[RISCV] Break the (czero_eqz x, (setne x, 0)) -> x combine into 2 combines.

[topperc]

We can think of this as two separate combines

(czero_eqz x, (setne y, 0)) -> (czero_eqz x, y)
and
(czero_eqz x, x) -> x

Similary the (czero_nez x, (seteq x, 0)) -> x combine can be broken into

(czero_nez x, (seteq y, 0)) -> (czero_eqz x, y)
and
(czero_eqz x, x) -> x

isel already does the (czero_eqz x, (setne y, 0)) -> (czero_eqz x, y) and (czero_nez x, (seteq y, 0)) -> (czero_eqz x, y) combines, but doing them early could expose other opportunities.

[llvmbot]

@llvm/pr-subscribers-backend-risc-v

Author: Craig Topper (topperc)

Changes

We can think of this as two separate combines

(czero_eqz x, (setne y, 0)) -> (czero_eqz x, y)
and
(czero_eqz x, x) -> x

Similary the (czero_nez x, (seteq x, 0)) -> x combine can be broken into

(czero_nez x, (seteq y, 0)) -> (czero_eqz x, y)
and
(czero_eqz x, x) -> x

isel already does the (czero_eqz x, (setne y, 0)) -> (czero_eqz x, y) and (czero_nez x, (seteq y, 0)) -> (czero_eqz x, y) combines, but doing them early could expose other opportunities.

---

Full diff: https://github.com/llvm/llvm-project/pull/90428.diff

1 Files Affected:

• (modified) llvm/lib/Target/RISCV/RISCVISelLowering.cpp (+28-20)

diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 3ab9e7d69105ca..069465a53a03e8 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -16165,28 +16165,36 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
     return performSELECTCombine(N, DAG, Subtarget);
   case RISCVISD::CZERO_EQZ:
   case RISCVISD::CZERO_NEZ: {
-    SDValue LHS = N->getOperand(0);
-    SDValue RHS = N->getOperand(1);
-    // czero_eq X, (xor Y, 1) -> czero_ne X, Y if Y is 0 or 1.
-    // czero_ne X, (xor Y, 1) -> czero_eq X, Y if Y is 0 or 1.
-    if (RHS.getOpcode() == ISD::XOR && isOneConstant(RHS.getOperand(1))) {
-      SDValue Cond = RHS.getOperand(0);
+    SDValue Val = N->getOperand(0);
+    SDValue Cond = N->getOperand(1);
+
+    unsigned Opc = N->getOpcode();
+
+    // czero_eqz x, x -> x
+    if (Opc && Val == Cond)
+      return Val;
+
+    unsigned InvOpc =
+        Opc == RISCVISD::CZERO_EQZ ? RISCVISD::CZERO_NEZ : RISCVISD::CZERO_EQZ;
+
+    // czero_eqz X, (xor Y, 1) -> czero_nez X, Y if Y is 0 or 1.
+    // czero_nez X, (xor Y, 1) -> czero_eqz X, Y if Y is 0 or 1.
+    if (Cond.getOpcode() == ISD::XOR && isOneConstant(Cond.getOperand(1))) {
+      Cond = Cond.getOperand(0);
       APInt Mask = APInt::getBitsSetFrom(Cond.getValueSizeInBits(), 1);
-      if (DAG.MaskedValueIsZero(Cond, Mask)) {
-        unsigned NewOpc = N->getOpcode() == RISCVISD::CZERO_EQZ
-                              ? RISCVISD::CZERO_NEZ
-                              : RISCVISD::CZERO_EQZ;
-        return DAG.getNode(NewOpc, SDLoc(N), N->getValueType(0), LHS, Cond);
-      }
+      if (DAG.MaskedValueIsZero(Cond, Mask))
+        return DAG.getNode(InvOpc, SDLoc(N), N->getValueType(0), Val, Cond);
+    }
+    // czero_eqz x, (setcc y, 0, ne) -> czero_eqz x, y
+    // czero_nez x, (setcc y, 0, ne) -> czero_nez x, y
+    // czero_eqz x, (setcc y, 0, eq) -> czero_nez x, y
+    // czero_nez x, (setcc y, 0, eq) -> czero_eqz x, y
+    if (Cond.getOpcode() == ISD::SETCC && isNullConstant(Cond.getOperand(1))) {
+      ISD::CondCode CCVal = cast<CondCodeSDNode>(Cond.getOperand(2))->get();
+      if (ISD::isIntEqualitySetCC(CCVal))
+        return DAG.getNode(CCVal == ISD::SETNE ? Opc : InvOpc, SDLoc(N),
+                           N->getValueType(0), Val, Cond.getOperand(0));
     }
-    // czero_eqz x, (setcc x, 0, ne) -> x
-    // czero_nez x, (setcc x, 0, eq) -> x
-    if (RHS.getOpcode() == ISD::SETCC && isNullConstant(RHS.getOperand(1)) &&
-        cast<CondCodeSDNode>(RHS.getOperand(2))->get() ==
-            (N->getOpcode() == RISCVISD::CZERO_EQZ ? ISD::CondCode::SETNE
-                                                   : ISD::CondCode::SETEQ) &&
-        LHS == RHS.getOperand(0))
-      return LHS;
     return SDValue();
   }
   case RISCVISD::SELECT_CC: {

[topperc]

CC @zengdage

[dtcxzyw]

I think the following logic can be removed now.

llvm-project/llvm/lib/Target/RISCV/RISCVInstrInfoZicond.td

Lines 42 to 49 in 3785d74

	def : Pat<(XLenVT (riscv_czero_eqz GPR:$rs1, (riscv_setne (XLenVT GPR:$rc)))),
	(CZERO_EQZ GPR:$rs1, GPR:$rc)>;
	def : Pat<(XLenVT (riscv_czero_eqz GPR:$rs1, (riscv_seteq (XLenVT GPR:$rc)))),
	(CZERO_NEZ GPR:$rs1, GPR:$rc)>;
	def : Pat<(XLenVT (riscv_czero_nez GPR:$rs1, (riscv_setne (XLenVT GPR:$rc)))),
	(CZERO_NEZ GPR:$rs1, GPR:$rc)>;
	def : Pat<(XLenVT (riscv_czero_nez GPR:$rs1, (riscv_seteq (XLenVT GPR:$rc)))),
	(CZERO_EQZ GPR:$rs1, GPR:$rc)>;

[topperc]

Those can't remove removed. They handle cases other than RHS being 0.

[dtcxzyw]

Hmm, this patch seems like a NFC(I) change :) If not, please add some tests.

[dtcxzyw]

LGTM.

[topperc]

Hmm, this patch seems like a NFC(I) change :) If not, please add some tests.

I don't think it's is quite NFC. Removing the setcc in DAGCombine can change use counts and enable other combines that may check that the setcc has only 1 use. I don't have a specific example in mind.