Give up on array allocas in getPointerDereferenceableBytes

[dotdash]

Summary

As suggested by Eli Friedman, don't try to handle array allocas here,
because of possible overflows, instead rely on instcombine converting
them to allocations of array types.

[efriedma]

LGTM with one minor change.

lib/IR/Value.cpp

659

The allocated type of an alloca is always sized (the verifier checks this).

lib/IR/Value.cpp

if (DerefBytes == 0) {
       if (MDNode *MD =
               LI->getMetadata(LLVMContext::MD_dereferenceable_or_null)) {
         ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(0));
         DerefBytes = CI->getLimitedValue();
       }
       CanBeNull = true;
     }
   } else if (auto *AI = dyn_cast<AllocaInst>(this)) {
-    const ConstantInt *ArraySize = dyn_cast<ConstantInt>(AI->getArraySize());
-    if (ArraySize && AI->getAllocatedType()->isSized()) {
-      DerefBytes = DL.getTypeStoreSize(AI->getAllocatedType()) *
-        ArraySize->getZExtValue();
+    if (AI->getAllocatedType()->isSized() && !AI->isArrayAllocation()) {
+      DerefBytes = DL.getTypeStoreSize(AI->getAllocatedType());
       CanBeNull = false;
     }
   } else if (auto *GV = dyn_cast<GlobalVariable>(this)) {
     if (GV->getValueType()->isSized() && !GV->hasExternalWeakLinkage()) {
       // TODO: Don't outright reject hasExternalWeakLinkage but set the
       // CanBeNull flag.
       DerefBytes = DL.getTypeStoreSize(GV->getValueType());
       CanBeNull = false;

test/Analysis/ValueTracking/memory-dereferenceable.ll

; CHECK: %alloca{{.*}}(aligned)
     %alloca = alloca i1
     %load2 = load i1, i1* %alloca
 
     ; Load from empty array alloca
 ; CHECK-NOT: %empty_alloca
     %empty_alloca = alloca i8, i64 0
     %empty_load = load i8, i8* %empty_alloca
 
-    ; Load from too small array alloca
-; CHECK-NOT: %small_array_alloca
-    %small_array_alloca = alloca i8, i64 2
-    %saa_cast = bitcast i8* %small_array_alloca to i32*
-    %saa_load = load i32, i32* %saa_cast
-
-    ; Load from array alloca
-; CHECK: %big_array_alloca{{.*}}(unaligned)
-    %big_array_alloca = alloca i8, i64 4
-    %baa_cast = bitcast i8* %big_array_alloca to i32*
-    %baa_load = load i32, i32* %baa_cast
-
     ; Loads from sret arguments
 ; CHECK: %sret_gep{{.*}}(aligned)
     %sret_gep = getelementptr inbounds %struct.A, %struct.A* %result, i64 0, i32 1, i64 2
     load i8, i8* %sret_gep
 
 ; CHECK-NOT: %sret_gep_outside
     %sret_gep_outside = getelementptr %struct.A, %struct.A* %result, i64 0, i32 1, i64 7
     load i8, i8* %sret_gep_outside