Introduce a CMake flag to disable fast-path context descriptor lookup based on short manglings

[kubamracek]

This is useful for codesize reductions. The fast-path is holding all the well-known short-mangling types referenced, so they cannot be optimized via GlobalDCE/dead-stripping.

[kubamracek]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with toolchain and preset

[kubamracek]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with toolchain and preset

[kubamracek]

preset=stdlib_S_standalone_minimal_macho_x86_64,build,test
@swift-ci please test with toolchain and preset

[kubamracek]

@swift-ci please test

[davezarzycki]

Why is it important to have the CMake variable default to TRUE but have build-script default to FALSE? This complicates building/testing for people not using build-script.

[kubamracek]

build-script default to FALSE

It's only set to FALSE in one particular special configuration ("freestanding"). TRUE is the correct default for (almost) everyone.

complicates building/testing for people not using build-script

I don't follow, would you mind explaining? It's a CMake config option, so you can either keep it set to the default (TRUE), or configure it yourself if you care about this setting.

[davezarzycki]

Ah, sorry. I didn't look at the build-script change in context and that's my fault.

I've just been bitten a few times when people tweak defaults at the build-script level but not the CMake level, which makes for confusing build failures and conversations.

Nothing to see here.

[kubamracek]

👍 Cool. Different defaults in CMake and build-script are certainly bad, and we need to avoid them, agree on that.