[llvm-bugs] [Bug 44346] New: Signed division with power of two - eliminate test instruction

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Dec 19 13:29:10 PST 2019


            Bug ID: 44346
           Summary: Signed division with power of two - eliminate test
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

After PR43197, we have a better codegen for signed division with power of two,
but it could be improved a bit.

int al2(int x) {
     return ((x + 3 )/4) *4;

We have:
al2(int):                                # @al2(int)
        lea     eax, [rdi + 3]
        add     edi, 6
        test    eax, eax
        cmovs   eax, edi
        and     eax, -4

But test instruction could be eliminated, like GCC does:
  lea eax, [rdi+6]
  add edi, 3
  cmovns eax, edi
  and eax, -4


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