feat(tests): cover ModExp gas before EIP-2565 - #3237
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The gas measurement harness lived in the EIP-7883 directory and every test using it started at Berlin, so the EIP-198 schedule that prices the precompile from Byzantium until Berlin had no coverage: the only ModExp tests reaching those forks call with all the gas available. Move the harness down to the EIP-198 directory, which already owns `ModExpInput` and is imported by the later repricing EIPs, and select the gas formula through a `modexp_spec` fixture that a repricing directory overrides. The formulas move with it: `Spec198` is new, `Spec2565` is the former `Spec` body, and EIP-7883 keeps overriding it. Two fixes were needed to reach Byzantium. The harness subtracted `gas_costs().WARM_ACCESS` as the cost of the call, but that field holds the EIP-2929 value on every fork, so before Berlin it under-counted the flat EIP-150 charge by 600 gas. And `get_exponent_head()` left-padded an exponent whose input is truncated, while reading past the end of the input yields zeros on the right; the EIP-2565 minimum of 200 gas had hidden the difference, which is observable under EIP-198 where a call can be free. With that in place, the legacy vectors and the exact-gas boundary test now run from Byzantium, and a new test pins each branch of the EIP-198 multiplication complexity formula at its exact cost and one gas less. Claude-Session: https://claude.ai/code/session_019YyttTB1mqHzWa77K3sEoT
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## forks/amsterdam #3237 +/- ##
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Coverage 93.41% 93.41%
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Files 624 624
Lines 37014 37014
Branches 3386 3386
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Hits 34576 34576
Misses 1668 1668
Partials 770 770
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Description
The gas measurement harness lived in the EIP-7883 directory and every test using it started at Berlin, so the EIP-198 schedule that prices the precompile from Byzantium until Berlin had no coverage: the only ModExp tests reaching those forks call with all the gas available.
Move the harness down to the EIP-198 directory, which already owns
ModExpInputand is imported by the later repricing EIPs, and select the gas formula through amodexp_specfixture that a repricing directory overrides. The formulas move with it:Spec198is new,Spec2565is the formerSpecbody, and EIP-7883 keeps overriding it.Two fixes were needed to reach Byzantium. The harness subtracted
gas_costs().WARM_ACCESSas the cost of the call, but that field holds the EIP-2929 value on every fork, so before Berlin it under-counted the flat EIP-150 charge by 600 gas. Andget_exponent_head()left-padded an exponent whose input is truncated, while reading past the end of the input yields zeros on the right; the EIP-2565 minimum of 200 gas had hidden the difference, which is observable under EIP-198 where a call can be free.With that in place, the legacy vectors and the exact-gas boundary test now run from Byzantium, and a new test pins each branch of the EIP-198 multiplication complexity formula at its exact cost and one gas less.
Related Issues or PRs
N/A.
Checklist
just static<type>(<area>): <title>, where<type>and<area>come from an appropriateC-<type>, respectivelyA-<area>, label. The title should match the target squash commit message.