On Fri, Mar 27, 2026 at 12:14 PM Richard Biener <[email protected]> wrote:
>
> The following fixes a confusion seen in the x86 backend by
> assign_parm_adjust_stack_rtl failing to trigger a local stack copy
> for an incoming stack parameter that is not aligned according to
> its type. The condition was introduced in r0-64961-gbfc45551d5ace4
> but there is the MEM_ALIGN (stack_parm) < PREFERRED_STACK_BOUNDARY
> condition not triggering for the case in question where both
> MEM_ALIGN and PREFERRED_STACK_BOUNDARY are 128. x86 supports
> stack-realignment so we can honor the declared alignment and this
> clears up the confusion. The following replaces the bound
> by MAX_SUPPORTED_STACK_ALIGNMENT if SUPPORTS_STACK_ALIGNMENT
> and the parameter has it's address taken and with BIGGEST_ALIGNMENT
> if SUPPORTS_STACK_ALIGNMENT otherwise, only affecting x86 and nvptx
> at this point.
>
> In addition to this it changes the i386 backends computation of
> the maximum stack alignment used to bound itself to BIGGEST_ALIGNMENT
> because the middle-end does not (and in general cannot) enforce actual
> alignment of all stack objects according to their type.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu.
>
> Compared to v2 this uses BIGGEST_ALIGNMENT if the parameter does not
> have its address taken and caps ix86_update_stack_alignment on
> BIGGEST_ALIGNMENT as well.
I don't think capping ix86_update_stack_alignment to BIGGEST_ALIGNMENT
is correct. For TARGET_IAMCU, biggest alignment is 32, but it still
can use FXSAVE that wants 16-byte (128-bit) alignment.
set_fast_math_sse in libgcc/config/i386/crtfastmath.c uses:
/* Check if DAZ is available. */
struct
{
unsigned short cwd;
unsigned short swd;
unsigned short twd;
unsigned short fop;
unsigned int fip;
unsigned int fcs;
unsigned int foo;
unsigned int fos;
unsigned int mxcsr;
unsigned int mxcsr_mask;
unsigned int st_space[32];
unsigned int xmm_space[32];
unsigned int padding[56];
} __attribute__ ((aligned (16))) fxsave;
which will probably fail with your x86 change due to unsatisfied
alignment requirements.
Uros.
