char> source, Span<byte> destination, out int bytesWritten)
+        {
+            var length = source.Length;


Suggested change

var length = source.Length;

int length = source.Length;

Use concrete types here. Same on other places.

Perf-wise this could be nuint, so all the division and modulo operations are treated as (fast) bit-operations.

Perf-wise this could be nuint, so all the division and modulo operations are treated as (fast) bit-operations.

That's interesting. We rarely use native width types except in interop contexts. @jkotas @EgorBo is it correct that in hot code of this type, it can have perf benefit for simple storage of counters and offsets?

Afair, JIT knows that Span.Length (and Array.Length as well) are never negative so it doesn't need extra efforts from the C# side 🙂 e.g.:

int Foo(Span<int> s) => s.Length % 4;

; Method Foo(System.Span`1[int]):int:this mov eax, dword ptr [rdx+08H] and eax, 3 ret ; Total bytes of code: 7

(some of the changes landed in .NET 8.0 so might be not visible on sharplab yet)

Right -- I'm not aware of patterns where nuint gives better code.

JIT knows that Span.Length (and Array.Length as well) are never negative

Thanks for the reminder -- I'll forget sometimes about the new JIT-goodness.

We rarely use native width types except in interop contexts

Quite a lot of vectorized code uses the native width types, because memory operations don't need (sign-) extending moves then (e.g. Unsafe.Add(ref ptr, intVariable) vs. Unsafe.Add(ref ptr, nuintVariable) -- https://godbolt.org/z/v8E31Wdsx).

Would Math.DivRem be more efficient here? Especially once it becomes an intrinsic.

Would Math.DivRem be more efficient here? Especially once it becomes an intrinsic.

I doubt it, since the JIT optimizes length % 2 != 0 to (length & 1) != 0.

That comment wasn't just about the single length check, but also the division done a few lines later. DivRem in theory combines the two operations.

the division done a few lines later

The JIT can optimize (uint)length / 2 to (uint)length >> 1. On X86, DivRem would likely be implemented using a div instruction, which is an relatively expensive operation.

danmoseley · 2023-05-21T19:57:23Z

went to see whether we have perf coverage and we may be missing the "from hex string" case here. if so might be good to add
https://github.com/dotnet/performance/blob/a39cc8085f5738f1bce11209ec06979003d84ce1/src/benchmarks/micro/libraries/System.Runtime.Extensions/Perf.Convert.cs#L89

hrrrrustic · 2023-05-21T21:18:41Z

@danmoseley Also created a PR with benchmark
This branch looks stable (previous CI passed with some unrelated errors on IOS), but there is a comment from Stephen #78472 (comment)

ghost · 2023-05-24T21:45:33Z

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Issue Details

Close #78472

Author:	hrrrrustic
Assignees:	-
Labels:	`area-System.Runtime`, `community-contribution`
Milestone:	-

xtqqczze · 2023-05-25T15:32:03Z

src/libraries/System.Private.CoreLib/src/System/Convert.cs

+                return true;
+            }
+
+            int requiredByteCount = length / 2;


Suggested change

int requiredByteCount = length / 2;

int requiredByteCount = (int)((uint)length / 2);

The JIT doesn't recognize length as never negative otherwise.

@EgorBo Should we open a JIT issue?

hrrrrustic · 2023-07-30T23:37:36Z

I'll try. Pushing updates that will probably fail CI since I didn't check them yet, just a checkpoint before sleep 😆

tannergooding · 2023-09-19T20:31:10Z

@hrrrrustic, we should be able to get this reviewed now. Could you merge in upstream so we can rerun CI?

danmoseley · 2023-09-25T04:34:31Z

src/libraries/System.Runtime.Extensions/tests/System/Convert.FromHexString.cs

+            Span<char> buffer = stackalloc char[loopCount * 2];
+            for (int i = 1; i < loopCount; i++)
+            {
+                byte[] data = Security.Cryptography.RandomNumberGenerator.GetBytes(i);


We usually use a seeded Random so the tests are reproducible.

I agree with Dan, but since the existing tests in this type were already using Security.Cryptography.RandomNumberGenerator it's acceptable to keep it.

adamsitnik

Overall it LGTM, I found some things that need to be adjusted before merging, but I've simply provided suggestions for all of them and I am about to apply them right now.

@hrrrrustic thank you for your contribution!

src/libraries/System.Private.CoreLib/src/System/Convert.cs

adamsitnik · 2023-10-31T07:36:13Z

src/libraries/System.Private.CoreLib/src/System/Convert.cs

+            if (destination.Length < quotient)
+            {
+                source = source.Slice(0, destination.Length * 2);
+                quotient = destination.Length;


subjective nit: using a variable called quotient to store both "quotient" and "bytesWritten" is confusing to me, we could simply assign bytesWritten to destination.Length here

src/libraries/System.Private.CoreLib/src/System/Convert.cs

src/libraries/System.Runtime.Extensions/tests/System/Convert.FromHexString.cs

adamsitnik · 2023-10-31T08:10:27Z

src/libraries/System.Runtime.Extensions/tests/System/Convert.FromHexString.cs

+            Span<char> buffer = stackalloc char[loopCount * 2];
+            for (int i = 1; i < loopCount; i++)
+            {
+                byte[] data = Security.Cryptography.RandomNumberGenerator.GetBytes(i);


I agree with Dan, but since the existing tests in this type were already using Security.Cryptography.RandomNumberGenerator it's acceptable to keep it.

adamsitnik

LGTM, thank you @hrrrrustic !

hrrrrustic added 7 commits May 19, 2023 01:22

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Few more tests

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var -> int

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Add Convert.FromHex benchmark dotnet/performance#3032

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Failed USB connection via port 54050, error 61, in tvOS arm64 Release AllSubsets_Mono #82637

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[API Proposal]: Convert.TryFromHexString #78472

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fix slicing

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[6.0] System.Random test failure: NextInt_AllValuesAreWithinSpecifiedRange #83421

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Fix tests?

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Timeout in System.Net.Quic.Functional.Tests #86019

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Merge remote-tracking branch 'origin/main' into issue-78472

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Convert.ToHexStringLower: lower variant for Convert.ToHexString #92483

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try fix target buffer size error

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CI error: System.Net.Quic.QuicException: The connection timed out from inactivity #91757

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Regressions in System.Perf_Convert #94477

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Convert.Try{From/To}HexString #86556

Convert.Try{From/To}HexString #86556

hrrrrustic commented May 21, 2023

gfoidl May 21, 2023

danmoseley May 21, 2023

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	int requiredByteCount = length / 2;
	int requiredByteCount = (int)((uint)length / 2);

Convert.Try{From/To}HexString #86556

Convert.Try{From/To}HexString #86556

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