[LLVMdev] Padding in Aggregates Useful ?

[Reid Kleckner]

Anything could live there. Run this IR through clang and you'll see an
alloca with a struct type where the padding is used by memset and
'use_bytes':

#include <string.h>
void use_bytes(char *);
int main() {
  union {
    char bytes[8];
    struct {
      char c;
      int i;
    } with_padding;
  } u = { .with_padding = {'a', 42} };
  memset(u.bytes, 8, 0);
  use_bytes(u.bytes);
}

On Thu, Apr 16, 2015 at 9:57 PM, Xin Tong <trent.tong at gmail.com> wrote:

> Hi
>
> I see that LLVM does not scalar replace aggregates used as the src or
> dst of memcpy. and the reason is  memcpy could be moving around
> elements that live in structure padding of the LLVM types. I wonder
> what could live in the padding, but may actually used ?
>
> 02605   // Okay, we know all the users are promotable.  If the
> aggregate is a memcpy
> 02606   // source and destination, we have to be careful.  In
> particular, the memcpy
> 02607   // could be moving around elements that live in structure
> padding of the LLVM
> 02608   // types, but may actually be used.  In these cases, we refuse
> to promote the
> 02609   // struct.
> 02610   if (Info.isMemCpySrc && Info.isMemCpyDst &&
> 02611       HasPadding(AI->getAllocatedType(), DL))
> 02612     return false;
>
> Thanks
> Trent
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