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This is the second patch to allow x86 code to be aligned with optimal nops.

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With the compiler changed to use EmitCodeAlignment() it does change the
functionality.  But X86 assembly code assembled with llvm-mc does not change
its output.  For that we will eventually change the assembler frontend to
detect a '.align x, 0x90' when used in a section that 'hasInstructions' and use
EmitCodeAlignment, but will wait until we have better target hooks.

llvm-svn: 96988
This commit is contained in:
Kevin Enderby 2010-02-23 21:41:24 +00:00
parent 92fb0be76b
commit 69d643ba20
1 changed files with 47 additions and 2 deletions
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49
lib/MC/MCAssembler.cpp
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@ -1066,9 +1066,54 @@ void MCAssembler::LayoutSection(MCSectionData &SD) {
///
/// FIXME this is X86 32-bit specific and should move to a better place.
static uint64_t WriteNopData(uint64_t Count, MachObjectWriter &MOW) {
// FIXME for now just use the 0x90 nop opcode byte.
static const uint8_t Nops[16][16] = {
// nop
{0x90},
// xchg %ax,%ax
{0x66, 0x90},
// nopl (%[re]ax)
{0x0f, 0x1f, 0x00},
// nopl 0(%[re]ax)
{0x0f, 0x1f, 0x40, 0x00},
// nopl 0(%[re]ax,%[re]ax,1)
{0x0f, 0x1f, 0x44, 0x00, 0x00},
// nopw 0(%[re]ax,%[re]ax,1)
{0x66, 0x0f, 0x1f, 0x44, 0x00, 0x00},
// nopl 0L(%[re]ax)
{0x0f, 0x1f, 0x80, 0x00, 0x00, 0x00, 0x00},
// nopl 0L(%[re]ax,%[re]ax,1)
{0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00},
// nopw 0L(%[re]ax,%[re]ax,1)
{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},
// nopl 0(%[re]ax,%[re]ax,1)
// nopw 0(%[re]ax,%[re]ax,1)
{0x0f, 0x1f, 0x44, 0x00, 0x00,
0x66, 0x0f, 0x1f, 0x44, 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}
};
if (Count > 15)
return 0;
for (uint64_t i = 0; i < Count; i++)
MOW.Write8 (uint8_t(0x90));
MOW.Write8 (uint8_t(Nops[Count - 1][i]));
return Count;
}