core/vm: memoize MULMOD modulus reciprocal

core/vm/evm.go

@@ -185,6 +185,10 @@ type EVM struct {
 	readOnly   bool   // Whether to throw on stateful modifications
 	returnData []byte // Last CALL's return data for subsequent reuse
 
+	// mulmodMemo caches the reciprocal of the last full-width MULMOD modulus so
+	// that repeated moduli skip uint256.MulMod's internal Reciprocal recompute.
+	mulmodMemo modReciprocal
+
 	// interrupt is the block-building timeout flag. When set, the interpreter
 	// checks it on every opcode across all call depths (Call, DelegateCall,
 	// StaticCall, CallCode, Create). Use SetInterrupt to configure.

core/vm/instructions.go

@@ -199,7 +199,7 @@ func opAddmod(pc *uint64, evm *EVM, scope *ScopeContext) ([]byte, error) {
 
 func opMulmod(pc *uint64, evm *EVM, scope *ScopeContext) ([]byte, error) {
 	x, y, z := scope.Stack.pop(), scope.Stack.pop(), scope.Stack.peek()
-	z.MulMod(&x, &y, z)
+	evm.mulmodMemo.mulmod(&x, &y, z)
 
 	return nil, nil
 }

core/vm/mulmod_memo.go

@@ -0,0 +1,66 @@
+// Copyright 2026 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package vm
+
+import "github.com/holiman/uint256"
+
+// modReciprocal memoizes uint256.Reciprocal for the last full-width (4-word)
+// MULMOD modulus seen by an EVM instance.
+//
+// uint256.MulMod, for a full-width modulus, computes Reciprocal(m) internally
+// and then reduces with it; the reciprocal is ~60% of that call's cost and is
+// thrown away every time. A common on-chain pattern is a long run of modular
+// multiplications against a single fixed-prime modulus (field arithmetic used
+// broadly by cryptographic and verification contracts), so a one-entry memo of
+// (modulus, reciprocal) hits almost always and elides the recompute.
+//
+// No synchronization: the memo is per-EVM scratch state, like depth and
+// returnData. The EVM "should never be reused and is not thread safe" (see the
+// EVM type doc); every construction site builds a fresh instance and each
+// executes on a single goroutine, so an unsynchronized field is safe here.
+type modReciprocal struct {
+	mod uint256.Int
+	mu  [5]uint64
+	ok  bool
+}
+
+// mulmod computes z = (x * y) % z in place, mirroring the exact semantics of
+// z.MulMod(x, y, z) — including m == 0 yielding 0 — but reuses the memoized
+// reciprocal when the modulus repeats.
+//
+// Correctness is a library guarantee, not a re-derivation: MulModWithReciprocal
+// with mu == Reciprocal(m) produces the identical result to MulMod, because
+// MulMod computes exactly that reciprocal and reduction internally. The memo
+// only changes how the reciprocal is obtained (cached vs recomputed), never the
+// arithmetic — the output is a pure function of (x, y, m), independent of cache
+// state. On a miss we recompute Reciprocal exactly as stock would.
+//
+// Only full-width (z[3] != 0) moduli take the memo path; narrower moduli — and
+// m == 0 — never reach the reciprocal reduction inside MulMod, so they fall
+// through to the stock call unchanged.
+func (mm *modReciprocal) mulmod(x, y, z *uint256.Int) {
+	if z[3] != 0 {
+		if !mm.ok || mm.mod != *z {
+			mm.mod = *z
+			mm.mu = uint256.Reciprocal(z)
+			mm.ok = true
+		}
+		z.MulModWithReciprocal(x, y, z, &mm.mu)
+		return
+	}
+	z.MulMod(x, y, z)
+}