Fix misleading comment for getOrEnforceKnownAlignment

[arsenm]

Summary

It does not return 0 to indicate failure, and returns the known alignment.

[jlebar]

include/llvm/Transforms/Utils/Local.h

165

This prep phrase is unwieldy. Can we rephrase? (I'd give a suggestion but I'm not actually sure what it means.)

170

What do we return if the alignment can be modified?

[arsenm]

Try to rephrase

[jlebar]

I'm happy with the edits below.

Thanks for fixing this.

include/llvm/Transforms/Utils/Local.h

166

I still can't tell what this function returns on "success". Perhaps s/increased/increased and PrefAlign returned/?

167

New paragraph before "It"?

[arsenm]

r281350

include/llvm/Transforms/Utils/Local.h

AllocaInst *DemoteRegToStack(Instruction &X,
                              bool VolatileLoads = false,
                              Instruction *AllocaPoint = nullptr);
 
 /// This function takes a virtual register computed by a phi node and replaces
 /// it with a slot in the stack frame, allocated via alloca. The phi node is
 /// deleted and it returns the pointer to the alloca inserted.
 AllocaInst *DemotePHIToStack(PHINode *P, Instruction *AllocaPoint = nullptr);
 
-/// If the specified pointer has an alignment that we can determine, return it,
-/// otherwise return 0. If PrefAlign is specified, and it is more than the
-/// alignment of the ultimate object, see if we can increase the alignment of
-/// the ultimate object, making this check succeed.
+/// If \p PrefAlign is more than the alignment of the ultimate object the
+/// specified pointer is derived from that we control, modify the object's
+/// alignment to PrefAlign. This isn't often possible though. If alignment is
+/// important, a more reliable approach is to simply align all global variables
+/// and allocation instructions to their preferred alignment from the beginning.
+///
+/// If the alignment cannot be modified, return the determined alignment value.
+///
 unsigned getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign,
                                     const DataLayout &DL,
                                     const Instruction *CxtI = nullptr,
                                     AssumptionCache *AC = nullptr,
                                     const DominatorTree *DT = nullptr);
 
 /// Try to infer an alignment for the specified pointer.
 static inline unsigned getKnownAlignment(Value *V, const DataLayout &DL,

lib/Transforms/Utils/Local.cpp

if (auto *GO = dyn_cast<GlobalObject>(V)) {
 
     GO->setAlignment(PrefAlign);
     return PrefAlign;
   }
 
   return Align;
 }
 
-/// getOrEnforceKnownAlignment - If the specified pointer has an alignment that
-/// we can determine, return it, otherwise return 0.  If PrefAlign is specified,
-/// and it is more than the alignment of the ultimate object, see if we can
-/// increase the alignment of the ultimate object, making this check succeed.
 unsigned llvm::getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign,
                                           const DataLayout &DL,
                                           const Instruction *CxtI,
                                           AssumptionCache *AC,
                                           const DominatorTree *DT) {
   assert(V->getType()->isPointerTy() &&
          "getOrEnforceKnownAlignment expects a pointer!");
   unsigned BitWidth = DL.getPointerTypeSizeInBits(V->getType());