[llvm-bugs] [Bug 51137] New: missed optimization for dead code elimination at -O3 (vs. -O2)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51137

            Bug ID: 51137
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -O2)
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: theodoros.theodoridis at inf.ethz.ch
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

$./bin/clang -v    
clang version 13.0.0 (https://github.com/llvm/llvm-project.git
96d8f2a1e0842b83b319a8daf3cc82dd5e1350fa)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /dev/shm/llvm-project/build/./bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
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
Selected multilib: .;@m64

$cat test.c   
extern void foo(void);
extern void bar(void);
int a, b, c, d, f;
int main() {
 int e[4] = {0,0,0,0}, g = 0, h, i;
 for (; g < 4; g++)
  for (h = 0; h < 6; h++)
   for (i = 0; i < 9; i++)
    bar();
 while (d) {
  f = e[3] << c;
  if (f)
   foo();
  b = e[a];
 }
 return 0;
}

$./bin/clang -O2 test.c -S -o O2.s      
$./bin/clang -O3 test.c -S -o O3.s      
$grep foo O2.s            
$grep foo O3.s       
        callq   foo

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