PCC failures on aarch64 for constant-heavy WebAssembly code usually come from a mismatch between how constants are lowered, encoded, or range-checked during verification and how the PCC pass expects those values to behave.

If the issue appears when compiling a Rust-generated hello-world Wasm module and reproduces with smaller test cases, the problem is typically not the source program itself. It is more often triggered by specific constant forms that the backend emits on AArch64, especially when PCC validation checks facts about values, immediates, or instruction semantics more strictly than the lowering path guarantees.

Understanding the Root Cause

On aarch64, constants are not always materialized as a single instruction. Depending on value size, signedness, relocation behavior, and the instruction selected by the backend, a constant may be split across multiple operations such as move-wide sequences, extended immediates, or address-forming instructions. When PCC is enabled, the compiler must prove that each transformed value still satisfies the checker’s rules.

This class of failure usually happens for one of these reasons:

• Constant materialization diverges from PCC assumptions: the verifier expects one value range, but the lowering sequence creates intermediate values that temporarily violate it.
• Signed vs unsigned interpretation: a constant that is valid as a raw bit pattern may fail when interpreted through a signed range or extension rule.
• Instruction-specific immediate constraints: some AArch64 instructions accept only particular encoded constants, forcing the backend to rewrite the value in a way PCC does not model correctly.
• Fact propagation gaps: the IR or machine-level pass loses value facts after legalization, optimization, or instruction selection, so the checker sees an under-specified constant.
• Wasm-to-backend lowering edge cases: Rust-generated Wasm can produce patterns like zero-extension, sign-extension, i64 splits, or symbolic offsets that are uncommon in simpler native frontends.

In practice, the failure means the PCC checker and the AArch64 lowering path disagree about a constant’s legality or proven properties. That is why tiny reproductions often succeed in isolating the bug: they expose one exact constant pattern that passes normal codegen but fails proof-carrying validation.

Step-by-Step Solution

The fastest path is to reduce the failing case, inspect the generated IR or machine code around the bad constant, and then patch either the lowering rule or the PCC fact model so both sides agree.

1. Reproduce the failure with the smallest test case.
2. Enable compiler debugging output for the function that fails.
3. Identify the exact constant form that triggers the PCC rejection.
4. Check how the constant is lowered on aarch64.
5. Compare the checker’s expected value facts with the actual instruction sequence.
6. Patch lowering or verifier logic depending on which side is incorrect.
7. Add regression tests for each constant pattern.

Use a workflow like this:

# Build the project in debug mode so verifier output is easier to inspect cargo build # Run the failing test or reproducer with PCC enabled ./your-compiler --target aarch64 --enable-pcc path/to/reproducer.wasm # If available, dump intermediate IR and machine lowering ./your-compiler --target aarch64 --enable-pcc --dump-ir --dump-vcode path/to/reproducer.wasm

Once you locate the failing function, inspect the constant-producing instructions. You are looking for patterns such as:

; Pseudocode patterns that often trigger verifier mismatches v0 = iconst.i64 0xffffffffffff0000 v1 = ireduce.i32 v0 v2 = uextend.i64 v1 ; or aarch64 materialization via move-wide instructions movz x0, #0x0 movk x0, #0xffff, lsl #16 movk x0, #0xffff, lsl #32 movk x0, #0xffff, lsl #48

Then verify whether PCC models the intermediate states correctly. If the checker incorrectly assumes the fully materialized constant is present after the first step, verification can fail even though final codegen is valid.

A typical fix falls into one of these categories:

1. Normalize constant facts before PCC checks

If a value is known only after a sequence completes, ensure the checker reasons about the final semantic value, not partially constructed temporaries.

// Example strategy in compiler logic: // - recognize multi-instruction constant materialization // - assign PCC facts after the full sequence is formed // - preserve signedness/bit-width metadata across transforms

2. Correct signedness or extension handling

If the bug appears only for negative-looking constants, high-bit values, or truncation/extension chains, audit sign extension and zero extension rules.

// Verify transformations like these are fact-preserving: i32_val -> uextend to i64 i32_val -> sextend to i64 i64_val -> ireduce to i32

3. Adjust AArch64 lowering for PCC-sensitive constants

Sometimes the backend selects a legal instruction sequence that is hard for PCC to prove. In that case, use a more verifier-friendly lowering path for affected constants.

// Example direction: // prefer explicit constant synthesis sequences that preserve checker facts // avoid folding into instruction forms with implicit semantics PCC cannot yet express

4. Add a focused regression test

Since the original issue reproduces with small cases, turn each one into a backend test that runs with PCC enabled on aarch64.

# Example test invocation cargo test pcc_aarch64_constants -- --nocapture

If you maintain the compiler backend, a solid fix strategy is:

1. Add the minimal failing Wasm or IR test. 2. Dump the selected AArch64 instruction sequence. 3. Confirm where PCC loses or misinterprets facts. 4. Patch the verifier or lowering rule. 5. Re-run all AArch64 PCC tests plus general codegen tests.

If you are only trying to unblock builds while waiting for an upstream fix, use one of these temporary mitigations:

• Disable PCC for the affected target or module.
• Rewrite the test to avoid the triggering constant pattern.
• Pin to a compiler revision before the regression, if known.
• Use a patched fork with additional constant-handling logic.

Common Edge Cases

• 64-bit constants with upper bits set: values near sign boundaries often reveal mismatches in extension semantics.
• Constant folding after legalization: a value proven safe in early IR may be re-expressed later in a way PCC does not recognize.
• Address-like constants: symbolic offsets, globals, and relocation-aware constants may be lowered differently from plain integers.
• Vector or lane constants: if your Wasm pipeline emits SIMD-related forms, constant encoding rules can differ substantially.
• Select or branch-derived constants: PCC may need merged facts from multiple predecessors, and one path may lose precision.
• Truncate then re-extend patterns: these are especially common in Wasm lowering and can expose incorrect bit-width assumptions.
• Backend-specific optimizations: peephole rewrites on AArch64 may create instruction forms that the checker was not originally designed to validate.

FAQ

Why does this fail only on aarch64 but not on other architectures?

Because AArch64 has its own immediate encoding rules and constant materialization sequences. Another backend may emit a simpler instruction or a different sequence that aligns with PCC’s current proof model.

Is the Rust hello-world Wasm module actually invalid?

No. The issue is usually not invalid Wasm. It is more often a compiler backend or verifier mismatch triggered by a specific lowering pattern for constants under PCC.

Should I fix the verifier or the instruction lowering?

Whichever side is semantically wrong. If lowering is producing a legal sequence that PCC should understand, improve the verifier model. If lowering is creating hard-to-prove or incorrectly annotated values, fix the backend and keep the verifier strict.

The durable solution is to make constant lowering, value facts, and PCC verification agree on the exact semantics of each transformed constant on aarch64. Once that alignment is restored and covered by regression tests, the hello-world Wasm case and the smaller repro cases should pass reliably.