[X86] Add comment about the sorting of the memory folding tables added in r335501.

llvm-svn: 335517

1 files changed, 15 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86InstrInfo.cpp b/llvm/lib/Target/X86/X86InstrInfo.cpp
index d5577acde90..94d381a1901 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.cpp
+++ b/llvm/lib/Target/X86/X86InstrInfo.cpp
@@ -120,6 +120,21 @@ struct X86MemoryFoldTableEntry {
   }
 };
 
+// These tables are sorted by their RegOp value allowing them to be binary
+// searched at runtime without the need for additional storage. The enum values
+// are currently emitted in X86GenInstrInfo.inc in alphabetical order. Which
+// makes sorting these tables a simple matter of alphabetizing the table.
+//
+// We also have a tablegen emitter that tries to autogenerate these tables
+// by comparing encoding information. This can be enabled by passing
+// X86_GEN_FOLD_TABLES=ON to cmake which fill produce X86GenFoldTables.inc
+// in the build area. There are currently some bugs in the autogenerated table
+// that require a manual review to copy them from the autogenerated table into
+// this table. It is unclear if we will ever be able to fully automate this
+// because as new instruction are added into holes in the X86 opcode map they
+// potentially pair up with old instructions and create new entries in the
+// tables that would be incorrect. The manual review process allows us a chance
+// to catch these before they become observable bugs.
 static const X86MemoryFoldTableEntry MemoryFoldTable2Addr[] = {
   { X86::ADC16ri,     X86::ADC16mi,    0 },
   { X86::ADC16ri8,    X86::ADC16mi8,   0 },