[X86] Adjust nop emission by compiler to consider target decode limitations

The primary motivation of this change is to bring the code more closely in sync behavior wise with the assembler's version of nop emission.  I'd like to eventually factor them into one, but that's hard to do when one has features the other doesn't.

The longest encodeable nop on x86 is 15 bytes, but many processors - for instance all intel chips - can't decode the 15 byte form efficiently.  On those processors, it's better to use either a 10 byte or 11 byte sequence depending.

1 files changed, 17 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index e20d1ee2ae5..2fc9a2af01d 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -1031,6 +1031,20 @@ void X86AsmPrinter::LowerTlsAddr(X86MCInstLower &MCInstLowering,
   }
 }
 
+/// Return the longest nop which can be efficiently decoded for the given
+/// target cpu.  15-bytes is the longest single NOP instruction, but some
+/// platforms can't decode the longest forms efficiently.
+static unsigned MaxLongNopLength(const MCSubtargetInfo &STI) {
+  uint64_t MaxNopLength = 10;
+  if (STI.getFeatureBits()[X86::ProcIntelSLM])
+    MaxNopLength = 7;
+  else if (STI.getFeatureBits()[X86::FeatureFast15ByteNOP])
+    MaxNopLength = 15;
+  else if (STI.getFeatureBits()[X86::FeatureFast11ByteNOP])
+    MaxNopLength = 11;
+  return MaxNopLength;
+}
+
 /// Emit the largest nop instruction smaller than or equal to \p NumBytes
 /// bytes.  Return the size of nop emitted.
 static unsigned EmitNop(MCStreamer &OS, unsigned NumBytes, bool Is64Bit,
@@ -1041,6 +1055,9 @@ static unsigned EmitNop(MCStreamer &OS, unsigned NumBytes, bool Is64Bit,
     return 1;
   }
 
+  // Cap a single nop emission at the profitable value for the target
+  NumBytes = std::min(NumBytes, MaxLongNopLength(STI));
+
   unsigned NopSize;
   unsigned Opc, BaseReg, ScaleVal, IndexReg, Displacement, SegmentReg;
   IndexReg = Displacement = SegmentReg = 0;