[llvm-bugs] [Bug 51136] New: missed optimization for dead code elimination at -O3 (vs. -O2)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51136

            Bug ID: 51136
           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);
int a, c, d, g;
volatile int b;
void e() {
 for (c = 0; c < 4; c++)
  for (g = 0; g < 3; g++)
   for (d = 0; d < 5; d++)
    b;
}
int main() {
 int h = 3;
 for (; h; h--)
  e();
 int i[0];
 if (a && !i[1])
  foo();
 return 0;
}

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

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