[X86] Limit maximum nop length on Silvermont
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.
Maximum nop length is limited to 7 bytes when used for padding on Silvermont.
For other x86 processors max nop length remains unchanged 15 bytes.
Differential Revision: http://reviews.llvm.org/D4374
llvm-svn: 212321
diff --git a/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp b/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
index 7840342..c12a57a 100644
--- a/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
+++ b/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp
@@ -73,11 +73,12 @@
};
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" &&
@@ -331,7 +332,7 @@
// 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);