Compiler Mid/Back: IR, SSA, Machine Code, Link

Compiler Mid/Back: IR, SSA, Machine Code, Link

Overview

After typecheck, the compiler optimizes in IR, lowers to SSA, emits machine code objects, then the linker produces a binary. This is where inlining, escape analysis, and PGO bite.

Diagram: mid and back end

  typed frontend
       │
       v
  IR (inline, escape, …)
       │
       v
  SSA opts + arch lower
       │
       v
  .o objects ──► linker ──► binary

IR responsibilities

Pass family	Effect
Inlining	Expand small callees; enable more opts
Escape analysis	Stack vs heap
Devirtualization	Concrete calls when type known
Walk / order	Prepare for SSA

Flags: go build -gcflags='-m -m' shows escape/inline decisions.

SSA

• Every value assigned once → easy dataflow
• Dozens of rewrite passes (nilcheck elim, bounds prove, …)
• Then lower to AMD64… / ARM64… ops

GOSSAFUNC=main go build .   # classic SSA HTML dump when supported
go build -gcflags='-S' . 2>&1 | head

Machine code + object files

• Encode instructions, relocations, PC↔︎line metadata
• One package → one (or more) object; still not runnable alone

Linker

resolve symbols across packages
  → apply relocations
  → layout sections
  → emit ELF/Mach-O/PE + buildid

go build -ldflags='-s -w -X main.version=dev' -trimpath -o app .

See also 210 link/race, 248 bootstrap.

Experiment

go build -gcflags='-m' . 2>&1 | head -40
go tool nm ./app | rg 'main\\.|runtime\\.main' | head

What to notice: Even tiny mains reference large runtime.* symbol sets after link.

Try next: Build twice with/without -pgo=auto after a CPU profile; bench a hot path.