[X86] Limit maximum nop length on Silvermont

[volkalexey]

Summary

Silvermont can only decode one instruction per cycle if the instruction exceeds 8 bytes.
Also in Silvermont instructions with more than 3 prefixes will cause 3 cycle penalty.
This patch introduces maximum nop length and limits it to 7 bytes when used for padding on Silvermont.
For other x86 processors max nop length remains unchanged 15 bytes.

[nadav]

LGTM.

[volkalexey]

Closed by commit rL212321 (authored by volkalex).

lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

 class X86ELFObjectWriter : public MCELFObjectTargetWriter {
 public:
   X86ELFObjectWriter(bool is64Bit, uint8_t OSABI, uint16_t EMachine,
                      bool HasRelocationAddend, bool foobar)
     : MCELFObjectTargetWriter(is64Bit, OSABI, EMachine, HasRelocationAddend) {}
 };
 
 class X86AsmBackend : public MCAsmBackend {
-  StringRef CPU;
+  const StringRef CPU;
   bool HasNopl;
+  const uint64_t MaxNopLength;
 public:
   X86AsmBackend(const Target &T, StringRef _CPU)
-    : MCAsmBackend(), CPU(_CPU) {
+    : MCAsmBackend(), CPU(_CPU), MaxNopLength(_CPU == "slm" ? 7 : 15) {
     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";
   }
 
   unsigned getNumFixupKinds() const override {
if (!HasNopl) {
     for (uint64_t i = 0; i < Count; ++i)
       OW->Write8(0x90);
     return true;
   }
 
   // 15 is the longest single nop instruction.  Emit as many 15-byte nops as
   // needed, then emit a nop of the remaining length.
   do {
-    const uint8_t ThisNopLength = (uint8_t) std::min(Count, (uint64_t) 15);
+    const uint8_t ThisNopLength = (uint8_t) std::min(Count, MaxNopLength);
     const uint8_t Prefixes = ThisNopLength <= 10 ? 0 : ThisNopLength - 10;
     for (uint8_t i = 0; i < Prefixes; i++)
       OW->Write8(0x66);
     const uint8_t Rest = ThisNopLength - Prefixes;
     for (uint8_t i = 0; i < Rest; i++)
       OW->Write8(Nops[Rest - 1][i]);
     Count -= ThisNopLength;
   } while (Count != 0);

test/MC/X86/x86_long_nop.s

 # RUN: llvm-mc -filetype=obj -arch=x86 -triple=x86_64-pc-linux-gnu %s | llvm-objdump -d -no-show-raw-insn - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu %s | llvm-objdump -d -no-show-raw-insn - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -arch=x86 -triple=x86_64-apple-darwin10.0 %s | llvm-objdump -d -no-show-raw-insn - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-apple-darwin8 %s | llvm-objdump -d -no-show-raw-insn - | FileCheck %s
+# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=slm %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=SLM %s
 
 # Ensure alignment directives also emit sequences of 15-byte NOPs on processors
 # capable of using long NOPs.
 inc %eax
 .p2align 5
 inc %eax
 # CHECK: 0:  inc
 # CHECK-NEXT: 1:  nop
 # CHECK-NEXT: 10:  nop
 # CHECK-NEXT: 1f:  nop
 # CHECK-NEXT: 20:  inc
+
+# On Silvermont we emit only 7 byte NOPs since longer NOPs are not profitable
+# SLM: 0:  inc
+# SLM-NEXT: 1:  nop
+# SLM-NEXT: 8:  nop
+# SLM-NEXT: f:  nop
+# SLM-NEXT: 16:  nop
+# SLM-NEXT: 1d:  nop
+# SLM-NEXT: 20:  inc

test/MC/X86/x86_nop.s

 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=generic %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=i386 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=i486 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=i586 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=pentium %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=pentium-mmx %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=geode %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=i686 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=k6 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=k6-2 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=k6-3 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=winchip-c6 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=winchip2 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=c3 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=c3-2 %s | llvm-objdump -d - | FileCheck %s
 # RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=core2 %s | llvm-objdump -d - | FileCheck --check-prefix=NOPL %s
+# RUN: llvm-mc -filetype=obj -triple=i686-pc-linux -mcpu=slm %s | llvm-objdump -d - | FileCheck --check-prefix=NOPL %s
 
 
 inc %eax
 .align 8
 inc %eax
 
 // CHECK: 0:  40                                            incl  %eax
 // CHECK: 1:  90                                            nop