[SDAG] Legalize all CondCodes by inverting them and/or swapping operands

[kparzysz]

Summary

When expanding a CondCode, try all combinations of inverting the condition code and swapping the operands.

[lebedev.ri]

I believe this either needs [NFC] in subject, or tests.

[kparzysz]

There is no test because nobody uses that yet. The additional situations that this handles used to abort compilation. I have a patch for Hexagon that uses this, but I want to keep it separate from the target-independent part.

[efriedma]

lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

1638–1654

Could you put the code to invert the condition code here, instead of inside the switch, so it doesn't get mixed up with the floating-point-specific stuff?

[kparzysz]

Place the added piece of swapping/inverting code together with the pre-existing part.

[efriedma]

LGTM

[dschuff]

So, it turns out that this actually does change functionality on targets (such as WebAssembly) that use the expansion for unordered comparisons because it tries inverting the condition before falling all the way through to line 1680. This actually results in better code for wasm; see https://reviews.llvm.org/rL324305

[kparzysz]

In the first version of the patch, the added code was executing last. In the committed version it takes precedence over the floating point code, so you're right, it wasn't really an NFC. I'm glad the result is a better code.

lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

 /// \returns true if the SetCC has been legalized, false if it hasn't.
 bool SelectionDAGLegalize::LegalizeSetCCCondCode(EVT VT, SDValue &LHS,
                                                  SDValue &RHS, SDValue &CC,
                                                  bool &NeedInvert,
                                                  const SDLoc &dl) {
   MVT OpVT = LHS.getSimpleValueType();
   ISD::CondCode CCCode = cast<CondCodeSDNode>(CC)->get();
   NeedInvert = false;
+  bool NeedSwap = false;
   switch (TLI.getCondCodeAction(CCCode, OpVT)) {
   default: llvm_unreachable("Unknown condition code action!");
   case TargetLowering::Legal:
     // Nothing to do.
     break;
   case TargetLowering::Expand: {
     ISD::CondCode InvCC = ISD::getSetCCSwappedOperands(CCCode);
     if (TLI.isCondCodeLegal(InvCC, OpVT)) {
       std::swap(LHS, RHS);
       CC = DAG.getCondCode(InvCC);
       return true;
     }
+    // Swapping operands didn't work. Try inverting the condition.
+    InvCC = getSetCCInverse(CCCode, OpVT.isInteger());
+    if (!TLI.isCondCodeLegal(InvCC, OpVT)) {
+      // If inverting the condition is not enough, try swapping operands
+      // on top of it.
+      InvCC = ISD::getSetCCSwappedOperands(InvCC);
+      NeedSwap = true;
+    }
+    if (TLI.isCondCodeLegal(InvCC, OpVT)) {
+      CC = DAG.getCondCode(InvCC);
+      NeedInvert = true;
+      if (NeedSwap)
+        std::swap(LHS, RHS);
+      return true;
+    }
+
     ISD::CondCode CC1 = ISD::SETCC_INVALID, CC2 = ISD::SETCC_INVALID;
     unsigned Opc = 0;
     switch (CCCode) {
     default: llvm_unreachable("Don't know how to expand this condition!");
     case ISD::SETO:
         assert(TLI.getCondCodeAction(ISD::SETOEQ, OpVT)
             == TargetLowering::Legal
             && "If SETO is expanded, SETOEQ must be legal!");
case ISD::SETULE:
           break;
         }
         // Fallthrough if we are unsigned integer.
         LLVM_FALLTHROUGH;
     case ISD::SETLE:
     case ISD::SETGT:
     case ISD::SETGE:
     case ISD::SETLT:
-      // We only support using the inverted operation, which is computed above
-      // and not a different manner of supporting expanding these cases.
-      llvm_unreachable("Don't know how to expand this condition!");
     case ISD::SETNE:
     case ISD::SETEQ:
-      // Try inverting the result of the inverse condition.
-      InvCC = CCCode == ISD::SETEQ ? ISD::SETNE : ISD::SETEQ;
-      if (TLI.isCondCodeLegal(InvCC, OpVT)) {
-        CC = DAG.getCondCode(InvCC);
-        NeedInvert = true;
-        return true;
-      }
-      // If inverting the condition didn't work then we have no means to expand
-      // the condition.
+      // If all combinations of inverting the condition and swapping operands
+      // didn't work then we have no means to expand the condition.
       llvm_unreachable("Don't know how to expand this condition!");
     }
 
     SDValue SetCC1, SetCC2;
     if (CCCode != ISD::SETO && CCCode != ISD::SETUO) {
       // If we aren't the ordered or unorder operation,
       // then the pattern is (LHS CC1 RHS) Opc (LHS CC2 RHS).
       SetCC1 = DAG.getSetCC(dl, VT, LHS, RHS, CC1);