Support little-endian encodings in the FixedLenDecoderEmitter

The convention used to specify the PowerPC ISA is that bits are numbered in
reverse order (0 is the index of the high bit). To support this "little endian"
encoding convention, CodeEmitterGen will reverse the bit numberings prior to
generating the encoding tables. In order to generate a disassembler,
FixedLenDecoderEmitter needs to do the same.

This moves the bit reversal logic out of CodeEmitterGen and into CodeGenTarget
(where it can be used by both CodeEmitterGen and FixedLenDecoderEmitter). This
is prep work for disassembly support in the PPC backend (which is the only
in-tree user of this little-endian encoding support).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197532 91177308-0d34-0410-b5e6-96231b3b80d8

4 files changed, 47 insertions, 37 deletions

diff --git a/utils/TableGen/CodeEmitterGen.cpp b/utils/TableGen/CodeEmitterGen.cpp
index c94d384901..b7dea1046b 100644
--- a/utils/TableGen/CodeEmitterGen.cpp
+++ b/utils/TableGen/CodeEmitterGen.cpp
@@ -43,7 +43,6 @@ public:
 private:
   void emitMachineOpEmitter(raw_ostream &o, const std::string &Namespace);
   void emitGetValueBit(raw_ostream &o, const std::string &Namespace);
-  void reverseBits(std::vector<Record*> &Insts);
   int getVariableBit(const std::string &VarName, BitsInit *BI, int bit);
   std::string getInstructionCase(Record *R, CodeGenTarget &Target);
   void AddCodeToMergeInOperand(Record *R, BitsInit *BI,
@@ -53,40 +52,6 @@ private:
 
 };
 
-void CodeEmitterGen::reverseBits(std::vector<Record*> &Insts) {
-  for (std::vector<Record*>::iterator I = Insts.begin(), E = Insts.end();
-       I != E; ++I) {
-    Record *R = *I;
-    if (R->getValueAsString("Namespace") == "TargetOpcode" ||
-        R->getValueAsBit("isPseudo"))
-      continue;
-
-    BitsInit *BI = R->getValueAsBitsInit("Inst");
-
-    unsigned numBits = BI->getNumBits();
- 
-    SmallVector<Init *, 16> NewBits(numBits);
- 
-    for (unsigned bit = 0, end = numBits / 2; bit != end; ++bit) {
-      unsigned bitSwapIdx = numBits - bit - 1;
-      Init *OrigBit = BI->getBit(bit);
-      Init *BitSwap = BI->getBit(bitSwapIdx);
-      NewBits[bit]        = BitSwap;
-      NewBits[bitSwapIdx] = OrigBit;
-    }
-    if (numBits % 2) {
-      unsigned middle = (numBits + 1) / 2;
-      NewBits[middle] = BI->getBit(middle);
-    }
-
-    BitsInit *NewBI = BitsInit::get(NewBits);
-
-    // Update the bits in reversed order so that emitInstrOpBits will get the
-    // correct endianness.
-    R->getValue("Inst")->setValue(NewBI);
-  }
-}
-
 // If the VarBitInit at position 'bit' matches the specified variable then
 // return the variable bit position.  Otherwise return -1.
 int CodeEmitterGen::getVariableBit(const std::string &VarName,
@@ -238,8 +203,7 @@ void CodeEmitterGen::run(raw_ostream &o) {
   std::vector<Record*> Insts = Records.getAllDerivedDefinitions("Instruction");
 
   // For little-endian instruction bit encodings, reverse the bit order
-  if (Target.isLittleEndianEncoding()) reverseBits(Insts);
-
+  Target.reverseBitsForLittleEndianEncoding();
 
   const std::vector<const CodeGenInstruction*> &NumberedInstructions =
     Target.getInstructionsByEnumValue();
diff --git a/utils/TableGen/CodeGenTarget.cpp b/utils/TableGen/CodeGenTarget.cpp
index dd17059558..4b8efff600 100644
--- a/utils/TableGen/CodeGenTarget.cpp
+++ b/utils/TableGen/CodeGenTarget.cpp
@@ -357,6 +357,46 @@ bool CodeGenTarget::isLittleEndianEncoding() const {
   return getInstructionSet()->getValueAsBit("isLittleEndianEncoding");
 }
 
+/// reverseBitsForLittleEndianEncoding - For little-endian instruction bit
+/// encodings, reverse the bit order of all instructions.
+void CodeGenTarget::reverseBitsForLittleEndianEncoding() {
+  if (!isLittleEndianEncoding())
+    return;
+
+  std::vector<Record*> Insts = Records.getAllDerivedDefinitions("Instruction");
+  for (std::vector<Record*>::iterator I = Insts.begin(), E = Insts.end();
+       I != E; ++I) {
+    Record *R = *I;
+    if (R->getValueAsString("Namespace") == "TargetOpcode" ||
+        R->getValueAsBit("isPseudo"))
+      continue;
+
+    BitsInit *BI = R->getValueAsBitsInit("Inst");
+
+    unsigned numBits = BI->getNumBits();
+ 
+    SmallVector<Init *, 16> NewBits(numBits);
+ 
+    for (unsigned bit = 0, end = numBits / 2; bit != end; ++bit) {
+      unsigned bitSwapIdx = numBits - bit - 1;
+      Init *OrigBit = BI->getBit(bit);
+      Init *BitSwap = BI->getBit(bitSwapIdx);
+      NewBits[bit]        = BitSwap;
+      NewBits[bitSwapIdx] = OrigBit;
+    }
+    if (numBits % 2) {
+      unsigned middle = (numBits + 1) / 2;
+      NewBits[middle] = BI->getBit(middle);
+    }
+
+    BitsInit *NewBI = BitsInit::get(NewBits);
+
+    // Update the bits in reversed order so that emitInstrOpBits will get the
+    // correct endianness.
+    R->getValue("Inst")->setValue(NewBI);
+  }
+}
+
 /// guessInstructionProperties - Return true if it's OK to guess instruction
 /// properties instead of raising an error.
 ///
diff --git a/utils/TableGen/CodeGenTarget.h b/utils/TableGen/CodeGenTarget.h
index 6271443029..d6458f41cf 100644
--- a/utils/TableGen/CodeGenTarget.h
+++ b/utils/TableGen/CodeGenTarget.h
@@ -177,6 +177,10 @@ public:
   ///
   bool isLittleEndianEncoding() const;
 
+  /// reverseBitsForLittleEndianEncoding - For little-endian instruction bit
+  /// encodings, reverse the bit order of all instructions.
+  void reverseBitsForLittleEndianEncoding();
+
   /// guessInstructionProperties - should we just guess unset instruction
   /// properties?
   bool guessInstructionProperties() const;
diff --git a/utils/TableGen/FixedLenDecoderEmitter.cpp b/utils/TableGen/FixedLenDecoderEmitter.cpp
index 87d18cdb9d..d2d3f3dd07 100644
--- a/utils/TableGen/FixedLenDecoderEmitter.cpp
+++ b/utils/TableGen/FixedLenDecoderEmitter.cpp
@@ -2012,6 +2012,8 @@ void FixedLenDecoderEmitter::run(raw_ostream &o) {
 
   emitFieldFromInstruction(OS);
 
+  Target.reverseBitsForLittleEndianEncoding();
+
   // Parameterize the decoders based on namespace and instruction width.
   NumberedInstructions = &Target.getInstructionsByEnumValue();
   std::map<std::pair<std::string, unsigned>,