crypto: Handle modexp with exponent of 1 separately

[chfast]

This allows optimizing the main loop and avoid 2 Montgomery multiplications.

                                                     │ old              │           new                        │
                                                     │      gas/s       │    gas/s      vs base                │
modexp<expmod_execute>/mod_len:8/exp_bits:33-14             875.8M ± 7%    891.7M ± 0%   +1.81% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:16/exp_bits:33-14            868.9M ± 0%    877.8M ± 1%   +1.02% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:24/exp_bits:33-14            217.1M ± 0%    223.9M ± 0%   +3.14% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:32/exp_bits:33-14            218.0M ± 0%    225.0M ± 0%   +3.24% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:32/exp_bits:256-14           237.4M ± 0%    239.4M ± 0%   +0.87% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:32/exp_bits:8192-14          472.1M ± 0%    475.8M ± 0%   +0.77% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:40/exp_bits:11-14            182.7M ± 0%    196.5M ± 1%   +7.52% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:48/exp_bits:8-14             239.6M ± 0%    265.1M ± 2%  +10.63% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:48/exp_bits:256-14           336.5M ± 0%    337.5M ± 0%   +0.30% (p=0.001 n=11)
modexp<expmod_execute>/mod_len:56/exp_bits:6-14             297.9M ± 0%    339.2M ± 0%  +13.87% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:64/exp_bits:5-14             348.9M ± 1%    407.0M ± 0%  +16.65% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:72/exp_bits:4-14             116.0M ± 2%    144.1M ± 0%  +24.21% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:112/exp_bits:4-14            271.0M ± 0%    335.1M ± 1%  +23.66% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:136/exp_bits:3-14            86.38M ± 0%   114.13M ± 0%  +32.12% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:192/exp_bits:2-14            107.2M ± 1%    153.1M ± 1%  +42.91% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:504/exp_bits:2-14            47.21M ± 1%    67.21M ± 1%  +42.37% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:512/exp_bits:2-14            48.79M ± 0%    69.45M ± 1%  +42.34% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:512/exp_bits:8192-14         348.0M ± 0%    347.5M ± 1%        ~ (p=0.478 n=11)
modexp<expmod_execute>/mod_len:520/exp_bits:2-14            50.24M ± 0%    71.75M ± 1%  +42.81% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:1016/exp_bits:2-14           188.3M ± 0%    267.9M ± 0%  +42.25% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:1024/exp_bits:2-14           191.5M ± 0%    271.7M ± 1%  +41.93% (p=0.000 n=11)
modexp<expmod_execute>/mod_len:1024/exp_bits:256-14         700.3M ± 0%    700.7M ± 0%        ~ (p=0.748 n=11)
modexp<expmod_execute>/mod_len:1024/exp_bits:2048-14        1.327G ± 0%    1.320G ± 1%   -0.52% (p=0.003 n=11)
geomean                                                     231.5M         268.3M       +15.89%

[Copilot]

Pull request overview

This PR optimizes the modular exponentiation (modexp) implementation by handling the exponent value of 1 as a special case, avoiding 2 Montgomery multiplications. The optimization initializes the result with the base in Montgomery form and reduces the loop iterations by 1.

Changes:

• Modified modexp_odd to initialize result with base instead of 1, and loop from bit_width() - 1 instead of bit_width()
• Added special handling for exponent = 0 in modexp_impl to return 1 (or 0 if modulus is 1) without computation
• Added test vectors covering edge cases including exponent 0, exponent with modulus 1, and modulus 2

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/evmone_precompiles/modexp.cpp	Optimized modexp_odd to skip first iteration, added exponent=0 special case, improved performance by 15-42% for small exponents
test/unittests/precompiles_expmod_test.cpp	Added test vectors for edge cases: exponent 0, exponent with modulus 0/1/2 to ensure correctness

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[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.68%. Comparing base (d8c01be) to head (7117f8d).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/evmone_precompiles/modexp.cpp	92.30%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1424   +/-   ##
=======================================
  Coverage   81.68%   81.68%           
=======================================
  Files         152      152           
  Lines       13576    13584    +8     
  Branches     3217     3218    +1     
=======================================
+ Hits        11089    11096    +7     
  Misses        343      343           
- Partials     2144     2145    +1     

Flag	Coverage Δ
eest-develop	91.08% <92.30%> (-0.02%)	⬇️
eest-develop-gmp	29.01% <70.58%> (+<0.01%)	⬆️
eest-legacy	15.33% <0.00%> (-0.01%)	⬇️
eest-legacy-silkpre	24.37% <52.94%> (+<0.01%)	⬆️
evmone-unittests	76.25% <76.47%> (+<0.01%)	⬆️

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Components	Coverage Δ
core	94.36% <92.30%> (-0.02%)	⬇️
tooling	83.63% <ø> (ø)
tests	73.64% <100.00%> (+0.01%)	⬆️

Files with missing lines	Coverage Δ
test/unittests/precompiles_expmod_test.cpp	96.29% <100.00%> (+0.08%)	⬆️
lib/evmone_precompiles/modexp.cpp	98.76% <92.30%> (-1.24%)	⬇️

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