[llvm] [RISCV] Add BREV8 to SimplifyDemandedBitsForTargetNode. (PR #141898)

[Craig Topper via llvm-commits]

https://github.com/topperc created https://github.com/llvm/llvm-project/pull/141898

None

>From d80609c55947cc78185da10046c67f21addb007e Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper at sifive.com>
Date: Wed, 28 May 2025 23:08:46 -0700
Subject: [PATCH] [RISCV] Add BREV8 to SimplifyDemandedBitsForTargetNode.

---
 llvm/lib/Target/RISCV/RISCVISelLowering.cpp | 32 +++++++++++++++++++--
 llvm/lib/Target/RISCV/RISCVISelLowering.h   |  6 ++++
 llvm/test/CodeGen/RISCV/bswap-bitreverse.ll |  4 +--
 3 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
index 0a849f49116ee..f6cdd0d30aaf7 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.cpp
@@ -20575,8 +20575,7 @@ void RISCVTargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
     Known = DAG.computeKnownBits(Op.getOperand(0), Depth + 1);
     bool IsGORC = Op.getOpcode() == RISCVISD::ORC_B;
     // To compute zeros, we need to invert the value and invert it back after.
-    Known.Zero =
-        ~computeGREVOrGORC(~Known.Zero.getZExtValue(), 7, IsGORC);
+    Known.Zero = ~computeGREVOrGORC(~Known.Zero.getZExtValue(), 7, IsGORC);
     Known.One = computeGREVOrGORC(Known.One.getZExtValue(), 7, IsGORC);
     break;
   }
@@ -20717,6 +20716,35 @@ unsigned RISCVTargetLowering::ComputeNumSignBitsForTargetNode(
   return 1;
 }
 
+bool RISCVTargetLowering::SimplifyDemandedBitsForTargetNode(
+    SDValue Op, const APInt &OriginalDemandedBits,
+    const APInt &OriginalDemandedElts, KnownBits &Known, TargetLoweringOpt &TLO,
+    unsigned Depth) const {
+  unsigned BitWidth = OriginalDemandedBits.getBitWidth();
+
+  switch (Op.getOpcode()) {
+  case RISCVISD::BREV8: {
+    KnownBits Known2;
+    APInt DemandedBits =
+        APInt(BitWidth, computeGREVOrGORC(OriginalDemandedBits.getZExtValue(),
+                                          7, /*IsGORC=*/false));
+    if (SimplifyDemandedBits(Op.getOperand(0), DemandedBits,
+                             OriginalDemandedElts, Known2, TLO, Depth + 1))
+      return true;
+
+    // To compute zeros, we need to invert the value and invert it back after.
+    Known.Zero =
+        ~computeGREVOrGORC(~Known2.Zero.getZExtValue(), 7, /*IsGORC=*/false);
+    Known.One =
+        computeGREVOrGORC(Known2.One.getZExtValue(), 7, /*IsGORC=*/false);
+    return false;
+  }
+  }
+
+  return TargetLowering::SimplifyDemandedBitsForTargetNode(
+      Op, OriginalDemandedBits, OriginalDemandedElts, Known, TLO, Depth);
+}
+
 bool RISCVTargetLowering::canCreateUndefOrPoisonForTargetNode(
     SDValue Op, const APInt &DemandedElts, const SelectionDAG &DAG,
     bool PoisonOnly, bool ConsiderFlags, unsigned Depth) const {
diff --git a/llvm/lib/Target/RISCV/RISCVISelLowering.h b/llvm/lib/Target/RISCV/RISCVISelLowering.h
index 78f2044ba83a7..ed27e1ea5ef6d 100644
--- a/llvm/lib/Target/RISCV/RISCVISelLowering.h
+++ b/llvm/lib/Target/RISCV/RISCVISelLowering.h
@@ -153,6 +153,12 @@ class RISCVTargetLowering : public TargetLowering {
                                            const SelectionDAG &DAG,
                                            unsigned Depth) const override;
 
+  bool SimplifyDemandedBitsForTargetNode(SDValue Op, const APInt &DemandedBits,
+                                         const APInt &DemandedElts,
+                                         KnownBits &Known,
+                                         TargetLoweringOpt &TLO,
+                                         unsigned Depth) const override;
+
   bool canCreateUndefOrPoisonForTargetNode(SDValue Op,
                                            const APInt &DemandedElts,
                                            const SelectionDAG &DAG,
diff --git a/llvm/test/CodeGen/RISCV/bswap-bitreverse.ll b/llvm/test/CodeGen/RISCV/bswap-bitreverse.ll
index 40a5772142345..1afb03b346a1a 100644
--- a/llvm/test/CodeGen/RISCV/bswap-bitreverse.ll
+++ b/llvm/test/CodeGen/RISCV/bswap-bitreverse.ll
@@ -245,14 +245,14 @@ define i8 @test_bitreverse_i8(i8 %a) nounwind {
 ;
 ; RV32ZBKB-LABEL: test_bitreverse_i8:
 ; RV32ZBKB:       # %bb.0:
-; RV32ZBKB-NEXT:    rev8 a0, a0
+; RV32ZBKB-NEXT:    slli a0, a0, 24
 ; RV32ZBKB-NEXT:    brev8 a0, a0
 ; RV32ZBKB-NEXT:    srli a0, a0, 24
 ; RV32ZBKB-NEXT:    ret
 ;
 ; RV64ZBKB-LABEL: test_bitreverse_i8:
 ; RV64ZBKB:       # %bb.0:
-; RV64ZBKB-NEXT:    rev8 a0, a0
+; RV64ZBKB-NEXT:    slli a0, a0, 56
 ; RV64ZBKB-NEXT:    brev8 a0, a0
 ; RV64ZBKB-NEXT:    srli a0, a0, 56
 ; RV64ZBKB-NEXT:    ret