diff --git a/transform/allocs.go b/transform/allocs.go
index 8bc81783..ef5467bc 100644
--- a/transform/allocs.go
+++ b/transform/allocs.go
@@ -95,19 +95,36 @@ func OptimizeAllocs(mod llvm.Module, printAllocs *regexp.Regexp, logger func(tok
 		}
 		// The pointer value does not escape.
 
+		// Determine the appropriate alignment of the alloca. The size of the
+		// allocation gives us a hint what the alignment should be.
+		var alignment int
+		if size%2 != 0 {
+			alignment = 1
+		} else if size%4 != 0 {
+			alignment = 2
+		} else if size%8 != 0 {
+			alignment = 4
+		} else {
+			alignment = 8
+		}
+		if pointerAlignment := targetData.ABITypeAlignment(i8ptrType); pointerAlignment < alignment {
+			// Use min(alignment, alignof(void*)) as the alignment.
+			alignment = pointerAlignment
+		}
+
 		// Insert alloca in the entry block. Do it here so that mem2reg can
 		// promote it to a SSA value.
 		fn := bitcast.InstructionParent().Parent()
 		builder.SetInsertPointBefore(fn.EntryBasicBlock().FirstInstruction())
-		alignment := targetData.ABITypeAlignment(i8ptrType)
-		sizeInWords := (size + uint64(alignment) - 1) / uint64(alignment)
-		allocaType := llvm.ArrayType(mod.Context().IntType(alignment*8), int(sizeInWords))
+		allocaType := llvm.ArrayType(mod.Context().Int8Type(), int(size))
 		alloca := builder.CreateAlloca(allocaType, "stackalloc.alloca")
+		alloca.SetAlignment(alignment)
 
 		// Zero the allocation inside the block where the value was originally allocated.
 		zero := llvm.ConstNull(alloca.Type().ElementType())
 		builder.SetInsertPointBefore(bitcast)
-		builder.CreateStore(zero, alloca)
+		store := builder.CreateStore(zero, alloca)
+		store.SetAlignment(alignment)
 
 		// Replace heap alloc bitcast with stack alloc bitcast.
 		stackalloc := builder.CreateBitCast(alloca, bitcast.Type(), "stackalloc")
diff --git a/transform/testdata/allocs.out.ll b/transform/testdata/allocs.out.ll
index ec7c4c59..48f9b768 100644
--- a/transform/testdata/allocs.out.ll
+++ b/transform/testdata/allocs.out.ll
@@ -6,17 +6,17 @@ target triple = "armv7m-none-eabi"
 declare nonnull i8* @runtime.alloc(i32, i8*)
 
 define void @testInt() {
-  %stackalloc.alloca = alloca [1 x i32], align 4
-  store [1 x i32] zeroinitializer, [1 x i32]* %stackalloc.alloca, align 4
-  %stackalloc = bitcast [1 x i32]* %stackalloc.alloca to i32*
+  %stackalloc.alloca = alloca [4 x i8], align 4
+  store [4 x i8] zeroinitializer, [4 x i8]* %stackalloc.alloca, align 4
+  %stackalloc = bitcast [4 x i8]* %stackalloc.alloca to i32*
   store i32 5, i32* %stackalloc, align 4
   ret void
 }
 
 define i16 @testArray() {
-  %stackalloc.alloca = alloca [2 x i32], align 4
-  store [2 x i32] zeroinitializer, [2 x i32]* %stackalloc.alloca, align 4
-  %stackalloc = bitcast [2 x i32]* %stackalloc.alloca to i16*
+  %stackalloc.alloca = alloca [6 x i8], align 2
+  store [6 x i8] zeroinitializer, [6 x i8]* %stackalloc.alloca, align 2
+  %stackalloc = bitcast [6 x i8]* %stackalloc.alloca to i16*
   %1 = getelementptr i16, i16* %stackalloc, i32 1
   store i16 5, i16* %1, align 2
   %2 = getelementptr i16, i16* %stackalloc, i32 2
@@ -39,9 +39,9 @@ define void @testEscapingCall2() {
 }
 
 define void @testNonEscapingCall() {
-  %stackalloc.alloca = alloca [1 x i32], align 4
-  store [1 x i32] zeroinitializer, [1 x i32]* %stackalloc.alloca, align 4
-  %stackalloc = bitcast [1 x i32]* %stackalloc.alloca to i32*
+  %stackalloc.alloca = alloca [4 x i8], align 4
+  store [4 x i8] zeroinitializer, [4 x i8]* %stackalloc.alloca, align 4
+  %stackalloc = bitcast [4 x i8]* %stackalloc.alloca to i32*
   %1 = call i32* @noescapeIntPtr(i32* %stackalloc)
   ret void
 }
@@ -54,12 +54,12 @@ define i32* @testEscapingReturn() {
 
 define void @testNonEscapingLoop() {
 entry:
-  %stackalloc.alloca = alloca [1 x i32], align 4
+  %stackalloc.alloca = alloca [4 x i8], align 4
   br label %loop
 
 loop:                                             ; preds = %loop, %entry
-  store [1 x i32] zeroinitializer, [1 x i32]* %stackalloc.alloca, align 4
-  %stackalloc = bitcast [1 x i32]* %stackalloc.alloca to i32*
+  store [4 x i8] zeroinitializer, [4 x i8]* %stackalloc.alloca, align 4
+  %stackalloc = bitcast [4 x i8]* %stackalloc.alloca to i32*
   %0 = call i32* @noescapeIntPtr(i32* %stackalloc)
   %1 = icmp eq i32* null, %0
   br i1 %1, label %loop, label %end