[RISCV] Implement getIPRACSRegs hook (#125586)

Fixes #124932.

This patch implements the getIPRACSRegs hook for RISC-V, similar to its introduction for x86 in commit 14b567d. This hook is necessary for correct code generation when Interprocedural Register Allocation (IPRA) is enabled, ensuring that the return address register (ra / x1) is correctly saved and restored when needed.

Unlike the x86 implementation, this patch only saves ra and does not yet include the frame pointer (fp). Further investigation is required to determine whether fp should also be preserved in all cases.

The test case is representative of a miscompile observed in the GCC torture suite (20090113-3.c), though similar failures occur in SPEC’s xz benchmark.

Author: mikhailramalho

llvm/lib/Target/RISCV/RISCVCallingConv.td

@@ -42,6 +42,8 @@ def CSR_ILP32D_LP64D_V
 // Needed for implementation of RISCVRegisterInfo::getNoPreservedMask()
 def CSR_NoRegs : CalleeSavedRegs<(add)>;
 
+def CSR_IPRA : CalleeSavedRegs<(add X1)>;
+
 // Interrupt handler needs to save/restore all registers that are used,
 // both Caller and Callee saved registers.
 def CSR_Interrupt : CalleeSavedRegs<(add X1, (sequence "X%u", 5, 31))>;

llvm/lib/Target/RISCV/RISCVRegisterInfo.cpp

@@ -55,6 +55,11 @@ RISCVRegisterInfo::RISCVRegisterInfo(unsigned HwMode)
     : RISCVGenRegisterInfo(RISCV::X1, /*DwarfFlavour*/0, /*EHFlavor*/0,
                            /*PC*/0, HwMode) {}
 
+const MCPhysReg *
+RISCVRegisterInfo::getIPRACSRegs(const MachineFunction *MF) const {
+  return CSR_IPRA_SaveList;
+}
+
 const MCPhysReg *
 RISCVRegisterInfo::getCalleeSavedRegs(const MachineFunction *MF) const {
   auto &Subtarget = MF->getSubtarget<RISCVSubtarget>();

llvm/lib/Target/RISCV/RISCVRegisterInfo.h

@@ -62,6 +62,8 @@ struct RISCVRegisterInfo : public RISCVGenRegisterInfo {
 
   const MCPhysReg *getCalleeSavedRegs(const MachineFunction *MF) const override;
 
+  const MCPhysReg *getIPRACSRegs(const MachineFunction *MF) const override;
+
   BitVector getReservedRegs(const MachineFunction &MF) const override;
   bool isAsmClobberable(const MachineFunction &MF,
                         MCRegister PhysReg) const override;

llvm/test/CodeGen/RISCV/ipra.ll

@@ -63,26 +63,29 @@ entry:
   ret i32 0
 }
 
-; FIXME: this function calls another function but doesn't save/restore ra
 define internal void @foobar(ptr %live_throughout.0.val) norecurse nounwind {
 ; RV64-LABEL: foobar:
 ; RV64:       # %bb.0: # %entry
 ; RV64-NEXT:    addi sp, sp, -48
+; RV64-NEXT:    sd ra, 40(sp) # 8-byte Folded Spill
 ; RV64-NEXT:    mv a1, a0
-; RV64-NEXT:    addi a0, sp, 16
-; RV64-NEXT:    addi a2, sp, 12
+; RV64-NEXT:    addi a0, sp, 8
+; RV64-NEXT:    addi a2, sp, 4
 ; RV64-NEXT:    call bmp_iter_set_init
+; RV64-NEXT:    ld ra, 40(sp) # 8-byte Folded Reload
 ; RV64-NEXT:    addi sp, sp, 48
 ; RV64-NEXT:    ret
 ;
 ; RV32-LABEL: foobar:
 ; RV32:       # %bb.0: # %entry
-; RV32-NEXT:    addi sp, sp, -32
+; RV32-NEXT:    addi sp, sp, -48
+; RV32-NEXT:    sw ra, 44(sp) # 4-byte Folded Spill
 ; RV32-NEXT:    mv a1, a0
-; RV32-NEXT:    addi a0, sp, 8
-; RV32-NEXT:    addi a2, sp, 4
+; RV32-NEXT:    addi a0, sp, 16
+; RV32-NEXT:    addi a2, sp, 12
 ; RV32-NEXT:    call bmp_iter_set_init
-; RV32-NEXT:    addi sp, sp, 32
+; RV32-NEXT:    lw ra, 44(sp) # 4-byte Folded Reload
+; RV32-NEXT:    addi sp, sp, 48
 ; RV32-NEXT:    ret
 ;
 ; RV64-WITHFP-LABEL: foobar: