The current Intel Atom microarchitecture has a feature whereby when a function

returns early then it is slightly faster to execute a sequence of NOP
instructions to wait until the return address is ready,
as opposed to simply stalling on the ret instruction
until the return address is ready.

When compiling for X86 Atom only, this patch will run a pass, called
"X86PadShortFunction" which will add NOP instructions where less than four
cycles elapse between function entry and return.

It includes tests.

Patch by Andy Zhang.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171524 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 5 insertions, 0 deletions

diff --git a/lib/Target/X86/X86Subtarget.h b/lib/Target/X86/X86Subtarget.h
index 44f38a1a91..080f4cfeca 100644
--- a/lib/Target/X86/X86Subtarget.h
+++ b/lib/Target/X86/X86Subtarget.h
@@ -146,6 +146,10 @@ protected:
   /// PostRAScheduler - True if using post-register-allocation scheduler.
   bool PostRAScheduler;
 
+  /// PadShortFunctions - True if the short functions should be padded to prevent
+  /// a stall when returning too early.
+  bool PadShortFunctions;
+
   /// stackAlignment - The minimum alignment known to hold of the stack frame on
   /// entry to the function and which must be maintained by every function.
   unsigned stackAlignment;
@@ -231,6 +235,7 @@ public:
   bool hasCmpxchg16b() const { return HasCmpxchg16b; }
   bool useLeaForSP() const { return UseLeaForSP; }
   bool hasSlowDivide() const { return HasSlowDivide; }
+  bool padShortFunctions() const { return PadShortFunctions; }
 
   bool isAtom() const { return X86ProcFamily == IntelAtom; }