[LLVMdev] LLVM compile speed significantly slower than GCC (w/ test case) (original) (raw)
Jim Grosbach grosbach at apple.com
Thu Jul 19 16:31:40 PDT 2012
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Thanks, Matt. This is great information. Sounds like Chandler is looking into the details of what's going on.
-Jim
On Jul 19, 2012, at 3:55 PM, Matt Fischer <mattfischer84 at gmail.com> wrote:
I've been doing some profiling of LLVM on our codebase, to see how it stacks up to the existing GCC build that we do. The primary thing I'm focusing on at the moment is build speed, and in this regard LLVM seems to be pretty all over the map. On some files it seems to go quite a bit faster than GCC, and on others it's slower, leading to an aggregate build time for our repository that's roughly the same as GCC.
Some IRC discussions suggested that you guys might be interested in seeing an example of a file that goes appreciably slower, so I managed to isolate one that's completely self-contained. It's a relatively stock implementation of the SHA1 algorithm, so it should be a pretty straightforward file to follow, as well as being a relatively data-intensive piece of code. I compiled the file with both compilers for the arm-none-eabi triple. The numbers I get are as follows: GCC (4.5.2, Windows build from CodeSourcery) - With -O0: 110ms, with -O2: 215ms Clang/LLVM (Release mode, LLVM git hash 7f5714f4..., clang git hash 9d9cf5...) - With -O0: 110ms, with -O2: 640ms The compilers are essentially identical for the -O0 case, but when compiling with -O2, LLVM takes almost three times as long as GCC. I'm not sure whether this file is unusual in some way, such that fixing whatever makes this slow wouldn't have much of an effect on other files, or if this is evidence of some problem that's broad enough to improve the compile speed of a wide variety of files. If anybody is interested in investigating the discrepancy, though, I'd love to hear about it. Thanks, Matt <sha1test.c>_______________________ LLVM Developers mailing list LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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