A proposal to align GCC stack

[Robert Dewar]

Ross Ridge wrote:
> Ye, Joey writes: 
>> i. STACK_BOUNDARY in bits, which is enforced by hardware, 32 for i386
>> and 64 for x86_64. It is the minimum stack boundary. It is fixed.
> 
> Ross Ridge wrote:
>> Strictly speaking by the above definition it would be 8 for i386.
>> The hardware doesn't force the stack to be 32-bit aligned, it just
>> performs poorly if it isn't.
> 
> Robert Dewar writes:
>> First, although for some types, the accesses may work, the optimizer
>> is allowed to assume that data is properly aligned, and could possibly
>> generate incorrect code ...
> 
> That's not enforced by hardware.

But suppose we have something like int(&k) & 1. The optimizer
is permitted to replace this with 0 if it knows that the type
of k is four byte aligned.
> 
>> Second, I am pretty sure there are SSE types that require
>> alignment at the hardware levell, even on the i386
> 
> This isn't a restriction on stack aligment.  It's a restriction on what
> kinds of machine types can be accessed on the stack.

Well if we have local variables of type float (and we have specified
use of SSE), we are in trouble, no?

> This is different than the proposed ABI_STACK_BOUNDARY macro which defines
> the possibily stricter aligment the ABI requires at function entry.  Since
> most i386 ABIs don't require a stricter alignment, that has ment that
> SSE types couldn't be located on the stack.  Currently you can get around
> this problem by changing the ABI using -fperferred-stack-boundary or by
> forcing an SSE compatible alignment using -mstackrealign or __attribute__
> ((force_align_arg_pointer)).  Joey Ye's proposal is another solution
> to this problem where GCC would automatically force an SSE compatible
> aligment when SSE types are used on the stack.

right ...
> 
> 					Ross Ridge