Use DL preferred alignment for alloca in Value::getPointerAlignment

[apilipenko]

Summary

Teach Value::getPointerAlignment that allocas with no explicit alignment are aligned to preferred alignment of the allocated type.

[hfinkel]

LGTM.

lib/IR/Value.cpp

if (auto *GO = dyn_cast<GlobalObject>(this)) {
     Align = A->getParamAlignment();
 
     if (!Align && A->hasStructRetAttr()) {
       // An sret parameter has at least the ABI alignment of the return type.
       Type *EltTy = cast<PointerType>(A->getType())->getElementType();
       if (EltTy->isSized())
         Align = DL.getABITypeAlignment(EltTy);
     }
-  } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(this))
+  } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(this)) {
     Align = AI->getAlignment();
-  else if (auto CS = ImmutableCallSite(this))
+    if (Align == 0) {
+      Type *AllocatedType = AI->getAllocatedType();
+      if (AllocatedType->isSized())
+        Align = DL.getPrefTypeAlignment(AllocatedType);
+    }
+  } else if (auto CS = ImmutableCallSite(this))
     Align = CS.getAttributes().getParamAlignment(AttributeSet::ReturnIndex);
   else if (const LoadInst *LI = dyn_cast<LoadInst>(this))
     if (MDNode *MD = LI->getMetadata(LLVMContext::MD_align)) {
       ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(0));
       Align = CI->getLimitedValue();
     }
 
   return Align;

test/Analysis/ValueTracking/memory-dereferenceable.ll

 ; RUN: opt -print-memderefs -analyze -S <%s | FileCheck %s
 
 ; Uses the print-deref (+ analyze to print) pass to run
 ; isDereferenceablePointer() on many load instruction operands
 
-target datalayout = "e"
+target datalayout = "e-i32:32:64"
 
 %TypeOpaque = type opaque
 
 declare zeroext i1 @return_i1()
 
 declare i32* @foo()
 @globalstr = global [6 x i8] c"hello\00"
 @globali32ptr = external global i32*
; CHECK: %deref_and_aligned_return{{.*}}(aligned)
     ; Load from a dereferenceable and aligned load
 ; CHECK: %d4_unaligned_load{{.*}}(unaligned)
 ; CHECK: %d4_aligned_load{{.*}}(aligned)
     %d4_unaligned_load = load i32*, i32** @globali32ptr, !dereferenceable !0
     %d4_aligned_load = load i32*, i32** @globali32ptr, !dereferenceable !0, !align !{i64 16}
     %load26 = load i32, i32* %d4_unaligned_load, align 16
     %load27 = load i32, i32* %d4_aligned_load, align 16
 
+   ; Alloca with no explicit alignment is aligned to preferred alignment of
+   ; the type (specified by datalayout string).
+; CHECK: %alloca.noalign{{.*}}(aligned)
+    %alloca.noalign = alloca i32
+    %load28 = load i32, i32* %alloca.noalign, align 8
+
     ret void
 }
 
 ; Just check that we don't crash.
 ; CHECK-LABEL: 'opaque_type_crasher'
 define void @opaque_type_crasher(%TypeOpaque* dereferenceable(16) %a) {
 entry:
   %bc = bitcast %TypeOpaque* %a to i8*