[X86] Restrict max long nop length for Lakemont

[aturetsk]

Summary

Restrict the max length of long nops for Lakemont to 7. Experiments on MCU benchmarks (Dhrystone, Coremark) show that this is the most optimal length.

[bruno]

LGTM, one comment below

lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

75 ↗

(On Diff #53014)

clang-format this one.

[aturetsk]

Thanks for the review. The formatting was fixed.

llvm/trunk/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

 };
 
 class X86AsmBackend : public MCAsmBackend {
   const StringRef CPU;
   bool HasNopl;
   const uint64_t MaxNopLength;
 public:
   X86AsmBackend(const Target &T, StringRef CPU)
-      : MCAsmBackend(), CPU(CPU), MaxNopLength(CPU == "slm" ? 7 : 15) {
+      : MCAsmBackend(), CPU(CPU),
+        MaxNopLength((CPU == "slm" || CPU == "lakemont") ? 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 {

llvm/trunk/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
+# 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=LNOP7 %s
+# RUN: llvm-mc -filetype=obj -arch=x86 -triple=i686-pc-linux-gnu -mcpu=lakemont %s | llvm-objdump -d -no-show-raw-insn - | FileCheck --check-prefix=LNOP7 %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
+# On Silvermont and Lakemont we emit only 7 byte NOPs since longer NOPs
+# are not profitable.
+# LNOP7: 0:  inc
+# LNOP7-NEXT: 1:  nop
+# LNOP7-NEXT: 8:  nop
+# LNOP7-NEXT: f:  nop
+# LNOP7-NEXT: 16:  nop
+# LNOP7-NEXT: 1d:  nop
+# LNOP7-NEXT: 20:  inc