[PATCH] D12882: [SimplifyCFG] do not speculate fdiv by default; it's expensive (PR24818)

[Dimitry Andric via llvm-commits]

dim added a subscriber: dim.
dim added a comment.

FWIW, this works for my test case in bug 24343 <https://llvm.org/bugs/show_bug.cgi?id=24343>.  Before http://reviews.llvm.org/D12282:

  double foo(int n, double d)
  {
    if (n > 0)
      d = 1.0 / d;
    return d;
  }

compiles to:

  foo:                                    # @foo
  # BB#0:                                 # %entry
          pushl       %ebp
          movl        %esp, %ebp
          fldl        12(%ebp)
          cmpl        $0, 8(%ebp)
          fld1
          fdiv        %st(1)
          jg  .LBB0_2
  # BB#1:                                 # %entry
          fstp        %st(0)
          fldz
          fxch        %st(1)
  .LBB0_2:                                # %entry
          fstp        %st(1)
          popl        %ebp
          retl

After http://reviews.llvm.org/D12282, it compiles to:

  foo:                                    # @foo
  # BB#0:                                 # %entry
          pushl       %ebp
          movl        %esp, %ebp
          fldl        12(%ebp)
          cmpl        $0, 8(%ebp)
          jle .LBB0_2
  # BB#1:                                 # %if.then
          fld1
          fdivp       %st(1)
  .LBB0_2:                                # %if.end
          popl        %ebp
          retl

So the fdiv is no longer executed before the comparison.


http://reviews.llvm.org/D12882