[LLVMdev] LLVM 3.4 performance regressed?

[David Blaikie]

On Tue, Apr 8, 2014 at 1:05 AM, Jonas Wagner <jonas.wagner at epfl.ch> wrote:
> Hi,
>
> (adding llvm-dev again)
>
>> I did enable optimization, but that didn't have an effect on the runtime
>> performance numbers.
>
>
> Can you elaborate? For a program such as bzip2, I'd expect the program to be
> at least twice as fast with -O3 than with -O0.
>
> I also noticed that you use LLC in the final step.

This is a thing I'd find suspect as well - the correct sequence of
passes changes from time to time (certainly over several revisions).
I'd be more inclined to do a normal clang (all the way to object files
- or lto as described below) comparison, that'd rule out any dated
pass sequencing you might have.

> An alternative that works
> well for me is to use the gold linker plugin. This way, you can link bitcode
> files directly into the program by using
>
>     clang -flto $(LDFLAGS) <bitcode files> -o <output file> $(LDLIBS)
>
> If your bitcode files have the extension .o, clang will only run LTO
> optimizations, code generation, and linking. If they have the extension .bc,
> it will run a full set of compilation passes (in which case you might want
> to add $(CFLAGS) to the command line).
>
> Hope this helps,
> Jonas
>
>
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