Commit 886d2c2c authored by Fangrui Song's avatar Fangrui Song
Browse files

[BranchRelaxation] Simplify offset computation and fix a bug in adjustBlockOffsets() 

If Start!=0, adjustBlockOffsets() may unnecessarily adjust the offset of
Start. There is no correctness issue, but it can create more block
splits.
parent a7818e6f

llvm/lib/CodeGen/BranchRelaxation.cpp

+4 −9
Original line number Diff line number Diff line
@@ -67,16 +67,13 @@ class BranchRelaxation : public MachineFunctionPass { 
    unsigned postOffset(const MachineBasicBlock &MBB) const {

      const unsigned PO = Offset + Size;

      const Align Alignment = MBB.getAlignment();

      if (Alignment == 1)

        return PO;



      const Align ParentAlign = MBB.getParent()->getAlignment();

      if (Alignment <= ParentAlign)

        return PO + offsetToAlignment(PO, Alignment);

        return alignTo(PO, Alignment);



      // The alignment of this MBB is larger than the function's alignment, so we

      // can't tell whether or not it will insert nops. Assume that it will.

      return PO + Alignment.value() + offsetToAlignment(PO, Alignment);

      return alignTo(PO, Alignment) + Alignment.value() - ParentAlign.value();

    }

  };
@@ -129,7 +126,6 @@ void BranchRelaxation::verify() { 
  unsigned PrevNum = MF->begin()->getNumber();

  for (MachineBasicBlock &MBB : *MF) {

    const unsigned Num = MBB.getNumber();

    assert(isAligned(MBB.getAlignment(), BlockInfo[Num].Offset));

    assert(!Num || BlockInfo[PrevNum].postOffset(MBB) <= BlockInfo[Num].Offset);

    assert(BlockInfo[Num].Size == computeBlockSize(MBB));

    PrevNum = Num;
@@ -195,10 +191,9 @@ unsigned BranchRelaxation::getInstrOffset(const MachineInstr &MI) const { 


void BranchRelaxation::adjustBlockOffsets(MachineBasicBlock &Start) {

  unsigned PrevNum = Start.getNumber();

  for (auto &MBB : make_range(MachineFunction::iterator(Start), MF->end())) {

  for (auto &MBB :

       make_range(std::next(MachineFunction::iterator(Start)), MF->end())) {

    unsigned Num = MBB.getNumber();

    if (!Num) // block zero is never changed from offset zero.

      continue;

    // Get the offset and known bits at the end of the layout predecessor.

    // Include the alignment of the current block.

    BlockInfo[Num].Offset = BlockInfo[PrevNum].postOffset(MBB);

llvm/test/CodeGen/AArch64/branch-relax-bcc.ll

+3 −9
Original line number Diff line number Diff line
@@ -2,13 +2,10 @@ 


; CHECK-LABEL: invert_bcc:

; CHECK:      fcmp s0, s1

; CHECK-NEXT: b.eq [[JUMP_BB1:LBB[0-9]+_[0-9]+]]

; CHECK-NEXT: b [[JUMP_BB2:LBB[0-9]+_[0-9]+]]



; CHECK-NEXT: [[JUMP_BB1]]:

; CHECK-NEXT: b.ne [[JUMP_BB1:LBB[0-9]+_[0-9]+]]

; CHECK-NEXT: b [[BB1:LBB[0-9]+_[0-9]+]]



; CHECK-NEXT: [[JUMP_BB2]]:

; CHECK-NEXT: [[JUMP_BB1]]:

; CHECK-NEXT: b.vc [[BB2:LBB[0-9]+_[0-9]+]]

; CHECK-NEXT: b [[BB1]]
@@ -41,10 +38,7 @@ declare i32 @foo() #0 


; CHECK-LABEL: _block_split:

; CHECK: cmp w0, #5

; CHECK-NEXT: b.eq [[LONG_BR_BB:LBB[0-9]+_[0-9]+]]

; CHECK-NEXT: b [[LOR_LHS_FALSE_BB:LBB[0-9]+_[0-9]+]]



; CHECK: [[LONG_BR_BB]]:

; CHECK-NEXT: b.ne [[LOR_LHS_FALSE_BB:LBB[0-9]+_[0-9]+]]

; CHECK-NEXT: b [[IF_THEN_BB:LBB[0-9]+_[0-9]+]]



; CHECK: [[LOR_LHS_FALSE_BB]]:

llvm/test/CodeGen/AMDGPU/branch-relaxation.ll

+1 −2
Original line number Diff line number Diff line
@@ -491,8 +491,7 @@ ret: 


; GCN-LABEL: {{^}}long_branch_hang:

; GCN: s_cmp_lt_i32 s{{[0-9]+}}, 6

; GCN: s_cbranch_scc1 {{BB[0-9]+_[0-9]+}}

; GCN-NEXT: s_branch [[LONG_BR_0:BB[0-9]+_[0-9]+]]

; GCN: s_cbranch_scc0 [[LONG_BR_0:BB[0-9]+_[0-9]+]]

; GCN-NEXT: BB{{[0-9]+_[0-9]+}}:



; GCN: s_add_u32 s{{[0-9]+}}, s{{[0-9]+}}, [[LONG_BR_DEST0:BB[0-9]+_[0-9]+]]-(