EmitAsmX64 has no [base+index-register] (SIB) form — fails safely, but with an unhelpful error

• Type: bug (asmtext.inc — diagnostics/coverage gap, low severity) — Track A
• Status: done — part 1 (clearer error) landed 2026-07-01, pin v132. Part 2 (real SIB encoding) left undone, not needed by any current call site.
• Opened: 2026-07-01 (found during the ongoing ir_codegen.inc → EmitAsmX64 migration, [[feature-asm-structured-ir-library]]; corrected same day — see Log)

What's missing

EmitAsmX64's memory-operand grammar (compiler/asmtext.inc, AsmTextOperand) supports [base], [base+disp], [base-disp], [base+%] (hole) — a single base register plus an optional constant/hole displacement. It has no support for a second, index register in the bracket ([rsi+rcx], real x86-64 SIB addressing with an actual index register, not just a displacement).

Corrected understanding (read this first)

This ticket originally claimed the gap was a silent miscompile hazard ([rsi+rcx] quietly becoming [rsi+0] with no diagnostic). That was wrong — caught and empirically verified same-day before landing on this description. AsmTextParseInt (compiler/asmtext.inc, ~line 84) already validates every character of a displacement string and calls the (fatal — Halt(1)) Error proc the moment it sees a non-digit, so [rsi+rcx] fails immediately:

$ EmitAsmX64('cmp byte [rsi+rcx], 0')
ERROR: EmitAsmX64: bad integer literal

Verified directly against the real asmtext.inc/x64enc.inc via the test_asm_emit_x64.pas harness scaffold, not just re-read the source. So there is no silent-corruption risk — the actual issue is smaller: the error message says "bad integer literal", which doesn't tell the caller why (that they used an index register, a form that isn't supported yet) — a confusing message for whoever hits it next, not a safety hazard.

Where it was found

While converting EmitArgvToStringManaged (compiler/ir_codegen.inc) to EmitAsmX64 — its inline strlen loop uses [rsi+rcx] for the byte scan, left as raw EmitB (not mechanically convertible) specifically because of this gap, plus separately because the same loop uses hardcoded (non-Patch-tracked) short jump deltas whose values depend on the loop's exact byte length.

Scope

1. Cheap: a clearer error. When the displacement-position parse fails specifically because a register name appears there (as opposed to genuine garbage), raise Error('EmitAsmX64: [base+index-register] (SIB) addressing not supported — write it as raw EmitB') instead of the generic "bad integer literal". Low value, mostly a courtesy.
2. Fuller (optional, only if a real site needs it): add real [base+index]/[base+index*scale] support. Needs a second register token, an optional *1/2/4/8 scale suffix, and real SIB-byte encoding (mod/base/index/scale) instead of the current base-only path. The one known site (EmitArgvToStringManaged's strlen loop) doesn't need this — it's staying raw EmitB regardless, blocked also by its hardcoded jump deltas.

Acceptance

• (If part 1 done) attempting [base+index-register] gives a message that names the actual unsupported construct.
• (If part 2 done) [base+index]/[base+index*scale] encodes correctly, verified against llvm-mc, with oracle-test coverage in test_asm_emit_x64.pas.

Log

• 2026-07-01 — Opened with an incorrect "silent misparse" framing; corrected same day after empirically verifying AsmTextParseInt already rejects a register-name displacement with a fatal error. Rescoped from a safety bug to a minor diagnostics/coverage gap. Not blocking anything currently — the one site that hit it stays raw EmitB either way.
• 2026-07-01 — Part 1 landed: AsmTextOperand (compiler/asmtext.inc) now checks AsmRegNum on the displacement token before calling AsmTextParseInt, and raises EmitAsmX64: [base+index-register] (SIB) addressing not supported — write it as raw EmitB instead of the generic "bad integer literal". Verified via the test_asm_emit_x64.pas FPC oracle harness (all existing cases still pass; [rsi+rcx] now gives the new message). Self-host byte-identical, make test-asm green. Part 2 (real SIB encoding) left open — no current call site needs it.