[llvm] [RISCV] Check feature bits in getBrCond (PR #129859)

[Sudharsan Veeravalli via llvm-commits]

https://github.com/svs-quic updated https://github.com/llvm/llvm-project/pull/129859

>From 53c6f1d5618fd799a3d0fe586b8d3769fe1ec652 Mon Sep 17 00:00:00 2001
From: Sudharsan Veeravalli <quic_svs at quicinc.com>
Date: Wed, 5 Mar 2025 15:23:00 +0530
Subject: [PATCH 1/2] [RISCV] Check feature bits in getBrCond

The function currently only checks to see if the we compare against an
immediate before selecting the two branch immediate instructions that are
a part of the XCVbi vendor extension. This works at the moment since there are
no other extensions that have a branch immediate instruction but it would be better
if we explicitly check to see if the XCVbi extension is enabled before we return
the appropriate instruction. This is also done in preparation for the branch immediate
instructions that are a part of the Xqcibi vendor extension from Qualcomm.
---
 .../RISCV/GISel/RISCVInstructionSelector.cpp    |  3 ++-
 llvm/lib/Target/RISCV/RISCVInstrInfo.cpp        | 17 +++++++++++++----
 llvm/lib/Target/RISCV/RISCVInstrInfo.h          |  3 ++-
 3 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp b/llvm/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp
index 62fbe55dffba1..3550ab4c00476 100644
--- a/llvm/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp
+++ b/llvm/lib/Target/RISCV/GISel/RISCVInstructionSelector.cpp
@@ -789,7 +789,8 @@ bool RISCVInstructionSelector::select(MachineInstr &MI) {
     RISCVCC::CondCode CC;
     getOperandsForBranch(MI.getOperand(0).getReg(), CC, LHS, RHS, *MRI);
 
-    auto Bcc = MIB.buildInstr(RISCVCC::getBrCond(CC), {}, {LHS, RHS})
+    auto Bcc = MIB.buildInstr(RISCVCC::getBrCond(CC, STI.getFeatureBits()), {},
+                              {LHS, RHS})
                    .addMBB(MI.getOperand(1).getMBB());
     MI.eraseFromParent();
     return constrainSelectedInstRegOperands(*Bcc, TII, TRI, RBI);
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
index 913169c00b642..748b072e64969 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
@@ -953,14 +953,23 @@ static void parseCondBranch(MachineInstr &LastInst, MachineBasicBlock *&Target,
   Cond.push_back(LastInst.getOperand(1));
 }
 
-unsigned RISCVCC::getBrCond(RISCVCC::CondCode CC, bool Imm) {
+unsigned RISCVCC::getBrCond(RISCVCC::CondCode CC,
+                            const FeatureBitset &FeatureBits, bool Imm) {
   switch (CC) {
   default:
     llvm_unreachable("Unknown condition code!");
   case RISCVCC::COND_EQ:
-    return Imm ? RISCV::CV_BEQIMM : RISCV::BEQ;
+    if (Imm && FeatureBits[RISCV::FeatureVendorXCVbi])
+      return RISCV::CV_BEQIMM;
+    else
+      llvm_unreachable("Unknown branch immediate!");
+    return RISCV::BEQ;
   case RISCVCC::COND_NE:
-    return Imm ? RISCV::CV_BNEIMM : RISCV::BNE;
+    if (Imm && FeatureBits[RISCV::FeatureVendorXCVbi])
+      return RISCV::CV_BNEIMM;
+    else
+      llvm_unreachable("Unknown branch immediate!");
+    return RISCV::BNE;
   case RISCVCC::COND_LT:
     return RISCV::BLT;
   case RISCVCC::COND_GE:
@@ -974,7 +983,7 @@ unsigned RISCVCC::getBrCond(RISCVCC::CondCode CC, bool Imm) {
 
 const MCInstrDesc &RISCVInstrInfo::getBrCond(RISCVCC::CondCode CC,
                                              bool Imm) const {
-  return get(RISCVCC::getBrCond(CC, Imm));
+  return get(RISCVCC::getBrCond(CC, STI.getFeatureBits(), Imm));
 }
 
 RISCVCC::CondCode RISCVCC::getOppositeBranchCondition(RISCVCC::CondCode CC) {
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.h b/llvm/lib/Target/RISCV/RISCVInstrInfo.h
index 1c46d761a7e1e..0074c837eebfe 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.h
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.h
@@ -45,7 +45,8 @@ enum CondCode {
 };
 
 CondCode getOppositeBranchCondition(CondCode);
-unsigned getBrCond(CondCode CC, bool Imm = false);
+unsigned getBrCond(CondCode CC, const FeatureBitset &FeatureBits,
+                   bool Imm = false);
 
 } // end of namespace RISCVCC
 

>From 829bb24a089c1504f4a2c7c2f896d123a2bdf230 Mon Sep 17 00:00:00 2001
From: Sudharsan Veeravalli <quic_svs at quicinc.com>
Date: Wed, 5 Mar 2025 16:13:56 +0530
Subject: [PATCH 2/2] Early return

---
 llvm/lib/Target/RISCV/RISCVInstrInfo.cpp | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
index 748b072e64969..3ea8560efbd42 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
@@ -959,17 +959,17 @@ unsigned RISCVCC::getBrCond(RISCVCC::CondCode CC,
   default:
     llvm_unreachable("Unknown condition code!");
   case RISCVCC::COND_EQ:
-    if (Imm && FeatureBits[RISCV::FeatureVendorXCVbi])
+    if (!Imm)
+      return RISCV::BEQ;
+    if (FeatureBits[RISCV::FeatureVendorXCVbi])
       return RISCV::CV_BEQIMM;
-    else
-      llvm_unreachable("Unknown branch immediate!");
-    return RISCV::BEQ;
+    llvm_unreachable("Unknown branch immediate!");
   case RISCVCC::COND_NE:
-    if (Imm && FeatureBits[RISCV::FeatureVendorXCVbi])
+    if (!Imm)
+      return RISCV::BNE;
+    if (FeatureBits[RISCV::FeatureVendorXCVbi])
       return RISCV::CV_BNEIMM;
-    else
-      llvm_unreachable("Unknown branch immediate!");
-    return RISCV::BNE;
+    llvm_unreachable("Unknown branch immediate!");
   case RISCVCC::COND_LT:
     return RISCV::BLT;
   case RISCVCC::COND_GE: