RegBankSelect: Allow targets to introduce control flow for mapping

[arsenm]

Summary

For AMDGPU, if an operand requires an SGPR but is only available as a
VGPR, a loop needs to be introduced until to execute the instruction
with each unique combination of values across all lanes. The rest of
the instructions in the block will be moved to a new block following
the loop. Check if the next instruction's parent changed, and update
the iterators and insertion block if this happened.

Tests will be included in a future patch.

[qcolombet]

Nice catch!

LGTM. Question below.

lib/CodeGen/GlobalISel/RegBankSelect.cpp

709

Does this potentially breaks the iterators of RPOT?

[arsenm]

lib/CodeGen/GlobalISel/RegBankSelect.cpp

709

No, it builds a complete vector ahead of time. The newly created blocks aren’t visited but I don’t need them to be

[qcolombet]

lib/CodeGen/GlobalISel/RegBankSelect.cpp

709

Thanks for double checking!

[arsenm]

r354591

lib/CodeGen/GlobalISel/RegBankSelect.cpp

for (MachineBasicBlock::iterator MII = MBB->begin(), End = MBB->end();
       if (isTargetSpecificOpcode(MI.getOpcode()))
         continue;
 
       if (!assignInstr(MI)) {
         reportGISelFailure(MF, *TPC, *MORE, "gisel-regbankselect",
                            "unable to map instruction", MI);
         return false;
       }
+
+      // It's possible the mapping changed control flow, and moved the following
+      // instruction to a new block, so figure out the new parent.
+      if (MII != End) {
+        MachineBasicBlock *NextInstBB = MII->getParent();
+        if (NextInstBB != MBB) {
+          LLVM_DEBUG(dbgs() << "Instruction mapping changed control flow\n");
+          MBB = NextInstBB;
+          MIRBuilder.setMBB(*MBB);
+          End = MBB->end();
+        }
       }
     }
+  }
+
   OptMode = SaveOptMode;
   return false;
 }
 
 //------------------------------------------------------------------------------
 //                  Helper Classes Implementation
 //------------------------------------------------------------------------------
 RegBankSelect::RepairingPlacement::RepairingPlacement(