[llvm-bugs] [Bug 35709] New: Missed optimization in math expression: (x+x)/x == 2

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Dec 20 13:06:03 PST 2017


            Bug ID: 35709
           Summary: Missed optimization in math expression: (x+x)/x == 2
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            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 -ffast-math -funsafe-math-optimizations' for
this code:

int test(int x)
    return (x+x)/x;


test(int): # @test(int)
  lea eax, [rdi + rdi]
  idiv edi

Why? In this case we can return simply 2. Because there are only two corner
cases: when x is 0 and we have division by zero (it's UB), and when x+x is
integer overflow (it's also UB).

So we can simply optimize it. There are a lot of similar cases.

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