Allow reverse iterators for `MachineBasicBlock::findDebugLoc` and `MachineBasicBlock::findPrevDebugLoc`

[fhahn]

Are there any instances in the current codebase where this would be useful?

lib/CodeGen/MachineBasicBlock.cpp

1293

nit: return on new line.

1318

Should this be std::next here?

[m4yers]

In D56135#1343674, @fhahn wrote:

Are there any instances in the current codebase where this would be useful?

For example X86InstrInfo::AnalyzeBranchImpl can be rewritten with more natural reverse iterators.

lib/CodeGen/MachineBasicBlock.cpp

1318

ah, true, thanks!

[m4yers]

Change std::prev to std::next

[m4yers]

Newline the returns

[fhahn]

In D56135#1347626, @m4yers wrote:

In D56135#1343674, @fhahn wrote:

Are there any instances in the current codebase where this would be useful?

For example X86InstrInfo::AnalyzeBranchImpl can be rewritten with more natural reverse iterators.

Would it be possible to post that as a follow up patch?

[m4yers]

In D56135#1354342, @fhahn wrote:

In D56135#1347626, @m4yers wrote:

In D56135#1343674, @fhahn wrote:

Are there any instances in the current codebase where this would be useful?

For example X86InstrInfo::AnalyzeBranchImpl can be rewritten with more natural reverse iterators.

Would it be possible to post that as a follow up patch?

This is for a downstream backend target i'm writing, but yes, I can provide patches for X86 where using this interface is more suitable.

[fhahn]

marking as changes requested to clear up review queue. I think it would be good to have at least 1 or 2 in-tree users so this gets at least some test coverage.

include/llvm/CodeGen/MachineBasicBlock.h

bool CorrectExtraCFGEdges(MachineBasicBlock *DestA,
                             bool IsCond);
 
   /// Find the next valid DebugLoc starting at MBBI, skipping any DBG_VALUE
   /// and DBG_LABEL instructions.  Return UnknownLoc if there is none.
   DebugLoc findDebugLoc(instr_iterator MBBI);
   DebugLoc findDebugLoc(iterator MBBI) {
     return findDebugLoc(MBBI.getInstrIterator());
   }
+  DebugLoc findDebugLoc(reverse_instr_iterator MBBI);
+  DebugLoc findDebugLoc(reverse_iterator MBBI) {
+    return findDebugLoc(MBBI.getInstrIterator());
+  }
 
   /// Find the previous valid DebugLoc preceding MBBI, skipping and DBG_VALUE
   /// instructions.  Return UnknownLoc if there is none.
   DebugLoc findPrevDebugLoc(instr_iterator MBBI);
   DebugLoc findPrevDebugLoc(iterator MBBI) {
     return findPrevDebugLoc(MBBI.getInstrIterator());
   }
+  DebugLoc findPrevDebugLoc(reverse_instr_iterator MBBI);
+  DebugLoc findPrevDebugLoc(reverse_iterator MBBI) {
+    return findPrevDebugLoc(MBBI.getInstrIterator());
+  }
 
   /// Find and return the merged DebugLoc of the branch instructions of the
   /// block. Return UnknownLoc if there is none.
   DebugLoc findBranchDebugLoc();
 
   /// Possible outcome of a register liveness query to computeRegisterLiveness()
   enum LivenessQueryResult {
     LQR_Live,   ///< Register is known to be (at least partially) live.

lib/CodeGen/MachineBasicBlock.cpp

bool MachineBasicBlock::CorrectExtraCFGEdges(MachineBasicBlock *DestA,
   }
 
   if (Changed)
     normalizeSuccProbs();
   return Changed;
 }
 
 /// Find the next valid DebugLoc starting at MBBI, skipping any DBG_VALUE
-/// instructions.  Return UnknownLoc if there is none.
+/// instructions. Return UnknownLoc if there is none.
 DebugLoc
 MachineBasicBlock::findDebugLoc(instr_iterator MBBI) {
   // Skip debug declarations, we don't want a DebugLoc from them.
   MBBI = skipDebugInstructionsForward(MBBI, instr_end());
   if (MBBI != instr_end())
     return MBBI->getDebugLoc();
   return {};
 }
+DebugLoc
+MachineBasicBlock::findDebugLoc(reverse_instr_iterator MBBI) {
+  // Skip debug declarations, we don't want a DebugLoc from them.
+  MBBI = skipDebugInstructionsBackward(MBBI, instr_rbegin());
+  if (!MBBI->isDebugInstr())
+    return MBBI->getDebugLoc();
+  return {};
+}
 
 /// Find the previous valid DebugLoc preceding MBBI, skipping and DBG_VALUE
 /// instructions.  Return UnknownLoc if there is none.
 DebugLoc MachineBasicBlock::findPrevDebugLoc(instr_iterator MBBI) {
-  if (MBBI == instr_begin()) return {};
+  if (MBBI == instr_begin())
+    return {};
   // Skip debug declarations, we don't want a DebugLoc from them.
   MBBI = skipDebugInstructionsBackward(std::prev(MBBI), instr_begin());
-  if (!MBBI->isDebugInstr()) return MBBI->getDebugLoc();
+  if (!MBBI->isDebugInstr())
+    return MBBI->getDebugLoc();
+  return {};
+}
+DebugLoc MachineBasicBlock::findPrevDebugLoc(reverse_instr_iterator MBBI) {
+  // Skip debug declarations, we don't want a DebugLoc from them.
+  MBBI = skipDebugInstructionsForward(std::next(MBBI), instr_rend());
+  if (MBBI != instr_rend())
+    return MBBI->getDebugLoc();
   return {};
 }
 
 /// Find and return the merged DebugLoc of the branch instructions of the block.
 /// Return UnknownLoc if there is none.
 DebugLoc
 MachineBasicBlock::findBranchDebugLoc() {
   DebugLoc DL;
   auto TI = getFirstTerminator();
   while (TI != end() && !TI->isBranch())
     ++TI;
 
   if (TI != end()) {