[Analysis] Fix isSafeToLoadUnconditionally handling of volatile.

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

Differential Revision: https://reviews.llvm.org/D57040

llvm-svn: 352109

Author: Eli Friedman

llvm/lib/Analysis/Loads.cpp

@@ -280,9 +280,17 @@ bool llvm::isSafeToLoadUnconditionally(Value *V, unsigned Align,
     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

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

@@ -632,3 +632,15 @@ exit:
   %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
+}