Xcode needs to know that you're building in WMO mode, so rather than putting 
-whole-module-optimization in your "Other Swift Flags", put -Onone there. It's 
an ugly hack but it should work in the near term.

We do want to work to make this drastic speed difference go away, so if you're 
able we (at Apple) would love to have a source drop of your Swift 3 project, 
for additional data on where the problems are. Mind filing a Radar?

Best,
Jordan


> On Dec 1, 2016, at 11:51, Ben Asher via swift-dev <swift-dev@swift.org> wrote:
> 
> Hello! Someone recently tipped me off to using -whole-module-optimization 
> flag with -Onone for use during debug builds to speed up compile times. In 
> our project, the speedup feels quite dramatic, but when it gets to the 
> linking step (after compiling both Swift and Obj-C in the project) it fails 
> because ld can't find the individual object files that normally get emitted 
> during the debug-type build presumably because -whole-module-optimization 
> only emits one (and this isn't a normal "-Owholemodule"-type build which 
> works fine).
> 
> I can't seem to reproduce this outside of Xcode, but I was curious if anyone 
> has tried this and knows of a workaround to get -whole-module-optimization to 
> work with -Onone in Xcode?
> 
> I'm currently using Xcode 8.1 (App Store build) and Swift 3 on macOS Sierra.
> 
> Thanks!
> 
> Ben
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