Import of fastcomp commit 4105790f1549808c1f1daa5250b4ada5f41a5c02

[brson]

This is a minimal import of the emscripten "fastcomp" LLVM patchset.
All it contains is the target definitions necessary to create a
TargetMachine with the correct data layout. With this rustc can
emit LLVM IR that emcc will run through the PNaCl lagalizer and
the JS backend to generate asm.js.

r? @alexcrichton

[alexcrichton]

Looks good to me! Could you also document here (or in the commit message) the steps you took to trim the commit down? Other than that feel free to merge how you see fit (is the merge button right here?)

[brson]

I'd expect us not to have to make changes to this in the future, except for possibly updating the JSBackend data layout (emscripten is considering it), since acquiring the data layout is just about the only thing this patch does now. But here's what I did:

• Squashed the fastcomp branch from the merge base with our branch into one commit for easier diffing
• (Previously) removed the lib/Transform changes that we've been told aren't needed. There are only two remaining, and the InstcombinePatch may not actually be necessary.
• Preserved the asmjs changes to Triple.h and Triple.cpp
• Deleted the PNaCl legalizer from lib/Target/JSBackend/NaCl/
• ripgrepped for the three LLVMInitialize* functions in lib/Target/JSBackend. These are the three functions rustc calls to initalize the target information.
• From there I traced all the dependencies and deleted everything that wasn't needed
• Then removed the body of JSTargetMachine::addPassesToEmitFile
• Then updated the CMakeLists.txt and LLVMBuild.txt files.

[alexcrichton]

👍