[cfe-commits] r51673 - in /cfe/trunk: lib/CodeGen/CodeGenTypes.cpp test/CodeGen/struct-x86-darwin.c test/CodeGen/struct.c

[Eli Friedman]

On Thu, May 29, 2008 at 6:02 PM, Devang Patel <dpatel at apple.com> wrote:
> Consider
>
>        struct { char c; int i; }
>
> At llvm level you use either
>
>       { i8, i32 }
>
> or
>
>      <{ i8, i8, i8, i8, i32 }>
>
> The packed version involves 5 copies during  struct element by element copy.
> And it also raises the issue of dealing with undefined bytes.
>
> Plus, such layout change causes changes in the arithmetic involved to access
> bit fields.  I have myself once added two line patch in llvm-gcc to
> aggressively (but not all the time) use packed LLVM struct. This simplified
> code in llvm-gcc but I had to back out that patch after few weeks. Because
> our customer reported 25% performance regression and we tracked it down to
> this change in struct layout at llvm level, which introduced significant
> penalty while accessing union bitfields.
>
> You can argue that 1) the there is no guaranteed solution if the code
> performance relies on such behavior  and 2) the code that emits  bitfield
> access in llvm-gcc can be made smarter.
> But the point is, such changes have performance impact, so do not take it
> lightly.

Okay then, I'll take it into consideration.  But I'm more worried
about getting correct code than about the performance of said code at
the moment.

>> When LLVM packs a struct, it appears to use the tightest packing
>> possible, which is in fact 10 bytes for an x86 long double.  The issue
>> is that gcc-compatible C struct layout doesn't allow packing them that
>> tightly (might be considered a bug, but not really something that can
>> be fixed at this point).  Not sure how to deal with it (it doesn't
>> show up with types with power-of-two width).
>
> I am at lost. When you say "LLVM", you mean llvm-gcc ?

No, by LLVM I mean the LLVM assembly-level packed struct.  <{ x86_fp80
}> is 10 bytes.  struct __attribute((packed)) {long double x;} is 12
bytes on x86 Linux, and 16 bytes on x86-64 and x86 Darwin.

-Eli