Emscripten/WASM target — working build with fixes

[nasomers]

The Makefile's platform=emscripten target has been broken for years but is structurally intact. I got it building and running as a libretro core inside EmulatorJS — first known working Flycast WASM build.

Demo video: https://www.youtube.com/watch?v=VAGoy-kjqYA

Repo with all patches and full build guide: https://github.com/nasomers/flycast-wasm

Makefile issues

• HOST_CPU_FLAGS override fails — GNU Make $(filter) returns empty when HOST_CPU_FLAGS is unset, causing the conditional to fall through. The emscripten block never sets HOST_CPU_FLAGS, so TARGET_NO_REC and HAVE_GENERIC_JIT are never applied.
• Missing -s USE_ZLIB=1 — Emscripten provides zlib via ports, but the emscripten block doesn't enable it.
• -fexceptions needed — Flycast uses C++ exceptions but the emscripten block doesn't enable them.
• OpenMP flags leak into emscripten builds (not supported by Emscripten).

Source issues

• core/rend/gles/gles.cpp — glGetString(GL_VERSION) returns garbage in WASM (browser returns WebGL version string, not GLES). Flycast parses this to detect GLES3 and falls back to desktop GL #version 130 shaders, which fail in WebGL2. Fix: #ifdef __EMSCRIPTEN__ to force GLES3 code path.
• core/hw/sh4/sh4_core_regs.cpp — Missing CPU_GENERIC case in floating-point rounding mode switch. Build fails without TARGET_NO_REC.

Runtime issues (JavaScript/WebGL2 layer)

Three runtime patches are required for gameplay:

1. getParameter(GL_VERSION) override — return proper GLES3 string for RetroArch's GL driver detection
2. getError() suppression — GL_INVALID_ENUM from WebGL2 causes RetroArch to abort video driver init
3. texParameteri/f guard — calls on unbound textures cause massive console spam and lag

All patches documented with rationale in the technical writeup.

Current limitations

• SH4 interpreter only (no dynarec in WASM) — CPU-bound games are slow
• Single-threaded (pthreads disabled for stability)
• Built with emsdk 3.1.74 on the deprecated libretro/flycast fork

Performance optimization roadmap: https://github.com/nasomers/flycast-wasm/blob/main/PERFORMANCE.md

[barbudreadmon]

This repo is outdated and you shouldn't use it, real repo is at https://github.com/flyinghead/flycast