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fix(modexp): allow carry in saturated schoolbook quotient estimate #4

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Merged
kevaundray merged 1 commit into from
Sep 5, 2026
Merged

fix(modexp): allow carry in saturated schoolbook quotient estimate #4

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5 changes: 4 additions & 1 deletion src/modexp/LimbMath.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -300,7 +300,10 @@ library LimbMath {
bool doRefinement;
if (uHi >= vTop) {
qHat = type(uint256).max;
rHat = uLo + vTop;
// Algorithm D uses the wrapped sum below to detect carry in doRefinement.
unchecked {
rHat = uLo + vTop;
}
doRefinement = (rHat >= uLo);
} else {
(qHat, rHat) = div512by256(uHi, uLo, vTop);
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15 changes: 15 additions & 0 deletions test/modexp/ModexpDeployed.t.sol
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Expand Up @@ -93,6 +93,21 @@ contract ModexpDeployedTest is Test {
assertEq(outDep, outPre);
}

function test_identical_schoolbook_saturated_estimate_wrap() public view {
uint256 halfWord = uint256(1) << 255;
bytes memory input = _encodeInput(
abi.encodePacked(bytes32(halfWord), bytes32(halfWord), bytes32(0)),
hex"01",
abi.encodePacked(bytes32(halfWord), bytes32(halfWord + 1))
);

(bool okPre, bytes memory outPre) = _callPrecompile(input);
(bool okDep, bytes memory outDep) = _callDeployed(input);
assertTrue(okPre);
assertTrue(okDep);
assertEq(outDep, outPre);
}

function test_identical_even_modulus() public view {
// 3^7 mod 10
bytes memory input = _encodeInput(
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