[Webkit-unassigned] [Bug 188986] [CMake] Add support for LTO builds

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Wed May 1 15:43:09 PDT 2019


https://bugs.webkit.org/show_bug.cgi?id=188986

--- Comment #3 from Keith Rollin <krollin at apple.com> ---
I ran into some LTO issues when enabling it for Xcode, I want to make sure they are accounted for in your patch, too.

It's been a couple of years since I tried compiling JavaScriptCore with LTO, but when I did, I ran into a runtime crash. I don't understand the details, but the explanation I was given had something to do with JSC assuming there were implicit compiler fences that were abolished when LTO was applied. So if you're enabling LTO for JSC, be sure to run a full suite of tests. (If they pass, then I should probably try building JSC again under LTO to see if the bug I hit has gone away.)

Also, I had problems building for ARM and got an internal compiler error. I don't know if ARM is generally a target outside of Apple, so I don't know if I should assume you tested that configuration or not.

Finally, there were some issues with older clangs + lto.dylib's. In particular, there was a rare race condition that could lead to crashes at build time. I had to wait until Xcode 10.2 before I could actually turn on LTO. I don't know what the public versions of clang have on the inside, so I don't know how well-distributed the fix is. I could try to dig up some tool version information if this turns out to be something you want to account for. If you run `clang -v`, I might be able to find out if the fix is in there or not.

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