[llvm] [RISCV][GISel] Instruction selection for G_JUMP_TABLE and G_BRJT. (PR #71987)

[Michael Maitland via llvm-commits]

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@@ -493,6 +498,50 @@ bool RISCVInstructionSelector::select(MachineInstr &MI) {
     MI.eraseFromParent();
     return constrainSelectedInstRegOperands(*Bcc, TII, TRI, RBI);
   }
+  case TargetOpcode::G_BRJT: {
+    // FIXME: Move to legalization?
+    unsigned EntrySize =
+        MF.getJumpTableInfo()->getEntrySize(MF.getDataLayout());
+    assert((EntrySize == 4 || (Subtarget->is64Bit() && EntrySize == 8)) &&
+           "Unsupported jump-table entry size");
+
+    auto SLL =
+        MIB.buildInstr(RISCV::SLLI, {&RISCV::GPRRegClass}, {MI.getOperand(2)})
+            .addImm(Log2_32(EntrySize));
+    if (!SLL.constrainAllUses(TII, TRI, RBI))
+      return false;
+
+    // TODO: Use SHXADD. Moving to legalization would fix this automatically.
+    auto ADD = MIB.buildInstr(RISCV::ADD, {&RISCV::GPRRegClass},
+                              {MI.getOperand(0), SLL.getReg(0)});
+    if (!ADD.constrainAllUses(TII, TRI, RBI))
+      return false;
+
+    unsigned LdOpc = EntrySize == 8 ? RISCV::LD : RISCV::LW;
+    auto Dest =
+        MIB.buildInstr(LdOpc, {&RISCV::GPRRegClass}, {ADD.getReg(0)})
+            .addImm(0)
+            .addMemOperand(MF.getMachineMemOperand(
+                MachinePointerInfo::getJumpTable(MF), MachineMemOperand::MOLoad,
+                EntrySize, Align(EntrySize)));
+    if (!Dest.constrainAllUses(TII, TRI, RBI))
+      return false;
+
+    if (MF.getTarget().isPositionIndependent()) {
+      Dest = MIB.buildInstr(RISCV::ADD, {&RISCV::GPRRegClass},
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michaelmaitland wrote:

What does this code do? Why do we only do it for position independent?

https://github.com/llvm/llvm-project/pull/71987