Optimize ModExp

[benaadams]

Changes

• Do more maths in 64/32 bit space than 256 bit
• Vectorize overflow checks
• Don't allocate input arrays unless completely needed
• Don't throw and catch exception for overflow
• Don't try catch in CalculateIterationCount

Types of changes

What types of changes does your code introduce?

• Optimization

Testing

Requires testing

• No

[Copilot]

Pull Request Overview

This PR optimizes the ModExp precompile implementation to improve performance by reducing memory allocations and using more efficient data structures. The optimization focuses on doing more computations in 32/64-bit space rather than 256-bit space, vectorizing overflow checks, and minimizing array allocations.

• Vectorized overflow checks using SIMD instructions for faster input validation
• Changed data types from UInt256 to uint/ulong for length calculations to improve performance
• Optimized memory allocation patterns by avoiding unnecessary array creation and using ReadOnlySpan where possible

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
ModExpPrecompile.cs	Main optimization implementation with vectorized overflow checks and 32/64-bit arithmetic
ModExpPrecompilePreEip2565.cs	Updated to use ReadOnlySpan for consistency and removed unused import
ByteArrayExtensions.cs	Modified SliceWithZeroPaddingEmptyOnError to return ReadOnlySpan instead of byte array
Bytes.cs	Changed LeadingZerosCount parameter from Span to ReadOnlySpan for broader compatibility
EvmInstructions.Math2Param.cs	Changed variable from Span to ReadOnlySpan for consistency
Eip2565Tests.cs	Minor test refactoring to store input data and reorder assertion parameters
Eip7823Tests.cs	Updated test to return boolean instead of throwing exception for overflow case

Comments suppressed due to low confidence (1)

src/Nethermind/Nethermind.Evm.Test/Eip7823Tests.cs:64

• The test behavior has fundamentally changed from expecting an OverflowException to expecting a false return value. This suggests the underlying implementation no longer throws exceptions for overflow. Verify that this change maintains the same error handling semantics and that overflow conditions are still properly detected.

        Assert.That(TestSuccess(input, specEnabled), Is.EqualTo(false));