[RISCV] Add feature forced-sw-shadow-stack to decide use hw/sw implemenet for shadow stack.

Author: Yeting Kuo

llvm/lib/Target/RISCV/RISCVFeatures.td

@@ -1044,3 +1044,8 @@ def FeatureTaggedGlobals : SubtargetFeature<"tagged-globals",
     "AllowTaggedGlobals",
     "true", "Use an instruction sequence for taking the address of a global "
     "that allows a memory tag in the upper address bits">;
+
+def FeatureForcedSWShadowStack : SubtargetFeature<
+    "forced-sw-shadow-stack", "HasForcedSWShadowStack", "true",
+    "Implement shadow stack with software.">;
+def HasForcedSWShadowStack : Predicate<"Subtarget->hasForcedSWShadowStack()">;

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

@@ -27,11 +27,6 @@
 
 using namespace llvm;
 
-static cl::opt<bool>
-    HardwareShadowStack("riscv-hardware-shadow-stack", cl::init(false),
-                        cl::Hidden,
-                        cl::desc("Enable hardware shadow stack with Zicfiss."));
-
 static const Register AllPopRegs[] = {
     RISCV::X1,  RISCV::X8,  RISCV::X9,  RISCV::X18, RISCV::X19,
     RISCV::X20, RISCV::X21, RISCV::X22, RISCV::X23, RISCV::X24,
@@ -57,9 +52,8 @@ static void emitSCSPrologue(MachineFunction &MF, MachineBasicBlock &MBB,
     return;
 
   const RISCVInstrInfo *TII = STI.getInstrInfo();
-  if (HardwareShadowStack) {
-    if (!STI.hasFeature(RISCV::FeatureStdExtZicfiss))
-      report_fatal_error("Hardware shadow stack needs Zicfiss to be enabled");
+  if (!STI.hasForcedSWShadowStack() &&
+      STI.hasFeature(RISCV::FeatureStdExtZicfiss)) {
     BuildMI(MBB, MI, DL, TII->get(RISCV::SSPUSH)).addReg(RAReg);
     return;
   }
@@ -119,9 +113,8 @@ static void emitSCSEpilogue(MachineFunction &MF, MachineBasicBlock &MBB,
     return;
 
   const RISCVInstrInfo *TII = STI.getInstrInfo();
-  if (HardwareShadowStack) {
-    if (!STI.hasFeature(RISCV::FeatureStdExtZicfiss))
-      report_fatal_error("Hardware shadow stack needs Zicfiss to be enabled");
+  if (!STI.hasForcedSWShadowStack() &&
+      STI.hasFeature(RISCV::FeatureStdExtZicfiss)) {
     BuildMI(MBB, MI, DL, TII->get(RISCV::SSPOPCHK)).addReg(RAReg);
     return;
   }

llvm/test/CodeGen/RISCV/shadowcallstack.ll

@@ -3,10 +3,14 @@
 ; RUN:   | FileCheck %s --check-prefix=RV32
 ; RUN: llc -mtriple=riscv64 -verify-machineinstrs < %s \
 ; RUN:   | FileCheck %s --check-prefix=RV64
-; RUN: llc -mtriple=riscv32 -mattr=+experimental-zicfiss -riscv-hardware-shadow-stack -verify-machineinstrs < %s \
-; RUN:   | FileCheck %s --check-prefix=RV32-ZICFISS
-; RUN: llc -mtriple=riscv64 -mattr=+experimental-zicfiss -riscv-hardware-shadow-stack -verify-machineinstrs < %s \
-; RUN:   | FileCheck %s --check-prefix=RV64-ZICFISS
+; RUN: llc -mtriple=riscv32 -mattr=+experimental-zicfiss < %s \
+; RUN:   -verify-machineinstrs | FileCheck %s --check-prefix=RV32-ZICFISS
+; RUN: llc -mtriple=riscv64 -mattr=+experimental-zicfiss < %s \
+; RUN:   -verify-machineinstrs | FileCheck %s --check-prefix=RV64-ZICFISS
+; RUN: llc -mtriple=riscv32 -mattr=+experimental-zicfiss,forced-sw-shadow-stack \
+; RUN:    -verify-machineinstrs < %s | FileCheck %s --check-prefix=RV32
+; RUN: llc -mtriple=riscv64 -mattr=+experimental-zicfiss,forced-sw-shadow-stack \
+; RUN:    -verify-machineinstrs < %s | FileCheck %s --check-prefix=RV64
 
 define void @f1() shadowcallstack {
 ; RV32-LABEL: f1: