core/vm: refactor memory resize

[MariusVanDerWijden]

Looks like (in some very EVM specific tests) we spent a lot of time resizing memory.
If the underlying array if big enough, we can speed it up a bit though:

goos: linux
goarch: amd64
pkg: github.com/ethereum/go-ethereum/core/vm
cpu: Intel(R) Core(TM) Ultra 7 155U
           │ /tmp/old.txt │              /tmp/new.txt              │
           │    sec/op    │    sec/op     vs base                  │
Resize-14     6.145n ± 9%   1.854n ± 14%  -69.83% (p=0.000 n=10)

           │ /tmp/old.txt │            /tmp/new.txt             │
           │     B/op     │    B/op     vs base                 │
Resize-14      5.000 ± 0%   5.000 ± 0%       ~ (p=1.000 n=10)

           │ /tmp/old.txt │             /tmp/new.txt              │
           │  allocs/op   │ allocs/op   vs base                   │
Resize-14    0.000 ± 0%     0.000 ± 0%       ~ (p=1.000 n=10) ¹

From the blocktest benchmark:

     620ms     10.93s (flat, cum)  9.92% of Total
         .          .     80:func (m *Memory) Resize(size uint64) {
      30ms       60ms     81:	if uint64(m.Len()) < size {
     590ms     10.87s     82:		m.store = append(m.store, make([]byte, size-uint64(m.Len()))...)
         .          .     83:	}
         .          .     84:}

[rjl493456442]

As we always reuse the Memory instance without clearing.

// Free returns the memory to the pool.
func (m *Memory) Free() {
	// To reduce peak allocation, return only smaller memory instances to the pool.
	const maxBufferSize = 16 << 10
	if cap(m.store) <= maxBufferSize {
		m.store = m.store[:0]
		m.lastGasCost = 0
		memoryPool.Put(m)
	}
}

After the resize, the junks will also be visible, for instance:

• T0; memory is empty (the underlying slice is 0x1,0x2)
• T1; memory is resized to 2, the memory becomes [0x1, 0x2]
• T2; the EVM only use the first byte somehow (not sure if it's possible), then the memory becomes [correct-data, junk]

[rjl493456442]

diff --git a/core/vm/memory.go b/core/vm/memory.go
index 5e11e83748..2e71c16c1a 100644
--- a/core/vm/memory.go
+++ b/core/vm/memory.go
@@ -44,6 +44,8 @@ func (m *Memory) Free() {
 	// To reduce peak allocation, return only smaller memory instances to the pool.
 	const maxBufferSize = 16 << 10
 	if cap(m.store) <= maxBufferSize {
+		clear(m.store) // zero out the bytes before truncating the length
+
 		m.store = m.store[:0]
 		m.lastGasCost = 0
 		memoryPool.Put(m)
@@ -79,7 +81,11 @@ func (m *Memory) Set32(offset uint64, val *uint256.Int) {
 // Resize resizes the memory to size
 func (m *Memory) Resize(size uint64) {
 	if uint64(m.Len()) < size {
-		m.store = append(m.store, make([]byte, size-uint64(m.Len()))...)
+		if uint64(cap(m.store)) > size {
+			m.store = m.store[:size]
+		} else {
+			m.store = append(m.store, make([]byte, size-uint64(m.Len()))...)
+		}
 	}
 }