[Analysis] Fix isSafeToLoadUnconditionally handling of volatile.

[efriedma]

Summary

A volatile operation cannot be used to prove an address points to normal memory. (LangRef was recently updated to state this explicitly.)

This issue was found by inspection; I think it's unlikely to show up in real code.

[hfinkel]

LGTM

llvm/trunk/lib/Analysis/Loads.cpp

while (BBI != E) {
     // a free) the pointer could be marked invalid.
     if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
         !isa<DbgInfoIntrinsic>(BBI))
       return false;
 
     Value *AccessedPtr;
     unsigned AccessedAlign;
     if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
+      // Ignore volatile loads. The execution of a volatile load cannot
+      // be used to prove an address is backed by regular memory; it can,
+      // for example, point to an MMIO register.
+      if (LI->isVolatile())
+        continue;
       AccessedPtr = LI->getPointerOperand();
       AccessedAlign = LI->getAlignment();
     } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
+      // Ignore volatile stores (see comment for loads).
+      if (SI->isVolatile())
+        continue;
       AccessedPtr = SI->getPointerOperand();
       AccessedAlign = SI->getAlignment();
     } else
       continue;
 
     Type *AccessedTy = AccessedPtr->getType()->getPointerElementType();
     if (AccessedAlign == 0)
       AccessedAlign = DL.getABITypeAlignment(AccessedTy);

llvm/trunk/test/Transforms/SROA/phi-and-select.ll

 
 exit:
 ; CHECK: %phi = phi i32* [ {{.*}}, %then ], [ %z, %entry ]
 ; CHECK-NEXT: %result = load i32, i32* %phi, align 1
   %phi = phi i32* [ %a1, %then ], [ %z, %entry ]
   %result = load i32, i32* %phi, align 4
   ret i32 %result
 }
+
+; Don't speculate a load based on an earlier volatile operation.
+define i8 @volatile_select(i8* %p, i1 %b) {
+; CHECK-LABEL: @volatile_select(
+; CHECK: select i1 %b, i8* %p, i8* %p2
+  %p2 = alloca i8
+  store i8 0, i8* %p2
+  store volatile i8 0, i8* %p
+  %px = select i1 %b, i8* %p, i8* %p2
+  %v2 = load i8, i8* %px
+  ret i8 %v2
+}