Use multiple 0x66 prefixes so that all nops up to 15 bytes are a single instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120147 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 8 insertions, 24 deletions

diff --git a/lib/Target/X86/X86AsmBackend.cpp b/lib/Target/X86/X86AsmBackend.cpp
index fd823bf524..ab8f628745 100644
--- a/lib/Target/X86/X86AsmBackend.cpp
+++ b/lib/Target/X86/X86AsmBackend.cpp
@@ -211,10 +211,8 @@ void X86AsmBackend::RelaxInstruction(const MCInst &Inst, MCInst &Res) const {
 /// WriteNopData - Write optimal nops to the output file for the \arg Count
 /// bytes.  This returns the number of bytes written.  It may return 0 if
 /// the \arg Count is more than the maximum optimal nops.
-///
-/// FIXME this is X86 32-bit specific and should move to a better place.
 bool X86AsmBackend::WriteNopData(uint64_t Count, MCObjectWriter *OW) const {
-  static const uint8_t Nops[16][16] = {
+  static const uint8_t Nops[10][10] = {
     // nop
     {0x90},
     // xchg %ax,%ax
@@ -235,30 +233,16 @@ bool X86AsmBackend::WriteNopData(uint64_t Count, MCObjectWriter *OW) const {
     {0x66, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00},
     // nopw %cs:0L(%[re]ax,%[re]ax,1)
     {0x66, 0x2e, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00},
-    // nopw %cs:0L(%[re]ax,%[re]ax,1)
-    {0x66, 0x66, 0x2e, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00},
-    // nopw 0(%[re]ax,%[re]ax,1)
-    // nopw 0(%[re]ax,%[re]ax,1)
-    {0x66, 0x0f, 0x1f, 0x44, 0x00, 0x00,
-     0x66, 0x0f, 0x1f, 0x44, 0x00, 0x00},
-    // nopw 0(%[re]ax,%[re]ax,1)
-    // nopl 0L(%[re]ax) */
-    {0x66, 0x0f, 0x1f, 0x44, 0x00, 0x00,
-     0x0f, 0x1f, 0x80, 0x00, 0x00, 0x00, 0x00},
-    // nopl 0L(%[re]ax)
-    // nopl 0L(%[re]ax)
-    {0x0f, 0x1f, 0x80, 0x00, 0x00, 0x00, 0x00,
-     0x0f, 0x1f, 0x80, 0x00, 0x00, 0x00, 0x00},
-    // nopl 0L(%[re]ax)
-    // nopl 0L(%[re]ax,%[re]ax,1)
-    {0x0f, 0x1f, 0x80, 0x00, 0x00, 0x00, 0x00,
-     0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00}
   };
 
   // Write an optimal sequence for the first 15 bytes.
-  uint64_t OptimalCount = (Count < 16) ? Count : 15;
-  for (uint64_t i = 0, e = OptimalCount; i != e; i++)
-    OW->Write8(Nops[OptimalCount - 1][i]);
+  const uint64_t OptimalCount = (Count < 16) ? Count : 15;
+  const uint64_t Prefixes = OptimalCount <= 10 ? 0 : OptimalCount - 10;
+  for (uint64_t i = 0, e = Prefixes; i != e; i++)
+    OW->Write8(0x66);
+  const uint64_t Rest = OptimalCount - Prefixes;
+  for (uint64_t i = 0, e = Rest; i != e; i++)
+    OW->Write8(Nops[Rest - 1][i]);
 
   // Finish with single byte nops.
   for (uint64_t i = OptimalCount, e = Count; i != e; ++i)