[llvm] r253557 - Alternative to long nops for X86 CPUs, by Andrey Turetsky
Alexey Bataev via llvm-commits
llvm-commits at lists.llvm.org
Thu Nov 19 03:44:35 PST 2015
Author: abataev
Date: Thu Nov 19 05:44:35 2015
New Revision: 253557
URL: http://llvm.org/viewvc/llvm-project?rev=253557&view=rev
Log:
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
Modified:
llvm/trunk/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
llvm/trunk/test/MC/X86/x86_nop.s
Modified: llvm/trunk/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp?rev=253557&r1=253556&r2=253557&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp (original)
+++ llvm/trunk/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp Thu Nov 19 05:44:35 2015
@@ -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(con
/// 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_
{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},
+ };
+
+ // 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);
- // 15 is the longest single nop instruction. Emit as many 15-byte nops as
- // needed, then emit a nop of the remaining length.
+ // 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;
Modified: llvm/trunk/test/MC/X86/x86_nop.s
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/X86/x86_nop.s?rev=253557&r1=253556&r2=253557&view=diff
==============================================================================
--- llvm/trunk/test/MC/X86/x86_nop.s (original)
+++ llvm/trunk/test/MC/X86/x86_nop.s Thu Nov 19 05:44:35 2015
@@ -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
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