// This checks whether the transformation is legal. // Also returns false in cases where it's potentially legal, but // we don't even want to try. bool X86CallFrameOptimization::isLegal(MachineFunction &MF) { if (NoX86CFOpt.getValue()) return false; // We can't encode multiple DW_CFA_GNU_args_size or DW_CFA_def_cfa_offset // in the compact unwind encoding that Darwin uses. So, bail if there // is a danger of that being generated. if (STI->isTargetDarwin() && (!MF.getLandingPads().empty() || (MF.getFunction().needsUnwindTableEntry() && !TFL->hasFP(MF)))) return false; // It is not valid to change the stack pointer outside the prolog/epilog // on 64-bit Windows. if (STI->isTargetWin64()) return false; // You would expect straight-line code between call-frame setup and // call-frame destroy. You would be wrong. There are circumstances (e.g. // CMOV_GR8 expansion of a select that feeds a function call!) where we can // end up with the setup and the destroy in different basic blocks. // This is bad, and breaks SP adjustment. // So, check that all of the frames in the function are closed inside // the same block, and, for good measure, that there are no nested frames. unsigned FrameSetupOpcode = TII->getCallFrameSetupOpcode(); unsigned FrameDestroyOpcode = TII->getCallFrameDestroyOpcode(); for (MachineBasicBlock &BB : MF) { bool InsideFrameSequence = false; for (MachineInstr &MI : BB) { if (MI.getOpcode() == FrameSetupOpcode) { if (InsideFrameSequence) return false; InsideFrameSequence = true; } else if (MI.getOpcode() == FrameDestroyOpcode) { if (!InsideFrameSequence) return false; InsideFrameSequence = false; } } if (InsideFrameSequence) return false; } return true; }