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

[Shi, Steven via cfe-dev]

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?

2.       Is the compiler domain optimization (e.g. clang/llvm) or the linker (e.g. ld) domain optimization make these difference?

3.       How can I explicitly enable or disable these specific optimizations besides using -O0, -O1, -O2?


$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?

$ 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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