[RISCV] Implement isZextFree

This returns true for 8-bit and 16-bit loads, allowing LBU/LHU to be selected
and avoiding unnecessary masks.

llvm-svn: 330943

Author: asb

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -211,6 +211,20 @@ bool RISCVTargetLowering::isTruncateFree(EVT SrcVT, EVT DstVT) const {
   return (SrcBits == 64 && DestBits == 32);
 }
 
+bool RISCVTargetLowering::isZExtFree(SDValue Val, EVT VT2) const {
+  // Zexts are free if they can be combined with a load.
+  if (auto *LD = dyn_cast<LoadSDNode>(Val)) {
+    EVT MemVT = LD->getMemoryVT();
+    if ((MemVT == MVT::i8 || MemVT == MVT::i16 ||
+         (Subtarget.is64Bit() && MemVT == MVT::i32)) &&
+        (LD->getExtensionType() == ISD::NON_EXTLOAD ||
+         LD->getExtensionType() == ISD::ZEXTLOAD))
+      return true;
+  }
+
+  return TargetLowering::isZExtFree(Val, VT2);
+}
+
 // Changes the condition code and swaps operands if necessary, so the SetCC
 // operation matches one of the comparisons supported directly in the RISC-V
 // ISA.

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -46,6 +46,7 @@ class RISCVTargetLowering : public TargetLowering {
   bool isLegalAddImmediate(int64_t Imm) const override;
   bool isTruncateFree(Type *SrcTy, Type *DstTy) const override;
   bool isTruncateFree(EVT SrcVT, EVT DstVT) const override;
+  bool isZExtFree(SDValue Val, EVT VT2) const override;
 
   // Provide custom lowering hooks for some operations.
   SDValue LowerOperation(SDValue Op, SelectionDAG &DAG) const override;

llvm/test/CodeGen/RISCV/zext-with-load-is-free.ll

@@ -15,8 +15,7 @@ define i32 @test_zext_i8() {
 ; RV32I-NEXT:    bne a0, a1, .LBB0_3
 ; RV32I-NEXT:  # %bb.1: # %entry
 ; RV32I-NEXT:    lui a0, %hi(bytes+1)
-; RV32I-NEXT:    lb a0, %lo(bytes+1)(a0)
-; RV32I-NEXT:    andi a0, a0, 255
+; RV32I-NEXT:    lbu a0, %lo(bytes+1)(a0)
 ; RV32I-NEXT:    addi a1, zero, 7
 ; RV32I-NEXT:    bne a0, a1, .LBB0_3
 ; RV32I-NEXT:  # %bb.2: # %if.end
@@ -46,15 +45,13 @@ define i32 @test_zext_i16() {
 ; RV32I-LABEL: test_zext_i16:
 ; RV32I:       # %bb.0: # %entry
 ; RV32I-NEXT:    lui a0, 16
-; RV32I-NEXT:    addi a1, a0, -120
-; RV32I-NEXT:    lui a2, %hi(shorts)
-; RV32I-NEXT:    lhu a2, %lo(shorts)(a2)
-; RV32I-NEXT:    bne a2, a1, .LBB1_3
+; RV32I-NEXT:    addi a0, a0, -120
+; RV32I-NEXT:    lui a1, %hi(shorts)
+; RV32I-NEXT:    lhu a1, %lo(shorts)(a1)
+; RV32I-NEXT:    bne a1, a0, .LBB1_3
 ; RV32I-NEXT:  # %bb.1: # %entry
-; RV32I-NEXT:    lui a1, %hi(shorts+2)
-; RV32I-NEXT:    lh a1, %lo(shorts+2)(a1)
-; RV32I-NEXT:    addi a0, a0, -1
-; RV32I-NEXT:    and a0, a1, a0
+; RV32I-NEXT:    lui a0, %hi(shorts+2)
+; RV32I-NEXT:    lhu a0, %lo(shorts+2)(a0)
 ; RV32I-NEXT:    addi a1, zero, 7
 ; RV32I-NEXT:    bne a0, a1, .LBB1_3
 ; RV32I-NEXT:  # %bb.2: # %if.end