Minor tweak to MDA

[reames]

Summary

Two minor tweaks I noticed when reading through the code:

• No need to recompute begin() on every iteration. We're not modifying the instructions in this loop.
• We can ignore PHINodes and Dbg intrinsics. The current code does this anyways, but it will spend slightly more time doing so and will count towards the limit of instructions in the block. It seems really silly to give up due the presence of PHIs...

[jfb]

lgtm otherwise.

lib/Analysis/MemoryDependenceAnalysis.cpp

426

Missing period.

lib/Analysis/MemoryDependenceAnalysis.cpp

 
   if (isLoad && QueryInst) {
     LoadInst *LI = dyn_cast<LoadInst>(QueryInst);
     if (LI && LI->getMetadata(LLVMContext::MD_invariant_load) != nullptr)
       isInvariantLoad = true;
   }
 
   // Walk backwards through the basic block, looking for dependencies.
-  while (ScanIt != BB->begin()) {
+  // We can stop before processing PHIs or dbg intrinsics
+  const BasicBlock::iterator Begin(BB->getFirstNonPHIOrDbg());
+  while (ScanIt != Begin) {
     Instruction *Inst = --ScanIt;
 
     if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(Inst))
       // Debug intrinsics don't (and can't) cause dependencies.
       if (isa<DbgInfoIntrinsic>(II)) continue;
 
     // Limit the amount of scanning we do so we don't end up with quadratic
     // running time on extreme testcases.