[llvm-bugs] [Bug 49731] New: missed optimization for dead code elimination at -O3 (vs. -O1)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49731

            Bug ID: 49731
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -O1)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Interprocedural Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: zhendong.su at inf.ethz.ch
                CC: llvm-bugs at lists.llvm.org

[654] % clangtk -v
clang version 13.0.0 (https://github.com/llvm/llvm-project.git
dc46783f7f61f54618b4cf1b0b325fdad304d109)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /local/suz-local/opfuzz/bin
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.5.0
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
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
[655] % 
[655] % clangtk -O1 -S -o O1.s small.c
[656] % clangtk -O3 -S -o O3.s small.c
[657] % 
[657] % wc O1.s O3.s
  17   45  546 O1.s
  46  140 1298 O3.s
  63  185 1844 total
[658] % 
[658] % grep foo O1.s
[659] % grep foo O3.s
        callq   foo
[660] % 
[660] % cat small.c
extern void foo(void);

static long a = 78240;
static int b, d;
static short e;

static short c(short f, short h) { return h == 0 || (f && h == 1) ? 0 : f % h;
}

int main() {
  short g = a;
  for (b = 0; b < 1; b++) {
    e = a;
    d = c((e == a) ^ g, a);
  }
  if (d) {
    foo();
    for (; a; a++)
      ;
  }
  return 0;
}

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