[PATCH] D119099: [RISCV][DAGCombiner] Custom lower ISD::ABS to sra (X, size(X)-1); sub (xor (X, Y), Y).

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.
craig.topper added reviewers: RKSimon, spatel, lebedev.ri, asb, luismarques.
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Default lowering does sra (X, size(X)-1); Z = add(X, Y); xor(Z, Y)

If we put the sub at the end we give opportunities to use subw if
the later ops want the value sign extended.

I had to limit DAGCombiner to prevent an infinite loop.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D119099

Files:
  llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
  llvm/lib/Target/RISCV/RISCVISelLowering.cpp
  llvm/test/CodeGen/RISCV/rv32zbb.ll
  llvm/test/CodeGen/RISCV/rv64zbb.ll


Index: llvm/test/CodeGen/RISCV/rv64zbb.ll
===================================================================
--- llvm/test/CodeGen/RISCV/rv64zbb.ll
+++ llvm/test/CodeGen/RISCV/rv64zbb.ll
@@ -942,8 +942,8 @@
 ; RV64I:       # %bb.0:
 ; RV64I-NEXT:    sext.w a0, a0
 ; RV64I-NEXT:    srai a1, a0, 63
-; RV64I-NEXT:    add a0, a0, a1
 ; RV64I-NEXT:    xor a0, a0, a1
+; RV64I-NEXT:    sub a0, a0, a1
 ; RV64I-NEXT:    ret
 ;
 ; RV64ZBB-LABEL: abs_i32:
@@ -961,9 +961,8 @@
 ; RV64I-LABEL: abs_i32_sext:
 ; RV64I:       # %bb.0:
 ; RV64I-NEXT:    srai a1, a0, 63
-; RV64I-NEXT:    add a0, a0, a1
 ; RV64I-NEXT:    xor a0, a0, a1
-; RV64I-NEXT:    sext.w a0, a0
+; RV64I-NEXT:    subw a0, a0, a1
 ; RV64I-NEXT:    ret
 ;
 ; RV64ZBB-LABEL: abs_i32_sext:
@@ -982,8 +981,8 @@
 ; RV64I-LABEL: abs_i64:
 ; RV64I:       # %bb.0:
 ; RV64I-NEXT:    srai a1, a0, 63
-; RV64I-NEXT:    add a0, a0, a1
 ; RV64I-NEXT:    xor a0, a0, a1
+; RV64I-NEXT:    sub a0, a0, a1
 ; RV64I-NEXT:    ret
 ;
 ; RV64ZBB-LABEL: abs_i64:
Index: llvm/test/CodeGen/RISCV/rv32zbb.ll
===================================================================
--- llvm/test/CodeGen/RISCV/rv32zbb.ll
+++ llvm/test/CodeGen/RISCV/rv32zbb.ll
@@ -732,8 +732,8 @@
 ; RV32I-LABEL: abs_i32:
 ; RV32I:       # %bb.0:
 ; RV32I-NEXT:    srai a1, a0, 31
-; RV32I-NEXT:    add a0, a0, a1
 ; RV32I-NEXT:    xor a0, a0, a1
+; RV32I-NEXT:    sub a0, a0, a1
 ; RV32I-NEXT:    ret
 ;
 ; RV32ZBB-LABEL: abs_i32:
Index: llvm/lib/Target/RISCV/RISCVISelLowering.cpp
===================================================================
--- llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -250,6 +250,9 @@
   setOperationAction(ISD::SRL_PARTS, XLenVT, Custom);
   setOperationAction(ISD::SRA_PARTS, XLenVT, Custom);
 
+  if (!Subtarget.hasStdExtZbb())
+    setOperationAction(ISD::ABS, XLenVT, Custom);
+
   if (Subtarget.hasStdExtZbb() || Subtarget.hasStdExtZbp() ||
       Subtarget.hasStdExtZbkb()) {
     if (Subtarget.is64Bit()) {
@@ -5735,6 +5738,20 @@
   MVT VT = Op.getSimpleValueType();
   SDValue X = Op.getOperand(0);
 
+  if (VT.isScalarInteger()) {
+    assert(!Subtarget.hasStdExtZbb() && "Unexpected custom legalisation");
+    // Default lowering uses
+    //   Y = sra (X, size(X)-1); Z = add(X, Y); xor(Z, Y);
+    // We want to use
+    //   Y = sra (X, size(X)-1); sub (xor (X, Y), Y)
+    // This allows sign_extend_inreg to be combined with the sub.
+    SDValue Shift =
+        DAG.getNode(ISD::SRA, DL, VT, X,
+                    DAG.getConstant(VT.getSizeInBits() - 1, DL, VT));
+    SDValue Xor = DAG.getNode(ISD::XOR, DL, VT, X, Shift);
+    return DAG.getNode(ISD::SUB, DL, VT, Xor, Shift);
+  }
+
   assert(VT.isFixedLengthVector() && "Unexpected type");
 
   MVT ContainerVT = getContainerForFixedLengthVector(VT);
Index: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -3523,7 +3523,7 @@
   }
 
   // fold Y = sra (X, size(X)-1); sub (xor (X, Y), Y) -> (abs X)
-  if (TLI.isOperationLegalOrCustom(ISD::ABS, VT)) {
+  if (hasOperation(ISD::ABS, VT)) {
     if (N0.getOpcode() == ISD::XOR && N1.getOpcode() == ISD::SRA) {
       SDValue X0 = N0.getOperand(0), X1 = N0.getOperand(1);
       SDValue S0 = N1.getOperand(0);


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