пт, 28 авг. 2026 г. в 15:55, Richard Biener <[email protected]>:
>
> On Thu, 27 Aug 2026, Oleg Tolmatcev wrote:
>
> > Targets that cannot realign the stack cap their stack alignment and rely
> > on the middle end to align over-aligned stack objects inside their slots.
> > Win64 SEH caps MAX_STACK_ALIGNMENT at 128 bits; aarch64 does not define
> > MAX_STACK_ALIGNMENT at all and is capped at STACK_BOUNDARY, also 128.
> >
> > In several places the middle end does not do that alignment. A 256-bit
> > vector requires 32-byte alignment, its slot gets the capped 16 bytes, and
> > MEM_ALIGN is set to 32 anyway. x86 selects vmovaps from MEM_ALIGN and
> > faults on the under-aligned slot (PR54412, MSYS2/MINGW-packages#1209).
> > There is an even bigger problem. Even with MEM_ALIGN and the slot's
> > alignment consistent, an escaped pointer loses the information about the
> > alignment cap: GCC assumes the object's natural alignment and uses an
> > aligned access on an under-aligned object, which faults on x86.
> >
> > aarch64 does not crash only because its loads and stores do not fault on
> > a misaligned address; the object is under-aligned there just the same.
> >
> > Add assign_stack_local_aligned, which preserves the requested alignment by
> > overallocating a slot and aligning an address inside it, and use it
> > wherever an object gets a stack slot of its own. This is what
> > assign_parm_setup_block already did for BLKmode parameters and what
> > expand_stack_vars does for locals. Such a slot has no fixed frame offset,
> > so it cannot be reused as a temp slot.
>
> It seems to me that assign_stack_local itself guarantees alignment.
> Why does it (or rather assign_stack_local_1 with kind == ASLK_RECORD_PAD)
> not work?
>
> Richard.
assign_stack_local_1 silently caps the the alignment at
MAX_SUPPORTED_STACK_ALIGNMENT which for Win64 SEH is 16 bytes:
/* Ignore alignment if it exceeds MAX_SUPPORTED_STACK_ALIGNMENT. */
if (alignment_in_bits > MAX_SUPPORTED_STACK_ALIGNMENT)
{
alignment_in_bits = MAX_SUPPORTED_STACK_ALIGNMENT;
alignment = MAX_SUPPORTED_STACK_ALIGNMENT / BITS_PER_UNIT;
}
kind == ASLK_RECORD_PAD means that that the padding will be recorded
in a free list for later reuse. It has no influence on alignment and
assign_stack_local_1 does not even look at it before capping the
alignment.
Oleg
