Missing `isCall` and `isBranch` flags for RISCV instructions

[moste00]

The common root class Instruction defines several flags to roughly classify the semantics of an instruction, among them is an isCall and isBranch flags.

llvm-project/llvm/include/llvm/Target/Target.td

Line 646 in d297987

bit isBranch = false; // Is this instruction a branch instruction?

This flag is not set correctly in the definition of the JAL and the JALR instruction in RISCVInstrInfo.td

The instruction C_JR also doesn't have the isBranch flag set in RISCVInstrInfoC.td

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

Author: مصطفي محمود كمال الدين (moste00)

The common root class `Instruction` defines several flags to roughly classify the semantics of an instruction, among them is an `isCall` and `isBranch` flags. https://github.com/llvm/llvm-project/blob/d29798767cc10a0609b5f1f9f75f032b6760cf50/llvm/include/llvm/Target/Target.td#L646

<img width="1191" height="793" alt="Image" src="https://github.com/user-attachments/assets/1bb2f620-fa50-44f2-84ed-c8995d53cfac" />

This flag is not set correctly in the definition of the JAL and the JALR instruction in RISCVInstrInfo.td
<img width="1121" height="769" alt="Image" src="https://github.com/user-attachments/assets/c8d24fbf-9da1-4283-a218-ecd9d5a7262e" />

The instruction C_JR also doesn't have the isBranch flag set in RISCVInstrInfoC.td

<img width="1121" height="287" alt="Image" src="https://github.com/user-attachments/assets/c4104fa8-732b-4507-b1f4-e5c9f2b3e3d1" />

[lenary]

I believe these are not set for good reason.

JALR is not always a call, it might be a return, or an indirect branch, which it is depends on the register operands. These flags cannot depend on the (register) operands, so we use Pseudos in the backend, which have the information marked correctly.

JAL might also be a jump or a call, according to the same reasoning.

C.JR might be a return or an indirect branch.

[topperc]

The correction information is given by the functions in RISCVMCInstrAnalysis which check the registers.

[moste00]

@topperc @lenary Thanks for the quick response.

One other question: the compressed versions C_JAL and C_JALR have the isCall flag set, should it be removed or are they special cases and the reasoning here don't apply?

[lenary]

c.jal and c.jalr always set ra, so they're both definitely a call, all of the time.

Broadly:

• indirect jumps take their destination from a register
• direct jumps take their destination from a pc-relative immediate
• calls can either be direct or indirect jumps, but to be a call they have to set a "link address" (usually storing it to ra, but in some cases a different register) before jumping, hence "jump and link" or "jump and link register"
• returns are a kind of indirect jump that are categorised from control flow as much as from operands (though, an indirect jump to ra that does not set a link register is a return)