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
diff --git a/lib/Target/X86/X86AsmBackend.cpp b/lib/Target/X86/X86AsmBackend.cpp
index fd823bf..ab8f628 100644
--- a/lib/Target/X86/X86AsmBackend.cpp
+++ b/lib/Target/X86/X86AsmBackend.cpp
@@ -211,10 +211,8 @@
/// 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 @@
{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)