x87 floating point in 1995-2005 PC games murders ARM emulators — FEX's new stack optimization pass

lauriewired · x · 2026-10-23

• lauriewired highlights that almost every PC game from 1995-2005 relies on Intel's x87 floating-point extension, which uses 80-bit extended precision internally — a hidden performance trap when emulating x86 on ARM.
• FEX, an ARM64 emulator of Intel assembly, is doing the best public work in this space. Paulo Matos published a detailed post on FEX's new x87 stack optimization pass: the x87 FPU uses 8 registers operated as a stack, with the top tracked via the TOS field, and the emulator must reproduce this stack semantics efficiently.
• The optimization landed in FEX on July 22, 2024, with speedup results shown for Half-Life and other games.

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