llvm dev - May 2010 - [LLVMdev] Size of packed struct type <{<3 x i32>, i32}>

I have a packed struct <{<3 x i32>, i32}> type that LLVM determines
to be 20 bytes.
Is this the expected size for this type?  

I wrote a small example to illustrate:

; ModuleID = 'myexample.bc'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
target triple = "x86_64-pc-linux"

%mytype = type <{<3 x i32>, i32}>
define void @myexample(%mytype* %src, i64 %index, i32* %result) {
entry:
  %vectoraddr = getelementptr %mytype* %src, i64 %index, i32 0 
  %vector = load <3 x i32>* %vectoraddr                        
  %tmp1 = extractelement <3 x i32> %vector, i32 2
  store i32 %tmp1, i32* %result
  ret void
}

When I generate code (llc revision: 103084) i get:

.Leh_func_begin0:
# BB#0:                                 # %entry
        leaq    (%rsi,%rsi,4), %rax      <- Multiply index by 5
        pshufd  $2, (%rdi,%rax,4), %xmm0 <- multiply again by 4 (element size
20) and add base pointer
        movd    %xmm0, (%rdx)
        ret

My guess was that the size should be 16 because I thought there should be no
padding between elements.

- Jan

On 05/05/10 17:29, Jan Sjodin wrote:
> I have a packed struct<{<3 x i32>, i32}>  type that LLVM
determines to be 20 bytes.
> Is this the expected size for this type?
Yes.  LLVM systematically allocates the same amount of memory for an object of
a given type (here <3 x i32>), whether on the stack, in an array, in a
struct,
or a packed struct etc.  The fact that the struct is packed is not relevant [*].
> My guess was that the size should be 16 because I thought there should be
no padding between elements.
There is no padding between elements, there is padding inside an element!

Ciao,

Duncan.

[*] This used to be different: when I first rationalized the notion of size in
LLVM, I allocated less memory to types when they are in a packed struct.  But I
later reverted that, since having this exceptional case seemed too confusing.