Clang: How to check if LTO was performed

Question

For gcc, this answer tells us how we can verify that Link-Time Optimization was performed. For clang, I cannot see any entries similar to .gnu.lto.

More specifically, I have a binary where I am quite sure that LTO should have a significant benefit, but I am seeing nothing. I wonder if cmake actually does the right thing.

Is there any way to tell if clang performed LTO by looking at a binary or an *.o file?

score 4 · Accepted Answer · answered Jun 29 '18 at 14:06

4

An option would be to try running llvm-dis on one of your .o files. If the LTO was actually performed, the .o files contain llvm bitcode and llvm-dis will produce the .ll file containing humain-readable llvm ir. Otherwise it will produce the error message "error: Invalid bitcode signature".

answered Jun 29 '18 at 14:06

mamai

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I had to re-read this to realize that no output is good news. Now it works, thanks! – mrks Jul 02 '18 at 09:52

score 4 · Answer 2 · answered Mar 08 '19 at 03:36

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The file utility will report that the file is "LLVM IR bitcode". LLVM/clang's LTO facilities don't produce "fat" object files, it's just IR.

answered Mar 08 '19 at 03:36

jberryman

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Clang: How to check if LTO was performed

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