Use -O2 instead of -O3 when linking libGD.js/wasm to speed up CI builds

[4ian]

Summary

Lower the optimization level applied during the final Emscripten link of libGD.js / libGD.wasm from -O3 to -O2. The link step (specifically the wasm-opt pass over the ~3.4 MB wasm output) is the bottleneck of the build-gdevelop_js-wasm-only CircleCI job — even with ccache speeding up the compile phase, linking still takes 3–4 minutes.

Benchmarks

Measured on a 4-vCPU Linux box with emsdk 3.1.21 (same version as CI), emmake make -j 8, release variant. The CI runner (medium+, 3 vCPU) should see a comparable speedup.

Link / build time

Phase	-O3 (current)	-O2 (this PR)	Δ
Pure relink (delete libGD.*, re-make)	2m52s	0m14.7s	~11.7× faster
Recompile 4 bindings + link	3m18s	0m56s	~3.5× faster

The wasm-opt -O3 pass over the ~3.4 MB wasm dominates link cost. -O2 skips the most expensive passes (aggressive inlining, full code-flow optimizations) and finishes in ~15 s instead of ~170 s.

Output size

Artifact	-O3 (current)	-O2 (this PR)	Δ
libGD.js	2,087,516 B	2,254,690 B	+167 KB (+8.0%)
libGD.wasm	3,394,768 B	3,629,500 B	+235 KB (+6.9%)
libGD.js (gzip -9)	172,210 B	178,256 B	+5.9 KB (+3.5%)
libGD.wasm (gzip -9)	797,415 B	799,275 B	+1.8 KB (+0.2%)

Uncompressed grows ~7%, but the gzipped wasm (what actually goes over the wire) is essentially unchanged (+0.2%). Gzipped JS is +3.5% (~6 KB).

npm test runtime (3 runs each, --runInBand)

Build	jest reported	real time (avg)
-O3 (current)	9.63 / 9.30 / 9.07 s (avg 9.33 s)	10.09 s
-O2 (this PR)	9.51 / 8.60 / 8.18 s (avg 8.76 s)	9.63 s

1162 tests pass. Test runtime is unchanged (slight noise if anything favors -O2).

Notes

• Only the link-time -O level is changed. Compile-time -O3 -flto is kept, so all .cpp files are still compiled with full optimizations.
• Other variants (dev, debug, debug-assertions, debug-sanitizers) are unaffected.
• I separately confirmed that dropping -flto does almost nothing here (only the 4 Bindings/*.cpp files are scoped to it via the GDevelop.js/CMakeLists.txt-local add_compile_options; GDCore, GDJS, and Extensions are not), so it's left in place for now — the meaningful win is the link-time -O level.

Test plan

• CI build-gdevelop_js-wasm-only job completes noticeably faster (look at the "Build GDevelop.js" step duration)
• CI npm test step still passes (1162 tests)
• Manually open a few projects in newIDE built with the resulting libGD.js/libGD.wasm and confirm no perceptible perf regression

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Generated by Claude Code