replica
/
hanchenye-llvm-project
2
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity

Alternative to long nops for X86 CPUs, by Andrey Turetsky 

Make X86AsmBackend generate smarter nops instead of a bunch of 0x90 for code alignment for CPUs which don't support long nop instructions.
Differential Revision: http://reviews.llvm.org/D14178

llvm-svn: 253557
This commit is contained in:
Alexey Bataev 2015-11-19 11:44:35 +00:00
parent 26da45c2bb
commit b7b82bf33e
2 changed files with 33 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

46
llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
View File

@ -69,15 +69,19 @@ public:
class X86AsmBackend : public MCAsmBackend {
const StringRef CPU;
bool HasNopl;
const uint64_t MaxNopLength;
uint64_t MaxNopLength;
public:
X86AsmBackend(const Target &T, StringRef CPU)
: MCAsmBackend(), CPU(CPU), MaxNopLength(CPU == "slm" ? 7 : 15) {
X86AsmBackend(const Target &T, StringRef CPU) : MCAsmBackend(), CPU(CPU) {
HasNopl = CPU != "generic" && CPU != "i386" && CPU != "i486" &&
CPU != "i586" && CPU != "pentium" && CPU != "pentium-mmx" &&
CPU != "i686" && CPU != "k6" && CPU != "k6-2" && CPU != "k6-3" &&
CPU != "geode" && CPU != "winchip-c6" && CPU != "winchip2" &&
CPU != "c3" && CPU != "c3-2";
// Max length of true long nop instruction is 15 bytes.
// Max length of long nop replacement instruction is 7 bytes.
// Taking into account SilverMont architecture features max length of nops
// is reduced for it to achieve better performance.
MaxNopLength = (!HasNopl || CPU == "slm") ? 7 : 15;
}
unsigned getNumFixupKinds() const override {
@ -279,7 +283,7 @@ void X86AsmBackend::relaxInstruction(const MCInst &Inst, MCInst &Res) const {
/// bytes.
/// \return - true on success, false on failure
bool X86AsmBackend::writeNopData(uint64_t Count, MCObjectWriter *OW) const {
static const uint8_t Nops[10][10] = {
static const uint8_t TrueNops[10][10] = {
// nop
{0x90},
// xchg %ax,%ax
@ -302,17 +306,31 @@ bool X86AsmBackend::writeNopData(uint64_t Count, MCObjectWriter *OW) const {
{0x66, 0x2e, 0x0f, 0x1f, 0x84, 0x00, 0x00, 0x00, 0x00, 0x00},
};
// This CPU doesn't support long nops. If needed add more.
// FIXME: Can we get this from the subtarget somehow?
// FIXME: We could generated something better than plain 0x90.
if (!HasNopl) {
for (uint64_t i = 0; i < Count; ++i)
OW->write8(0x90);
return true;
}
// Alternative nop instructions for CPUs which don't support long nops.
static const uint8_t AltNops[7][10] = {
// nop
{0x90},
// xchg %ax,%ax
{0x66, 0x90},
// lea 0x0(%esi),%esi
{0x8d, 0x76, 0x00},
// lea 0x0(%esi),%esi
{0x8d, 0x74, 0x26, 0x00},
// nop + lea 0x0(%esi),%esi
{0x90, 0x8d, 0x74, 0x26, 0x00},
// lea 0x0(%esi),%esi
{0x8d, 0xb6, 0x00, 0x00, 0x00, 0x00 },
// lea 0x0(%esi),%esi
{0x8d, 0xb4, 0x26, 0x00, 0x00, 0x00, 0x00},
};
// 15 is the longest single nop instruction. Emit as many 15-byte nops as
// needed, then emit a nop of the remaining length.
// Select the right NOP table.
// FIXME: Can we get if CPU supports long nops from the subtarget somehow?
const uint8_t (*Nops)[10] = HasNopl ? TrueNops : AltNops;
assert(HasNopl || MaxNopLength <= 7);
// Emit as many largest nops as needed, then emit a nop of the remaining
// length.
do {
const uint8_t ThisNopLength = (uint8_t) std::min(Count, MaxNopLength);
const uint8_t Prefixes = ThisNopLength <= 10 ? 0 : ThisNopLength - 10;

8
llvm/test/MC/X86/x86_nop.s
View File

@ -22,13 +22,7 @@ inc %eax
inc %eax
// CHECK: 0: 40 incl %eax
// CHECK: 1: 90 nop
// CHECK: 2: 90 nop
// CHECK: 3: 90 nop
// CHECK: 4: 90 nop
// CHECK: 5: 90 nop
// CHECK: 6: 90 nop
// CHECK: 7: 90 nop
// CHECK: 1: 8d b4 26 00 00 00 00 leal (%esi), %esi
// CHECK: 8: 40 incl %eax