[swift-dev] Faster debug builds?

David Zarzycki zarzycki at icloud.com
Sun Aug 6 15:50:32 CDT 2017

> On Aug 6, 2017, at 16:16, Michael Gottesman <mgottesman at apple.com> wrote:
>> On Aug 6, 2017, at 11:11 AM, David Zarzycki via swift-dev <swift-dev at swift.org> wrote:
>> Hello,
>> Unless I’m missing a build-script flag, it seems to me that compiling the Swift stdlib with the unoptimized debug swift compiler takes about 15 minutes on a fast machine.
> I am assuming that you mean a debug swift compiler building an optimized stdlib?

Hi Michael,

I’m building the debug swift compiler via ./utils/build-script -r —debug-swift. I assume, perhaps wrongly, that implies a debug stdlib.

>> Other than forcing the type checker to be optimized, what if any tricks can I use to building the stdlib faster with the debug compiler? Is there a way to tell Clang to enable the inliner and only the inliner during -O0 builds? I have an anecdotal experiment[1] that suggests that this would yield appreciably faster Swift stdlib builds with the debug compiler (and selfishly speaking, I can tolerate the minor impact on debugging that inlining does to otherwise unoptimized code).
> Are building LLVM in release + Swift in debug? I.e.:
> —release-debuginfo --debug-swift --force-optimized-typechecker

Yes, with the exception that I cannot use —force-optimized-typechecker because I’m hacking on the type checker. Otherwise, this is what I’m doing to make debug builds go as fast as possible:

./utils/build-script \
	--llvm-targets-to-build X86 \
	--skip-ios --skip-tvos --skip-watchos \
	--skip-build-benchmarks true \
	--build-swift-static-stdlib false \
	--build-swift-static-sdk-overlay false \
	--build-swift-dynamic-sdk-overlay false \
	--build-swift-stdlib-unittest-extra false \
	--extra-cmake-options \\-DCMAKE_CXX_FLAGS=-Werror=switch \
	-r \
	--debug-swift  \

>> Thanks!
>> Dave
>> [1] – If one force inlines LLVM’s casting logic and associated callbacks (like classof() and getKind()), then the Swift stdlib builds 18% faster on my machine with the debug Swift compiler. One can imagine how much faster the whole stdlib would compile if all trivial functions were inlined automatically.
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