Re: Pentium 4 and misaligned doubles

[Rici Lake <lua@...>]

On 16-Aug-05, at 9:35 AM, Mike Pall wrote:

> Of course compiling with -malign-double is the easiest thing to
> solve this with GCC. Alas, this breaks the x86 ABI. I think this
> doesn't matter since the whole Lua core never passes structures
> or unions that contain doubles to C library functions or back.

Doesn't that change the stack alignment of double arguments? That would 
affect programs that called, for example, lua_pushnumber(), no? Or am I 
misreading something?

> The only way I could make it work is with:
>
>   typedef struct lua_TValue {
>     TValuefields;
>   } __attribute__ ((aligned(16))) TValue;
>
> A bit awkward, but solves both the stack alignment and the array
> alignment problem.

Wouldn't aligned(8) be sufficient? (At least, that wouldn't be lying to 
the compiler about the results of malloc() on x86 :) )

> On a related note: the lua_number2int() optimization should be
> turned off if __SSE2__ is defined (which is the case with
> -march=pentium4).

I had to specify -msse2 for this to work on gcc 3.3.3

PS: Before anyone else notices that I'm fond of making an idiot of 
myself in public, it is quite clear why array alignment isn't important 
(but constant alignment is). The GETTABLE/SETTABLE ops (and the 
equivalent API calls) copy the table data onto the stack with a union 
copy (see the setobj macro in lobject.h), so the fp unit is presumably 
not involved. The key object, on the other hand, is not copied (if the 
constant is in the acceptable range for RK operands) and is then used 
directly as a number, so it's alignment is important.