- // Moving EFLAGS to / from another register requires a push and a pop.
- // Notice that we have to adjust the stack if we don't want to clobber the
- // first frame index. See X86FrameLowering.cpp - clobbersTheStack.
- if (SrcReg == X86::EFLAGS) {
- if (X86::GR64RegClass.contains(DestReg)) {
- BuildMI(MBB, MI, DL, get(X86::PUSHF64));
- BuildMI(MBB, MI, DL, get(X86::POP64r), DestReg);
- return;
+ bool FromEFLAGS = SrcReg == X86::EFLAGS;
+ bool ToEFLAGS = DestReg == X86::EFLAGS;
+ int Reg = FromEFLAGS ? DestReg : SrcReg;
+ bool is32 = X86::GR32RegClass.contains(Reg);
+ bool is64 = X86::GR64RegClass.contains(Reg);
+ if ((FromEFLAGS || ToEFLAGS) && (is32 || is64)) {
+ // The flags need to be saved, but saving EFLAGS with PUSHF/POPF is
+ // inefficient. Instead:
+ // - Save the overflow flag OF into AL using SETO, and restore it using a
+ // signed 8-bit addition of AL and INT8_MAX.
+ // - Save/restore the bottom 8 EFLAGS bits (CF, PF, AF, ZF, SF) to/from AH
+ // using LAHF/SAHF.
+ // - When RAX/EAX is live and isn't the destination register, make sure it
+ // isn't clobbered by PUSH/POP'ing it before and after saving/restoring
+ // the flags.
+ // This approach is ~2.25x faster than using PUSHF/POPF.
+ //
+ // This is still somewhat inefficient because we don't know which flags are
+ // actually live inside EFLAGS. Were we able to do a single SETcc instead of
+ // SETO+LAHF / ADDB+SAHF the code could be 1.02x faster.
+ //
+ // PUSHF/POPF is also potentially incorrect because it affects other flags
+ // such as TF/IF/DF, which LLVM doesn't model.
+ //
+ // Notice that we have to adjust the stack if we don't want to clobber the
+ // first frame index. See X86FrameLowering.cpp - clobbersTheStack.
+
+ int Mov = is64 ? X86::MOV64rr : X86::MOV32rr;
+ int Push = is64 ? X86::PUSH64r : X86::PUSH32r;
+ int Pop = is64 ? X86::POP64r : X86::POP32r;
+ int AX = is64 ? X86::RAX : X86::EAX;
+
+ bool AXDead = (Reg == AX) ||
+ (MachineBasicBlock::LQR_Dead ==
+ MBB.computeRegisterLiveness(&getRegisterInfo(), AX, MI));
+
+ if (!AXDead)
+ BuildMI(MBB, MI, DL, get(Push)).addReg(AX, getKillRegState(true));
+ if (FromEFLAGS) {
+ BuildMI(MBB, MI, DL, get(X86::SETOr), X86::AL);
+ BuildMI(MBB, MI, DL, get(X86::LAHF));
+ BuildMI(MBB, MI, DL, get(Mov), Reg).addReg(AX);