[x86] invert logic for attribute 'FeatureFastUAMem'

This is a 'no functional change intended' patch. It removes one FIXME, but adds several more.

Motivation: the FeatureFastUAMem attribute may be too general. It is used to determine if any
sized misaligned memory access under 32-bytes is 'fast'. From the added FIXME comments, however,
you can see that we're not consistent about this. Changing the name of the attribute makes it
clearer to see the logic holes.

Changing this to a 'slow' attribute also means we don't have to add an explicit 'fast' attribute
to new chips; fast unaligned accesses have been standard for several generations of CPUs now.

Differential Revision: http://reviews.llvm.org/D12154

llvm-svn: 245729

1 files changed, 8 insertions, 3 deletions

diff --git a/llvm/lib/Target/X86/X86InstrInfo.cpp b/llvm/lib/Target/X86/X86InstrInfo.cpp
index d964bf17a0f..9d799d8e780 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.cpp
+++ b/llvm/lib/Target/X86/X86InstrInfo.cpp
@@ -5508,9 +5508,10 @@ bool X86InstrInfo::unfoldMemoryOperand(MachineFunction &MF, MachineInstr *MI,
 
   const MCInstrDesc &MCID = get(Opc);
   const TargetRegisterClass *RC = getRegClass(MCID, Index, &RI, MF);
+  // TODO: Check if 32-byte or greater accesses are slow too?
   if (!MI->hasOneMemOperand() &&
       RC == &X86::VR128RegClass &&
-      !Subtarget.isUnalignedMemAccessFast())
+      Subtarget.isUnalignedMemUnder32Slow())
     // Without memoperands, loadRegFromAddr and storeRegToStackSlot will
     // conservatively assume the address is unaligned. That's bad for
     // performance.
@@ -5658,9 +5659,11 @@ X86InstrInfo::unfoldMemoryOperand(SelectionDAG &DAG, SDNode *N,
                             cast<MachineSDNode>(N)->memoperands_end());
     if (!(*MMOs.first) &&
         RC == &X86::VR128RegClass &&
-        !Subtarget.isUnalignedMemAccessFast())
+        Subtarget.isUnalignedMemUnder32Slow())
       // Do not introduce a slow unaligned load.
       return false;
+    // FIXME: If a VR128 can have size 32, we should be checking if a 32-byte
+    // memory access is slow above.
     unsigned Alignment = RC->getSize() == 32 ? 32 : 16;
     bool isAligned = (*MMOs.first) &&
                      (*MMOs.first)->getAlignment() >= Alignment;
@@ -5701,9 +5704,11 @@ X86InstrInfo::unfoldMemoryOperand(SelectionDAG &DAG, SDNode *N,
                              cast<MachineSDNode>(N)->memoperands_end());
     if (!(*MMOs.first) &&
         RC == &X86::VR128RegClass &&
-        !Subtarget.isUnalignedMemAccessFast())
+        Subtarget.isUnalignedMemUnder32Slow())
       // Do not introduce a slow unaligned store.
       return false;
+    // FIXME: If a VR128 can have size 32, we should be checking if a 32-byte
+    // memory access is slow above.
     unsigned Alignment = RC->getSize() == 32 ? 32 : 16;
     bool isAligned = (*MMOs.first) &&
                      (*MMOs.first)->getAlignment() >= Alignment;