⏺ X86-64 Assembler Audit Results

  1. Incorrect IMUL3 Opcode

  Lines 419-424: The IMUL3 instruction uses opcodes 0x6a/0x68, but these are PUSH immediate opcodes. The correct opcodes should be 0x6b (8-bit
  immediate) and 0x69 (32-bit immediate).

  2. Missing REX.W Validation

  Line 119: The rex-prefix function doesn't validate that REX.W is only used with 64-bit operations. It could incorrectly emit REX.W for 32-bit
   operations.

  3. Bug in Zero-Extension Logic

  Lines 211-215: The zero-extendable? function checks 1 32 2^ 1 - between? which is checking if a value is between 1 and 2^32-1. This is
  incorrect - it should check if the value fits in 32 bits unsigned (0 to 2^32-1).

  4. Missing SIB Byte for [RSP+disp]

  Line 66: The code doesn't handle the special case where RSP as base register always requires a SIB byte, even without an index register. Test
   line 66 shows this is caught, but the implementation could be clearer.

  5. Incorrect FYL2XP1 Opcode

  Line 577: FYL2XP1 has the same opcode as FYL2X (0xD9 0xF1). The correct opcode for FYL2XP1 should be 0xD9 0xF9.

  6. Missing Validation in Shift Instructions

  Lines 400-404: The shift instructions don't validate that the immediate count is within valid range (0-63 for 64-bit, 0-31 for 32-bit).

  7. Potential Bug in TEST Immediate

  Line 364: TEST with immediate uses maybe-zero-extend but the encoding might be incorrect for certain edge cases with sign extension.

  8. Missing ModR/M Special Cases

  Lines 52-60: The code handles RBP/R13 requiring displacement when used as base (no displacement), but the logic could miss edge cases with
  SIB byte combinations.

  9. Incorrect Direction Bit Logic

  Lines 178-180: The direction bit logic swaps operands when dst is register and src is not, but this might not handle all cases correctly,
  especially with certain SSE instructions.

  10. Missing Prefix Ordering

  The assembler doesn't enforce correct prefix ordering (legacy prefixes, REX, opcode). While it may work, incorrect ordering can cause issues
  on some processors.

  Summary of Issues:

  - Wrong opcodes: IMUL3, FYL2XP1
  - Missing validation: REX.W usage, shift counts
  - Logic errors: zero-extension check, direction bit
  - Special case handling: RSP/RBP addressing modes
  - No prefix ordering enforcement

  The x86-64 assembler is more mature than the ARM64 one, but still has several issues that could lead to incorrect code generation or
  assembler errors.