Chromium Coverage for macOS Issue with Large Component Builds

[dis]

Hi, 

I'm encountering a coverage measurement issue on macOS.

Problem:

• Building with "use_clang_coverage=true" and "is_component_build=true" produces a ~3GB instrumented binary
• Runtime crashes with: "dyld[PID]: dyld cache '(null)' not loaded: syscall to map cache into shared region failed"
• The large binary conflicts with macOS kernel's dyld shared cache memory mapping and it should not be greater than 1.5GB.

What works:

• "is_component_build=false" runs but produces empty coverage (likely missing profdata merge) and `symbol_level=0`.

What I've tried:

• DYLD_SHARED_REGION=avoid and other dyld env vars
• Various build optimizations ("use_thin_lto", "optimize_for_size"). But it optimizes the size from 3.1GB to 2.9GB.
• Different coverage allowlists.

Question: 

Is there a recommended approach for coverage on macOS with large component builds?

Any guidance would be much appreciated.

[Hans Wennborg]

A 3GB large binary with the component build is surprising.

Can you file a bug on crbug.com under "Tools > LLVM" so this doesn't get lost?

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