Language: Null coalescing operator (??) in compiler pipeline

[PurHur]

Problem

Modern PHP web code relies heavily on ?? for defaults:

$name = $_GET['name'] ?? 'Guest';
$action = $_POST['action'] ?? 'index';

If Op\Expr\Coalesce (or equivalent in php-cfg) is not lowered, compilation throws Unsupported expression from lib/Compiler.php.

Goal

Support ?? in CFG → opcodes → VM → JIT.

Tasks

• Confirm php-cfg emits coalesce nodes; add compile path in Compiler::compileExpr
• VM: evaluate left; if null/undefined, evaluate right
• JIT: LLVM branch on null/undefined boxed value
• PHPT: test/real/cases/coalesce_get_default.phpt
• Interaction with Web: Dynamic isset/empty on $_GET and $_POST keys in JIT #70 (isset/empty on superglobals) and JIT: Dynamic hashtable read/write for $_GET[$key] and $_POST[$key] #86 (dynamic keys)

Acceptance criteria

bin/vm.php, bin/jit.php, and bin/compile.php compile and run scripts using ?? with same output as PHP 8.

Related

• Web: Dynamic isset/empty on $_GET and $_POST keys in JIT #70, JIT: Dynamic hashtable read/write for $_GET[$key] and $_POST[$key] #86, Web: Runtime-populated superglobals for JIT and AOT (per-request $_GET/$_POST) #49

Status on master (May 2026)

Layer	Status	Evidence
Compiler	✅	lib/Compiler.php — compileCoalesce(), Expr\BinaryOp\Coalesce, ??= assign
VM	✅	test/compliance/cases/language/coalesce.phpt, coalesce_assign.phpt
JIT	🚧 partial	lib/JIT/CoalesceHelper.php; coalesce_assign_jit.phpt; spine requires helper
AOT	✅ fixtures	test/fixtures/aot/cases/coalesce_*.phpt (MiniWebApp bisect ladder)
Lint	✅	UnsupportedRegistry no longer tracks Expr_AssignOp_Coalesce
CI gate	📋	Dedicated gate tracked in #1960

Remaining before closing #99

1. Audit JIT gaps: superglobal ?? chains, nested ?? with side effects
2. Land CI: COALESCE_COMPLIANCE_GATE default-on in ci-fast (audit + close #99) #1960 COALESCE_COMPLIANCE_GATE=1 in ci-fast.sh
3. Confirm bin/jit.php + bin/compile.php parity on coalesce_get_default.phpt

Implementation hints (JIT gaps)

Case	Files
Superglobal default	lib/Compiler.php + SuperglobalCoalesceVmTest
Branch targets	lib/JIT/CoalesceHelper.php
Assign coalesce	lib/JIT.php

Verification (local / Docker only)

./script/ci-fast.sh --filter Coalesce
COALESCE_COMPLIANCE_GATE=1 ./script/ci-fast.sh --filter Coalesce   # after #1960
docker run --rm -v "$(pwd):/compiler" -w /compiler php-compiler:22.04-dev \
  ./script/ci-fast.sh --filter Coalesce

Dependencies (updated)

• CI: COALESCE_COMPLIANCE_GATE default-on in ci-fast (audit + close #99) #1960 — ci-fast compliance gate + close-out audit
• Web: Dynamic isset/empty on $_GET and $_POST keys in JIT #70 / JIT: isset/empty on runtime-populated sg_GET keys (close #49 JIT slice) #1901 — isset/empty on dynamic keys (orthogonal)
• Web: Runtime-populated superglobals for JIT and AOT (per-request $_GET/$_POST) #49 — runtime superglobals for web ?? defaults

Links

• ROADMAP 🗺️ ROADMAP: Self-host + web-capable PHP subset (living) #78 Phase 1 Language
• Gate precedent: CI: Default NESTED_RETURN_COMPLIANCE_GATE=1 in ci-fast after #1885 #1888 (nested return call)

[PurHur]

Implementation hints (backlog batch 20)

CFG / compiler

1. Grep php-cfg for coalesce: Op\Expr\Coalesce or null-coalesce lowering in PHPCfg\Parser.
2. In lib/Compiler.php::compileExpr(), add branch before the final Unsupported expression:
   • Evaluate left operand; if result is null or undefined variable (match VM semantics for unset keys — pair with Runtime: Undefined array/superglobal key — Zend notice parity #273), evaluate right.
3. Prefer TYPE_JUMPIF + merge block over a new opcode if phi merge already exists for ternary (Language: Ternary operator (?:) in compiler pipeline #114).

VM (lib/VM.php)

• Reuse is_null() / undefined-key detection used by isset paths (Web: Dynamic isset/empty on $_GET and $_POST keys in JIT #70).
• Do not evaluate right side when left is non-null (short-circuit).

JIT (lib/JIT.php)

• LLVM: icmp on null tag + select or explicit branch to right-arm block.
• Boxed strings/ints: only null triggers fallback, not 0 or ''.

Lint

• Register Expr_Coalesce in lib/Lint/UnsupportedRegistry.php until done (mirror Lint: Register list() destructuring (Expr_List) in UnsupportedRegistry #297 list() pattern).

Tests

• test/compliance/cases/language/coalesce.phpt — $_GET['x'] ?? 'default', nested ??, right side must not run (side-effect counter).
• After JIT: Dynamic hashtable read/write for $_GET[$key] and $_POST[$key] #86: dynamic key $_GET[$k] ?? ''.

Verify (local / Docker)

docker run --rm -v "$(pwd):/compiler" -w /compiler php-compiler:22.04-dev \
  php bin/vm.php -r 'echo $_GET["missing"] ?? "Guest";'

Dependencies

• Runtime: Undefined array/superglobal key — Zend notice parity #273 undefined key notice parity (edge cases)
• Language: Ternary operator (?:) in compiler pipeline #114 ternary (different CFG shape)
• ROADMAP priority Stdlib: ceil, floor, intval and expanded PHPT specs #3 — ship before Reference: MiniWebApp lint-first skeleton (examples/003-MiniWebApp) #246 MiniWebApp templates