[GlobalOpt] Preven widenDestArray from shrinking an alloca.

If the destination alloca for one of the memcpy calls we are
modifying is already larger than our desired size, we shouldn't
replace it with a smaller alloca.

Author: topperc

llvm/lib/Transforms/IPO/GlobalOpt.cpp

@@ -2103,6 +2103,10 @@ static void widenDestArray(CallInst *CI, const unsigned NumBytesToPad,
     unsigned ElementByteWidth = SourceDataArray->getElementByteSize();
     unsigned int TotalBytes = NumBytesToCopy + NumBytesToPad;
     unsigned NumElementsToCopy = divideCeil(TotalBytes, ElementByteWidth);
+    // Don't change size if already wide enough.
+    if (Alloca->getAllocatedType()->getArrayNumElements() >= NumElementsToCopy)
+      return;
+
     // Update destination array to be word aligned (memcpy(X,...,...))
     IRBuilder<> BuildAlloca(Alloca);
     AllocaInst *NewAlloca = BuildAlloca.CreateAlloca(ArrayType::get(

llvm/test/Transforms/GlobalOpt/ARM/arm-widen-large-alloca.ll

@@ -7,7 +7,7 @@ define void @memcpy()  {
 ; CHECK-LABEL: define void @memcpy() local_unnamed_addr {
 ; CHECK-NEXT:  [[ENTRY:.*:]]
 ; CHECK-NEXT:    [[ALLOCA1:%.*]] = alloca [4 x i8], align 1
-; CHECK-NEXT:    [[ALLOCA2:%.*]] = alloca [4 x i8], align 1
+; CHECK-NEXT:    [[ALLOCA2:%.*]] = alloca [5 x i8], align 1
 ; CHECK-NEXT:    [[CALL1:%.*]] = call i32 @bar(ptr nonnull [[ALLOCA1]])
 ; CHECK-NEXT:    [[CALL2:%.*]] = call i32 @bar(ptr nonnull [[ALLOCA2]])
 ; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i32(ptr noundef nonnull align 1 dereferenceable(3) [[ALLOCA1]], ptr noundef nonnull align 1 dereferenceable(3) @.i8, i32 4, i1 false)