SSA

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.

Exercise 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}).

Show example 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}))
}

Test yourself

What does bounds check elimination (BCE) remove?
- Unused imports
- Comment lines
- The panic check on index accesses proven safe
- All error checks
Show answer

The panic check on index accesses proven safe — Every s[i] access normally includes an i < len(s) check; if the compiler proves it always holds, it removes the check.

Bounds hint

func f(b []byte) byte {
    _ = b[____]
    return b[0] ^ b[1] ^ b[2] ^ b[7]
}

Show answer

func f(b []byte) byte {
    _ = b[7]
    return b[0] ^ b[1] ^ b[2] ^ b[7]
}

Optimization

Sabit katlama

2*3 → 6
Dead code elimination

Unused computation dropped
BCE

Index check removed
GOSSAFUNC

Shows SSA passes

Next lesson: Escape analysis

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