Re: [PATCH] Provide macro to override limiting field alignment

[Fred Forsman <forsman at apple dot com>]

On Monday, December 10, 2001, at 05:59  PM, Richard Henderson wrote:

> On Mon, Dec 10, 2001 at 04:26:44PM -0800, Fred Forsman wrote:
>> This patch introduces a macro (LIMIT_FIELD_ALIGN) to limit the 
>> alignment
>> of fields to maximum_field_alignment.
>
> Are you suggesting that perhaps "#pragma pack(N)" shouldn't do exactly
> what the user told us to do?  This doesn't seem well conceived.

In my proposed patch the LIMIT_FIELD_ALIGN macro is used to replace 
statements of the form

   x = MIN (x, maximum_field_alignment);

with

   x = LIMIT_FIELD_ALIGN (x);

As is, this does not change any behavior.  It does, however, provide a 
hook for handling the alignment of vector (AltiVec) data on the PowerPC, 
where there is a "usual" maximum_field_alignment of 32 bits which must 
be overridden for vector data.  In the case of such vector data, the 
vector alignment requirements take precedence above all else since the 
processor masks off any unaligned bits of address in a vector access.  
BTW, on Darwin we support two alignment modes, the default "power" 
alignment (in which maximum_field_alignment is 0, but in fact usually 
constrained to 32) and "mac68k" (in which maximum_field_alignment is 
16).  Vector data in both of these alignment modes is treated as an 
exception to the usual rules and is 128-bit aligned.

I'd appreciate your suggestions on how to treat this issue.

Fred