SSA
- Go course
- Expert
- Lesson 14/34
- 4 min
Learn the Static Single Assignment intermediate representation.
SSA
SSA (Static Single Assignment) is an intermediate form where each variable is assigned exactly once. x = 1; x = x + 2 becomes x1 = 1; x2 = x1 + 2. This form makes these optimizations easy for the compiler:
- Constant folding:
2 * 3→6 - Dead code elimination: unused computations are dropped
- Bounds check elimination (BCE): checks are removed on slice accesses that can be proven safe
A BCE hint
Accessing the largest index once before the reads lets the compiler prove the following accesses safe:
func topla4(b []byte) int {
_ = b[3] // the only check is here
return int(b[0]) + int(b[1]) + int(b[2]) + int(b[3])
}go build -gcflags="-d=ssa/check_bce" shows the remaining bounds checks line by line. GOSSAFUNC=topla4 go build draws every SSA pass as HTML.
Practice BCE-friendly function
Write oku32(b []byte) uint32: first _ = b[3], then combine the four bytes little-endian: uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24. Print oku32([]byte{1, 0, 0, 0}).
Starter code
package main
import "fmt"
func main() {
// buraya yaz
}Sample solution
package main
import "fmt"
func oku32(b []byte) uint32 {
_ = b[3]
return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
}
func main() {
fmt.Println(oku32([]byte{1, 0, 0, 0}))
}Expected output
1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does bounds check elimination (BCE) remove?
- Unused imports
- Comment lines
- The panic check on index accesses proven safe Correct answer
- All error checks
Explanation: Every s[i] access normally includes an i < len(s) check; if the compiler proves it always holds, it removes the check.
2 Bounds hint
func f(b []byte) byte {
_ = b[____]
return b[0] ^ b[1] ^ b[2] ^ b[7]
}func f(b []byte) byte {
_ = b[7]
return b[0] ^ b[1] ^ b[2] ^ b[7]
}Explanation: Accessing the largest index used (7) proves the other indexes safe too.
3 Optimization
| Term | Meaning |
|---|---|
Sabit katlama |
2*3 → 6 |
Dead code elimination |
Unused computation dropped |
BCE |
Index check removed |
GOSSAFUNC |
Shows SSA passes |