Intel APX introduces a new feature called Push-Pop Acceleration (PPX), which adds a performance hint to legacy PUSH and POP instructions, enabling the PUSHP and POPP forms. The PPX hint improves performance by accelerating register value forwarding between matching push/pop pairs.

Purpose: PPX is a performance hint that allows the processor to bypass memory and the training loop of Fast Store Forwarding Predictor (FSFP) by directly forwarding data between paired PUSHP and POPP instructions.

Requirements: Both the PUSH and its matching POP must be marked with PPX. A "matching" pair accesses the same stack address (e.g., typical function prolog/epilog). Standalone PUSH instructions (e.g. for argument passing) must not be marked.

Encoding: PUSHP/POPP is a replacement for legacy PUSH/POP (0x50+rd / 0x58+rd) and uses REX2.W = 1 (implies 64-bit operand size). PPX cannot be encoded with 16-bit operand size as REX2.W overrides the 0x66 prefix.