[RISCV] Allow RISCVAsmBackend::writeNopData to generate c.nop when supported

When the compressed instruction set is enabled, the 16-bit c.nop can be
generated if necessary.

Differential Revision: https://reviews.llvm.org/D41221
Patch by Shiva Chen.

llvm-svn: 322658

1 files changed, 18 insertions, 8 deletions

diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
index 6e06a4975e2..3dcd36f1b71 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
@@ -27,12 +27,13 @@ using namespace llvm;
 
 namespace {
 class RISCVAsmBackend : public MCAsmBackend {
+  const MCSubtargetInfo &STI;
   uint8_t OSABI;
   bool Is64Bit;
 
 public:
-  RISCVAsmBackend(uint8_t OSABI, bool Is64Bit)
-      : MCAsmBackend(), OSABI(OSABI), Is64Bit(Is64Bit) {}
+  RISCVAsmBackend(const MCSubtargetInfo &STI, uint8_t OSABI, bool Is64Bit)
+      : MCAsmBackend(), STI(STI), OSABI(OSABI), Is64Bit(Is64Bit) {}
   ~RISCVAsmBackend() override {}
 
   void applyFixup(const MCAssembler &Asm, const MCFixup &Fixup,
@@ -88,15 +89,24 @@ public:
 };
 
 bool RISCVAsmBackend::writeNopData(uint64_t Count, MCObjectWriter *OW) const {
-  // Once support for the compressed instruction set is added, we will be able
-  // to conditionally support 16-bit NOPs
-  if ((Count % 4) != 0)
+  bool HasStdExtC = STI.getFeatureBits()[RISCV::FeatureStdExtC];
+  unsigned MinNopLen = HasStdExtC ? 2 : 4;
+
+  if ((Count % MinNopLen) != 0)
     return false;
 
-  // The canonical nop on RISC-V is addi x0, x0, 0
-  for (uint64_t i = 0; i < Count; i += 4)
+  // The canonical nop on RISC-V is addi x0, x0, 0.
+  uint64_t Nop32Count = Count / 4;
+  for (uint64_t i = Nop32Count; i != 0; --i)
     OW->write32(0x13);
 
+  // The canonical nop on RVC is c.nop.
+  if (HasStdExtC) {
+    uint64_t Nop16Count = (Count - Nop32Count * 4) / 2;
+    for (uint64_t i = Nop16Count; i != 0; --i)
+      OW->write16(0x01);
+  }
+
   return true;
 }
 
@@ -235,5 +245,5 @@ MCAsmBackend *llvm::createRISCVAsmBackend(const Target &T,
                                           const MCTargetOptions &Options) {
   const Triple &TT = STI.getTargetTriple();
   uint8_t OSABI = MCELFObjectTargetWriter::getOSABI(TT.getOS());
-  return new RISCVAsmBackend(OSABI, TT.isArch64Bit());
+  return new RISCVAsmBackend(STI, OSABI, TT.isArch64Bit());
 }