[RISCV] Generate 32 bits jumptable entries when code model is small

The code can only address the whole RV32 address space or the lower 2 GiB
of the RV64 address space in small code model, so 32 bits entry is enough.
Cache hit ratio and code size have some improvements.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D116435

3 files changed, 28 insertions, 2 deletions

diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 5ad708be75be..fd295fedbb9e 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -24,6 +24,7 @@
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineJumpTableInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/TargetLoweringObjectFileImpl.h"
 #include "llvm/CodeGen/ValueTypes.h"
@@ -10127,6 +10128,24 @@ bool RISCVTargetLowering::shouldConvertFpToSat(unsigned Op, EVT FPVT,
   }
 }
 
+unsigned RISCVTargetLowering::getJumpTableEncoding() const {
+  // If we are using the small code model, we can reduce size of jump table
+  // entry to 4 bytes.
+  if (Subtarget.is64Bit() && !isPositionIndependent() &&
+      getTargetMachine().getCodeModel() == CodeModel::Small) {
+    return MachineJumpTableInfo::EK_Custom32;
+  }
+  return TargetLowering::getJumpTableEncoding();
+}
+
+const MCExpr *RISCVTargetLowering::LowerCustomJumpTableEntry(
+    const MachineJumpTableInfo *MJTI, const MachineBasicBlock *MBB,
+    unsigned uid, MCContext &Ctx) const {
+  assert(Subtarget.is64Bit() && !isPositionIndependent() &&
+         getTargetMachine().getCodeModel() == CodeModel::Small);
+  return MCSymbolRefExpr::create(MBB->getSymbol(), Ctx);
+}
+
 bool RISCVTargetLowering::isFMAFasterThanFMulAndFAdd(const MachineFunction &MF,
                                                      EVT VT) const {
   VT = VT.getScalarType();
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.h b/llvm/lib/Target/RISCV/RISCVISelLowering.h
index 23e5d89af081..4537dd7ba426 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.h
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.h
@@ -531,6 +531,13 @@ public:
   SDValue BuildSDIVPow2(SDNode *N, const APInt &Divisor, SelectionDAG &DAG,
                         SmallVectorImpl<SDNode *> &Created) const override;
 
+  unsigned getJumpTableEncoding() const override;
+
+  const MCExpr *LowerCustomJumpTableEntry(const MachineJumpTableInfo *MJTI,
+                                          const MachineBasicBlock *MBB,
+                                          unsigned uid,
+                                          MCContext &Ctx) const override;
+
 private:
   /// RISCVCCAssignFn - This target-specific function extends the default
   /// CCValAssign with additional information used to lower RISC-V calling
diff --git a/llvm/test/CodeGen/RISCV/jumptable.ll b/llvm/test/CodeGen/RISCV/jumptable.ll
index 10a7938c652a..ab638aaefead 100644
--- a/llvm/test/CodeGen/RISCV/jumptable.ll
+++ b/llvm/test/CodeGen/RISCV/jumptable.ll
@@ -239,11 +239,11 @@ define void @above_threshold(i32 %in, i32* %out) nounwind {
 ; RV64I-SMALL-NEXT:    li a2, 5
 ; RV64I-SMALL-NEXT:    bltu a2, a0, .LBB1_9
 ; RV64I-SMALL-NEXT:  # %bb.1: # %entry
-; RV64I-SMALL-NEXT:    slli a0, a0, 3
+; RV64I-SMALL-NEXT:    slli a0, a0, 2
 ; RV64I-SMALL-NEXT:    lui a2, %hi(.LJTI1_0)
 ; RV64I-SMALL-NEXT:    addi a2, a2, %lo(.LJTI1_0)
 ; RV64I-SMALL-NEXT:    add a0, a0, a2
-; RV64I-SMALL-NEXT:    ld a0, 0(a0)
+; RV64I-SMALL-NEXT:    lw a0, 0(a0)
 ; RV64I-SMALL-NEXT:    jr a0
 ; RV64I-SMALL-NEXT:  .LBB1_2: # %bb1
 ; RV64I-SMALL-NEXT:    li a0, 4