[llvm-bugs] [Bug 41025] New: Missed optimization: finding closed form of loop doesn't take advantage of assumption

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41025

            Bug ID: 41025
           Summary: Missed optimization: finding closed form of loop
                    doesn't take advantage of assumption
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: arthur.j.odwyer at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

Clang trunk does a great job turning this sum-the-area-of-a-trapezoid function
into its closed form. However, the closed form needs a branch to deal with the
possibility that the distance from 'b' to 'e' is negative: in that case, 0
should be returned as a special case.

You can eliminate the special case by adding `__builtin_assume(b < e)` to the
top of the function. This eliminates the branch from the codegen of `f1`.

However, Clang is not currently able to eliminate the branch from the codegen
of `f2`, even though it is identical to `f1` except that f2 uses `i != e` as
the loop termination condition instead of `i < e`.

I don't know if this is a very deep and complicated bug that's not worth
fixing, or just a simple one-line update somewhere. :)


// https://godbolt.org/z/aWAahD

int f1(unsigned b, unsigned e)
{
    __builtin_assume( b < e );

    int total = 0;
    for (unsigned i = b; i < e; ++i) {
        total += i;
    }
    return total;
}

int f2(unsigned b, unsigned e)
{
    __builtin_assume( b < e );

    int total = 0;
    for (unsigned i = b; i != e; ++i) {
        total += i;
    }
    return total;
}

====

_Z2f1jj: # @_Z2f1jj
  movl %edi, %ecx
  notl %ecx
  addl %esi, %ecx
  leal 1(%rdi), %eax
  imull %ecx, %eax
  addl $-2, %esi
  subl %edi, %esi
  imulq %rcx, %rsi
  shrq %rsi
  addl %edi, %eax
  addl %esi, %eax
  retq

_Z2f2jj: # @_Z2f2jj
  xorl %eax, %eax
  cmpl %esi, %edi
  je .LBB1_2        // THIS BRANCH should be unnecessary AFAICT
  movl %edi, %ecx
  notl %ecx
  addl %esi, %ecx
  leal 1(%rdi), %eax
  imull %ecx, %eax
  addl $-2, %esi
  subl %edi, %esi
  imulq %rcx, %rsi
  shrq %rsi
  addl %edi, %eax
  addl %esi, %eax
.LBB1_2:
  retq

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