[PATCH] D28593: Update loop branch_weight metadata after loop rotation.

[Dehao Chen via Phabricator via llvm-commits]

danielcdh added a comment.

Looks like this patch will make the "always call" worse:

Without this patch:

  pushq   %rbx
  movq    %rdi, %rbx
  cmpl    $0, (%rbx)
  jne     .LBB1_3

.LBB1_1:                                # =>This Inner Loop Header: Depth=1

  movq    %rbx, %rdi
  callq   call_me
  cmpl    $0, (%rbx)
  je      .LBB1_1

.LBB1_3:

  popq    %rbx
  retq

With this patch:

  pushq   %rbx
  movq    %rdi, %rbx
  cmpl    $0, (%rbx)
  je      .LBB1_1

.LBB1_3:

  popq    %rbx
  retq

.LBB1_1:                                # =>This Inner Loop Header: Depth=1

  movq    %rbx, %rdi
  callq   call_me
  cmpl    $0, (%rbx)
  jne     .LBB1_3
  jmp     .LBB1_1

As the trip count of this loop is always 1, the first code will have no taken branches, while with this patch, it will have 2 taken branches.


https://reviews.llvm.org/D28593