X86: Don't emit SAHF/LAHF for 64-bit targets unless explicitly supported

[oota-llvm.git] / lib / Target / X86 / X86InstrInfo.cpp

diff --git a/lib/Target/X86/X86InstrInfo.cpp b/lib/Target/X86/X86InstrInfo.cpp
index 12da3a9319e63ea9515477b2e9de4568e40ddfa9..ebe329064c50a5affe4379e4260db16b09e0803c 100644 (file)
--- a/lib/Target/X86/X86InstrInfo.cpp
+++ b/lib/Target/X86/X86InstrInfo.cpp
@@ -4385,7 +4385,32 @@ void X86InstrInfo::copyPhysReg(MachineBasicBlock &MBB,
   int Reg = FromEFLAGS ? DestReg : SrcReg;
   bool is32 = X86::GR32RegClass.contains(Reg);
   bool is64 = X86::GR64RegClass.contains(Reg);
+
   if ((FromEFLAGS || ToEFLAGS) && (is32 || is64)) {
+    int Mov = is64 ? X86::MOV64rr : X86::MOV32rr;
+    int Push = is64 ? X86::PUSH64r : X86::PUSH32r;
+    int PushF = is64 ? X86::PUSHF64 : X86::PUSHF32;
+    int Pop = is64 ? X86::POP64r : X86::POP32r;
+    int PopF = is64 ? X86::POPF64 : X86::POPF32;
+    int AX = is64 ? X86::RAX : X86::EAX;
+
+    if (!Subtarget.hasLAHFSAHF()) {
+      assert(is64 && "Not having LAHF/SAHF only happens on 64-bit.");
+      // 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 (FromEFLAGS) {
+        BuildMI(MBB, MI, DL, get(PushF));
+        BuildMI(MBB, MI, DL, get(Pop), DestReg);
+      }
+      if (ToEFLAGS) {
+        BuildMI(MBB, MI, DL, get(Push))
+            .addReg(SrcReg, getKillRegState(KillSrc));
+        BuildMI(MBB, MI, DL, get(PopF));
+      }
+      return;
+    }
+
     // 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
@@ -4407,14 +4432,20 @@ void X86InstrInfo::copyPhysReg(MachineBasicBlock &MBB,
     // 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));
+    bool AXDead = (Reg == AX);
+    // FIXME: The above could figure out that AX is dead in more cases with:
+    //          || (MachineBasicBlock::LQR_Dead ==
+    //            MBB.computeRegisterLiveness(&getRegisterInfo(), AX, MI));
+    //
+    //        Unfortunately this is slightly broken, see PR24535 and the likely
+    //        related PR25033 PR24991 PR24992 PR25201. These issues seem to
+    //        showcase sub-register / super-register confusion: a previous kill
+    //        of AH but no kill of AL leads computeRegisterLiveness to
+    //        erroneously conclude that AX is dead.
+    //
+    //        Once fixed, also update cmpxchg-clobber-flags.ll and
+    //        peephole-na-phys-copy-folding.ll.
 
     if (!AXDead)
       BuildMI(MBB, MI, DL, get(Push)).addReg(AX, getKillRegState(true));