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

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120147 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Rafael Espindola
2010-11-25 17:14:16 +00:00
parent c62feda741
commit 2ace1b68ac
2 changed files with 9 additions and 25 deletions
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32
lib/Target/X86/X86AsmBackend.cpp
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@@ -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)