[llvm-dev] LLVM trunk generates different machine code for JCC instruction w/ or w/o debug info

[Neil Nelson via llvm-dev]

Bug 37728 - [meta] Make llvm passes debug info invariant
https://bugs.llvm.org/show_bug.cgi?id=37728

Further discussion on methods.
https://groups.google.com/g/llvm-dev/c/yvbWr4azdh0/m/gy1tQIzIDwAJ

Neil Nelson

On 12/29/20 7:25 AM, 陈志伟 via llvm-dev wrote:
> Hi folks, it’s my first post in llvm-dev mailing list, and definitely 
> not the last :-)
>
> Recently, I found an elf file built with or without debug info has 
> different machine code generated. Sadly, it cannot be reproduced in a 
> piece of code. Here is my investigation.
>
> > clang -S -emit-llvm foo.cc <http://foo.cc> -O3 -ggdb3 -o dbg.ll
> > clang -S -emit-llvm foo.cc <http://foo.cc> -O3 -o rel.ll
>
> Where foo.cc <http://foo.cc> is a cc file in my company of 10k+ LOC 
> and depends on tons of 3rd libraries.
>
> The difference between dbg.ll and rel.ll are the llvm debug 
> intrinsics. Emmmm, looks fine.
>
> > llc dbg.ll -o dbg.s
> > llc rel.ll -o rel.s
>
> And the asm instructions are the same. Emmm, fine again.
>
> > llvm-mc -filetype=obj dbg.s -o dbg.o
> > llvm-mc -filetype=obj rel.s -o rel.o
>
> The 2 obj files generated by LLVM assembler has DIFFERENT machine codes.
>
> > 74 19                      je f20
>
> The obj compiled with debug info use 0x74 to represent a JE 
> instruction, while
>
> > 0f 84 15 00 00 00   je f20
>
> The obj compiled without debug info use 0x0f 0x84 instead.
>
> What? Why the debug info affects the generation of machine code? As a 
> LLVM beginner, I’m willing to dive deeper to find the root cause.
>
> Thanks in advance.
>
> --
> Zhiwei Chen
>
>
>
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