[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

diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVAsmBackend.cpp
index 6e06a49..3dcd36f 100644 (file)
--- 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());
 }

diff --git a/llvm/test/MC/RISCV/cnop.s b/llvm/test/MC/RISCV/cnop.s
new file mode 100644 (file)
index 0000000..8d52626
--- /dev/null
+++ b/llvm/test/MC/RISCV/cnop.s
@@ -0,0 +1,26 @@
+# RUN: llvm-mc -filetype=obj -triple riscv32 -mattr=+c < %s \
+# RUN:     | llvm-objdump -mattr=+c -d - | FileCheck -check-prefix=CHECK-INST %s
+
+# alpha and main are 8 byte alignment
+# but the alpha function's size is 6
+# So assembler will insert a c.nop to make sure 8 byte alignment.
+
+        .text
+       .p2align        3
+       .type   alpha,@function
+alpha:
+# BB#0:
+       addi    sp, sp, -16
+       c.lw    a0, 0(a0)
+# CHECK-INST: c.nop
+.Lfunc_end0:
+       .size   alpha, .Lfunc_end0-alpha
+                                        # -- End function
+       .globl  main
+       .p2align        3
+       .type   main,@function
+main:                                   # @main
+# BB#0:
+.Lfunc_end1:
+       .size   main, .Lfunc_end1-main
+                                        # -- End function