[llvm-bugs] [Bug 27694] New: WebAssembly backend speed comparison to fastcomp

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27694

            Bug ID: 27694
           Summary: WebAssembly backend speed comparison to fastcomp
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: WebAssembly
          Assignee: unassignedbugs at nondot.org
          Reporter: alonzakai at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

I compared the wasm backend's speed to the asm.js backend in emscripten
fastcomp. To build BananaBread, fastcomp takes 2.83 seconds, while the wasm
backend takes 14.24, which is 5.03x slower. Those are the times spent in the
respective LLVM backends, with -O2.

Fastcomp is a release+asserts build. wasm backend is a release build.

The fastcomp times includes legalization + compiling to asm.js. It doesn't
include asm.js optimization passes that happen later. Those are important for
perf so in a sense the above comparison is not fully fair. JS opts take 4.04
seconds, so that would make the wasm backend 2.07x slower, which decreases the
difference substantially, but still leaves it quite large.

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