[PATCH] D68898: JumpThreading: enhance JT to handle BB with no successor and address comparison

[David Li via Phabricator via llvm-commits]

davidxl created this revision.
davidxl added reviewers: efriedma, wmi.
Herald added a subscriber: jfb.

Current JT only process (clone) BBs with multiple successors in JT with the aim to thread the predecessor with a successor BB. This misses opportunities to to handle return BB where the return value can be simplified with threading (cloning).

Example:

#include <array>
#include <algorithm>

constexpr std::array<int, 3> x = {1, 7, 17};

bool Contains(int i) {

  return std::find(x.begin(), x.end(), i) != x.end();

}

Clang produces inefficient code:

_Z8Containsi:                           # @_Z8Containsi

  .cfi_startproc

1. %bb.0: cmpl    $1, %edi je      .LBB0_1
2. %bb.2: cmpl    $7, %edi jne     .LBB0_3
3. %bb.4: movl    $_ZL1x+4, %eax jmp     .LBB0_5

.LBB0_1:

  movl    $_ZL1x, %eax
  jmp     .LBB0_5

.LBB0_3:

  cmpl    $17, %edi
  movl    $_ZL1x+8, %ecx
  movl    $_ZL1x+12, %eax
  cmoveq  %rcx, %rax

.LBB0_5:

  movl    $_ZL1x+12, %ecx
  cmpq    %rcx, %rax
  setne   %al
  retq

While GCC produces:

_Z8Containsi:
.LFB1534:

  .cfi_startproc
  movl    $1, %eax
  cmpl    $1, %edi
  je      .L1
  cmpl    $7, %edi
  je      .L1
  cmpl    $17, %edi
  sete    %al

.L1 <https://reviews.llvm.org/L1>:

  ret

This patch address the issue. After  the fix, the generated code looks like:

_Z8Containsi:                           # @_Z8Containsi

  .cfi_startproc
  addl    $-1, %edi
  cmpl    $16, %edi
  ja      .LBB0_2
  movl    $65601, %eax            # imm = 0x10041
  movl    %edi, %ecx
  shrl    %cl, %eax
  andb    $1, %al
  retq

.LBB0_2:                                # %_ZSt4findIPKiiET_S2_S2_RKT0_.exit.thread

  xorl    %eax, %eax
  retq


https://reviews.llvm.org/D68898

Files:
  include/llvm/Transforms/Scalar/JumpThreading.h
  lib/Transforms/Scalar/JumpThreading.cpp
  test/Transforms/JumpThreading/addr.ll
  test/Transforms/JumpThreading/return.ll

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