[RISCV] Implement the TargetLowering::getRegisterByName hook

Summary: The hook should work for any RISC-V register. Non-allocatable registers
do not need to be reserved, for the remaining the hook will only succeed
if you pass clang the -ffixed-xX flag. This builds upon D67185, which
currently only allows reserving GPRs.

Reviewers: asb, lenary

Reviewed By: lenary

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69130

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -2894,3 +2894,22 @@ bool RISCVTargetLowering::shouldExtendTypeInLibCall(EVT Type) const {
 
   return true;
 }
+
+#define GET_REGISTER_MATCHER
+#include "RISCVGenAsmMatcher.inc"
+
+Register
+RISCVTargetLowering::getRegisterByName(const char *RegName, EVT VT,
+                                       const MachineFunction &MF) const {
+  Register Reg = MatchRegisterAltName(RegName);
+  if (Reg == RISCV::NoRegister)
+    Reg = MatchRegisterName(RegName);
+  if (Reg == RISCV::NoRegister)
+    report_fatal_error(
+        Twine("Invalid register name \"" + StringRef(RegName) + "\"."));
+  BitVector ReservedRegs = Subtarget.getRegisterInfo()->getReservedRegs(MF);
+  if (!ReservedRegs.test(Reg) && !Subtarget.isRegisterReservedByUser(Reg))
+    report_fatal_error(Twine("Trying to obtain non-reserved register \"" +
+                             StringRef(RegName) + "\"."));
+  return Reg;
+}

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -147,6 +147,13 @@ public:
 
   bool shouldExtendTypeInLibCall(EVT Type) const override;
 
+  /// Returns the register with the specified architectural or ABI name. This
+  /// method is necessary to lower the llvm.read_register.* and
+  /// llvm.write_register.* intrinsics. Allocatable registers must be reserved
+  /// with the clang -ffixed-xX flag for access to be allowed.
+  Register getRegisterByName(const char *RegName, EVT VT,
+                             const MachineFunction &MF) const override;
+
 private:
   void analyzeInputArgs(MachineFunction &MF, CCState &CCInfo,
                         const SmallVectorImpl<ISD::InputArg> &Ins,

llvm/test/CodeGen/RISCV/get-register-invalid.ll

@@ -0,0 +1,12 @@
+; RUN: not llc < %s -mtriple=riscv32 2>&1 | FileCheck %s
+
+define i32 @get_invalid_reg() nounwind {
+entry:
+; CHECK: Invalid register name "notareg".
+  %reg = call i32 @llvm.read_register.i32(metadata !0)
+  ret i32 %reg
+}
+
+declare i32 @llvm.read_register.i32(metadata) nounwind
+
+!0 = !{!"notareg\00"}

llvm/test/CodeGen/RISCV/get-register-noreserve.ll

@@ -0,0 +1,38 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=riscv32 | FileCheck %s
+
+define i32 @get_stack() nounwind {
+; CHECK-LABEL: get_stack:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    mv a0, sp
+; CHECK-NEXT:    ret
+entry:
+  %sp = call i32 @llvm.read_register.i32(metadata !0)
+  ret i32 %sp
+}
+
+define void @set_stack(i32 %val) nounwind {
+; CHECK-LABEL: set_stack:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    mv sp, a0
+; CHECK-NEXT:    ret
+entry:
+  call void @llvm.write_register.i32(metadata !0, i32 %val)
+  ret void
+}
+
+define i32 @get_tp_arch_name() nounwind {
+; CHECK-LABEL: get_tp_arch_name:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    mv a0, tp
+; CHECK-NEXT:    ret
+entry:
+  %sp = call i32 @llvm.read_register.i32(metadata !1)
+  ret i32 %sp
+}
+
+declare i32 @llvm.read_register.i32(metadata) nounwind
+declare void @llvm.write_register.i32(metadata, i32) nounwind
+
+!0 = !{!"sp\00"}
+!1 = !{!"x4\00"}

llvm/test/CodeGen/RISCV/get-register-reserve.ll

@@ -0,0 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: not llc < %s -mtriple=riscv32 -mattr +reserve-x8 2>&1 \
+; RUN:   | FileCheck -check-prefix=NO-RESERVE-A1 %s
+; RUN: not llc < %s -mtriple=riscv32 -mattr +reserve-x11 2>&1 \
+; RUN:   | FileCheck -check-prefix=NO-RESERVE-FP %s
+; RUN: llc < %s -mtriple=riscv32 -mattr +reserve-x8 -mattr +reserve-x11 \
+; RUN:   | FileCheck -check-prefix=RESERVE %s
+
+define i32 @get_reg_a1() nounwind {
+; NO-RESERVE-A1: Trying to obtain non-reserved register "a1".
+; RESERVE-LABEL: get_reg_a1:
+; RESERVE:       # %bb.0: # %entry
+; RESERVE-NEXT:    mv a0, a1
+; RESERVE-NEXT:    ret
+entry:
+  %a1 = call i32 @llvm.read_register.i32(metadata !0)
+  ret i32 %a1
+}
+
+define i32 @get_reg_fp() nounwind {
+; NO-RESERVE-FP: Trying to obtain non-reserved register "fp".
+; RESERVE-LABEL: get_reg_fp:
+; RESERVE:       # %bb.0: # %entry
+; RESERVE-NEXT:    mv a0, s0
+; RESERVE-NEXT:    ret
+entry:
+  %fp = call i32 @llvm.read_register.i32(metadata !1)
+  ret i32 %fp
+}
+
+declare i32 @llvm.read_register.i32(metadata) nounwind
+
+!0 = !{!"a1\00"}
+!1 = !{!"fp\00"}