[llvm-bugs] [Bug 51571] New: Emitted binary code changes at -O0 when compiling through -S

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51571

            Bug ID: 51571
           Summary: Emitted binary code changes at -O0 when compiling
                    through -S
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: tlwang at uwaterloo.ca
                CC: cnsun at uwaterloo.ca, llvm-bugs at lists.llvm.org

The .text section of program.c changes when the program is compiled through -S.

$ cat program.c
int a;
void b() {
  if (a)
    ;
}
int main() {}
$
$ clang-trunk -v
clang version 14.0.0 (https://github.com/llvm/llvm-project.git
3890ce708d4f94d0326172650ce22262f6b56661)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/cnsun/usr/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
$
$ clang-trunk -O0 -c program.c ; objdump --disassemble --section=.text
program.o > no_s.txt
$ clang-trunk -O0 -S program.c ; clang-trunk -O0 -c program.s ; objdump
--disassemble --section=.text program.o > s.txt
$ diff s.txt no_s.txt
12,18c12,16
<    c: 74 02                   je     10 <b+0x10>
<    e: eb 00                   jmp    10 <b+0x10>
<   10: 5d                      pop    %rbp
<   11: c3                      retq
<   12: 66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
<   19: 00 00 00
<   1c: 0f 1f 40 00             nopl   0x0(%rax)
---
>    c:	0f 84 05 00 00 00    	je     17 <b+0x17>
>   12:	e9 00 00 00 00       	jmpq   17 <b+0x17>
>   17:	5d                   	pop    %rbp
>   18:	c3                   	retq
>   19:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)

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