[llvm-bugs] [Bug 38344] New: Missed optimization: optimizing set of if statements

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38344

            Bug ID: 38344
           Summary: Missed optimization: optimizing set of if statements
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zamazan4ik at tut.by
                CC: llvm-bugs at lists.llvm.org

clang(trunk) with -O3 -std=c++17 for this code:

unsigned int foo(unsigned int x)
{
    if(x % 2 == 0)
    {
        return x * 2;
    }
    if(x % 4 == 0)
    {
        return x * 4;
    }
    if(x % 8 == 0)
    {
        return x * 8;
    }
    if(x % 16 == 0)
    {
        return x * 16;
    }
    if(x % 32 == 0)
    {
        return x * 32;
    }
    return 100;
}

generates this:

foo(unsigned int): # @foo(unsigned int)
  mov ecx, 1
  test dil, 1
  je .LBB0_7
  test dil, 3
  je .LBB0_2
  test dil, 7
  je .LBB0_4
  test dil, 15
  je .LBB0_6
  mov ecx, edi
  shl ecx, 5
  test dil, 31
  mov eax, 100
  cmove eax, ecx
  ret
.LBB0_2:
  mov ecx, 2
.LBB0_7:
  shl edi, cl
  mov eax, edi
  ret
.LBB0_4:
  mov ecx, 3
  shl edi, cl
  mov eax, edi
  ret
.LBB0_6:
  mov ecx, 4
  shl edi, cl
  mov eax, edi
  ret


As you see, generated code is suboptimal: here we can leave only first 'if'
statement and otherwise return 100.

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