[cfe-dev] [llvm-dev] How to debug if LTO generate wrong code?

[Umesh Kalappa via cfe-dev]

Steven,

As mehdi stated , the optimisation level is specific to linker and it
enables Inter-Pro  opts passes ,please  refer function

PassManagerBuilder::addLTOOptimizationPasses()  at
http://llvm.org/docs/doxygen/html/PassManagerBuilder_8cpp_source.html

internal options to disable to them ,i don't think ,you can do so.

Thank you
~Umesh

On Tue, May 17, 2016 at 9:21 PM, Mehdi Amini via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> On May 17, 2016, at 1:33 AM, Shi, Steven via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>
> Hello,
> Let me ask a LTO simple question again. For the llvm LTO example in the
> link:http://llvm.org/docs/LinkTimeOptimization.html, I use below build
> commands to generate three different optimization level binary: -O0, -O1,
> -O2. By nm listing the foo1~4 symbols , I can see these different
> optimizations really works.
> 1.       How can I know what different optimizations are used by the clang
> LTO among -O0, -O1 and -O2?
>
>
> LTO is linker specific, clang is only forwarding the option to the linker
> here.
>
> 2.       Is the compiler domain optimization (e.g. clang/llvm) or the linker
> (e.g. ld) domain optimization make these difference?
>
>
> In you case, you invoke clang with "emit-llvm", without any optimization
> level, so you get O0.
> For what the linker is doing at these optimizations levels, again this is
> linker specific.
>
> 3.       How can I explicitly enable or disable these specific optimizations
> besides using -O0, -O1, -O2?
>
>
> If you're talking about the LTO, this is linker specific again (ld is not
> the same program on every platform). For instance there is no such thing as
> O0/O1/O2 on OS X.
>
>
>
>
> $clang -emit-llvm -c main.c -o main.bc
> $clang -emit-llvm -c a.c -o a.bc
> $llvm-ar cr main.lib main.bc
> $llvm-ar cr a.lib a.bc
> $clang -O0 -flto main.lib a.lib -o main0
> $clang -O1 -flto main.lib a.lib -o main1
> $clang -O2 -flto main.lib a.lib -o main2
>
> $nm main0
> …
> 00000000004005a0 t foo1
> 0000000000400580 t foo2
> 00000000004005e0 t foo3
> 0000000000400530 t foo4
> 0000000000400500 t frame_dummy
> …
> $ nm main1
> …
> 0000000000400550 t foo1
> 0000000000400580 t foo3
> 0000000000400530 t foo4
> 0000000000400500 t frame_dummy
> …
> $ nm main2
> …
> 00000000004004d0 t frame_dummy
> …
>
> From blew verbose output, tt seems only linker( e.g. ld) is invovled to do
> the optimization?
>
>
> Yes.
> Usually the LTO pipeline is a bit different from what you're doing, I'm used
> to see:
>
> $clang -flto -O3 -c main.c -o main.o
> $clang -flto -O3 -c a.c -o a.o
> $clang -flto -O3 main.o a.o -o main0
>
>
> --
> Mehdi
>
>
>
>
> $ clang -O2 -flto main.lib a.lib -o main2 -v
> clang version 3.8.0 (tags/RELEASE_380/final)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> InstalledDir: /usr/local/bin
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.3
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.3.1
> Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.0.0
> Found candidate GCC installation:
> /usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0
> Selected GCC installation:
> /usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0
> Candidate multilib: .;@m64
> Candidate multilib: 32;@m32
> Selected multilib: .;@m64
> "/usr/bin/ld" -z relro --hash-style=gnu --build-id --eh-frame-hdr -m
> elf_x86_64 -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o main2
> /usr/lib/x86_64-linux-gnu/crt1.o /usr/lib/x86_64-linux-gnu/crti.o
> /usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0/crtbegin.o
> -L/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0
> -L/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0/../../../../lib64
> -L/usr/local/bin/../lib64 -L/lib/x86_64-linux-gnu -L/lib/../lib64
> -L/usr/lib/x86_64-linux-gnu
> -L/usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0/../../..
> -L/usr/local/bin/../lib -L/lib -L/usr/lib -plugin
> /usr/local/bin/../lib/LLVMgold.so -plugin-opt=mcpu=x86-64 -plugin-opt=O2
> main.lib a.lib -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc
> --as-needed -lgcc_s --no-as-needed
> /usr/local/bin/../lib/gcc/x86_64-pc-linux-gnu/7.0.0/crtend.o
> /usr/lib/x86_64-linux-gnu/crtn.o
>
>
> Steven Shi
> Intel\SSG\STO\UEFI Firmware
>
> Tel: +86 021-61166522
> iNet: 821-6522
>
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