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[lldb] Implement RISCV function unwinding using instruction emulation #147434

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[lldb] Implement RISCV function unwinding using instruction emulation #147434

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20 changes: 20 additions & 0 deletions lldb/source/Plugins/Instruction/RISCV/EmulateInstructionRISCV.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1899,4 +1899,24 @@ RISCVSingleStepBreakpointLocationsPredictor::HandleAtomicSequence(
return bp_addrs;
}

bool EmulateInstructionRISCV::CreateFunctionEntryUnwind(
UnwindPlan &unwind_plan) {
unwind_plan.Clear();
unwind_plan.SetRegisterKind(eRegisterKindLLDB);

UnwindPlan::Row row;

// Our previous Call Frame Address is the stack pointer
row.GetCFAValue().SetIsRegisterPlusOffset(gpr_sp_riscv, 0);
row.SetRegisterLocationToSame(gpr_fp_riscv, /*must_replace=*/false);

unwind_plan.AppendRow(std::move(row));
unwind_plan.SetSourceName("EmulateInstructionRISCV");
unwind_plan.SetSourcedFromCompiler(eLazyBoolNo);
unwind_plan.SetUnwindPlanValidAtAllInstructions(eLazyBoolYes);
unwind_plan.SetUnwindPlanForSignalTrap(eLazyBoolNo);
unwind_plan.SetReturnAddressRegister(gpr_ra_riscv);
return true;
}

} // namespace lldb_private
11 changes: 7 additions & 4 deletions lldb/source/Plugins/Instruction/RISCV/EmulateInstructionRISCV.h
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Expand Up @@ -57,11 +57,12 @@ class EmulateInstructionRISCV : public EmulateInstruction {

static bool SupportsThisInstructionType(InstructionType inst_type) {
switch (inst_type) {
case eInstructionTypeAny:
case eInstructionTypePCModifying:
case lldb_private::eInstructionTypeAny:
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This method is inside a namespace lldb_private, please don't add lldb_private:: specifiers.

case lldb_private::eInstructionTypePrologueEpilogue:
return true;
case eInstructionTypePrologueEpilogue:
case eInstructionTypeAll:

case lldb_private::eInstructionTypePCModifying:
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@jasonmolenda jasonmolenda Jul 9, 2025 •
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EmulateInstructionRISCV is currently used to correctly predict where a given instruction will branch to next, for instruction stepping on hardware that does not support instruction step. By changing eInstructionTypePCModifying to return false from SupportsThisInstructionType, you're going to break that use of this. Did you copy and paste this from another EmulateInstruction plugin? Modifying this plugin without a much deeper understanding of what it is doing & testing it carefully against a corpus of functions is not going to work; this isn't a minor edit.

case lldb_private::eInstructionTypeAll:
return false;
}
llvm_unreachable("Fully covered switch above!");
Expand Down Expand Up @@ -94,6 +95,8 @@ class EmulateInstructionRISCV : public EmulateInstruction {
std::optional<RegisterInfo> GetRegisterInfo(lldb::RegisterKind reg_kind,
uint32_t reg_num) override;

bool CreateFunctionEntryUnwind(UnwindPlan &unwind_plan) override;

std::optional<DecodeResult> ReadInstructionAt(lldb::addr_t addr);
std::optional<DecodeResult> Decode(uint32_t inst);
bool Execute(DecodeResult inst, bool ignore_cond);
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