From 4f26fab1e98a8a3e5dcb472dd3fc73a549149ffe Mon Sep 17 00:00:00 2001
From: Martin Holst Swende <martin@swende.se>
Date: Sun, 10 Jun 2018 22:33:50 +0200
Subject: [PATCH] core/vm: less alloc and copying for mstore

---
 core/vm/instructions.go |  2 +-
 core/vm/memory.go       | 21 ++++++++++++++++++++-
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/core/vm/instructions.go b/core/vm/instructions.go
index 3a67e1865f..69a04ffc8f 100644
--- a/core/vm/instructions.go
+++ b/core/vm/instructions.go
@@ -556,7 +556,7 @@ func opMload(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *St
 func opMstore(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *Stack) ([]byte, error) {
 	// pop value of the stack
 	mStart, val := stack.pop(), stack.pop()
-	memory.Set(mStart.Uint64(), 32, math.PaddedBigBytes(val, 32))
+	memory.Set32(mStart.Uint64(), val)
 
 	evm.interpreter.intPool.put(mStart, val)
 	return nil, nil
diff --git a/core/vm/memory.go b/core/vm/memory.go
index d5cc7870bc..a3530419d9 100644
--- a/core/vm/memory.go
+++ b/core/vm/memory.go
@@ -16,7 +16,12 @@
 
 package vm
 
-import "fmt"
+import (
+	"fmt"
+	"math/big"
+
+	"github.com/ethereum/go-ethereum/common/math"
+)
 
 // Memory implements a simple memory model for the ethereum virtual machine.
 type Memory struct {
@@ -43,6 +48,20 @@ func (m *Memory) Set(offset, size uint64, value []byte) {
 	}
 }
 
+// Set32 sets the 32 bytes starting at offset to the value of val, left-padded with zeroes to
+// 32 bytes.
+func (m *Memory) Set32(offset uint64, val *big.Int) {
+	// length of store may never be less than offset + size.
+	// The store should be resized PRIOR to setting the memory
+	if 32 > uint64(len(m.store)) {
+		panic("INVALID memory: store empty")
+	}
+	// Zero the memory area
+	copy(m.store[offset:offset+32], []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0})
+	// Fill in relevant bits
+	math.ReadBits(val, m.store[offset:offset+32])
+}
+
 // Resize resizes the memory to size
 func (m *Memory) Resize(size uint64) {
 	if uint64(m.Len()) < size {