[Bug target/79793] Incorrect stack alignment for interrupt handler in 64-bit

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79793

--- Comment #7 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to H.J. Lu from comment #5)
> In 64-bit mode, stack is 16-byte aligned when entering handler.  But if
> there is error code
> 
> SS
> RSP
> FLAGS
> CS
> RIP
> Error Code
> 
> the stack alignment is off by 8 bytes.  That is %rsp + 8 isn't multiple
> of 16 at the start of handler.

A testcase:

[hjl@gnu-skl-1 pr79793]$ cat y.c
#include <fxsrintrin.h>

typedef unsigned int uword_t __attribute__ ((mode (__word__)));

struct interrupt_frame
{
  uword_t ip;
  uword_t cs;
  uword_t flags;
  uword_t sp;
  uword_t ss;
};

__attribute__((interrupt))
void
fn (struct interrupt_frame *frame, uword_t error)
{
  char fxsave_region [512] __attribute__((aligned(16)));
  _fxsave64 (fxsave_region);
}
[hjl@gnu-skl-1 pr79793]$ make y.s
/export/build/gnu/gcc-x32-7/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-x32-7/build-x86_64-linux/gcc/ -O2 -mgeneral-regs-only
-S -o y.s y.c
[hjl@gnu-skl-1 pr79793]$ cat y.s
        .file   "y.c"
        .text
        .p2align 4,,15
        .globl  fn
        .type   fn, @function
fn:
.LFB4:
        .cfi_startproc
        subq    $400, %rsp
        .cfi_def_cfa_offset 408
        fxsave64        -120(%rsp)
        addq    $408, %rsp
        iretq
        .cfi_endproc
.LFE4:
        .size   fn, .-fn
        .ident  "GCC: (GNU) 7.1.1 20170724"
        .section        .note.GNU-stack,"",@progbits
[hjl@gnu-skl-1 pr79793]$ 

-120(%rsp) isn't aligned at 16 bytes.

For

[hjl@gnu-skl-1 pr79793]$ cat x.c
#include <fxsrintrin.h>

typedef unsigned int uword_t __attribute__ ((mode (__word__)));

struct interrupt_frame
{
  uword_t ip;
  uword_t cs;
  uword_t flags;
  uword_t sp;
  uword_t ss;
};

__attribute__((interrupt))
void
fn (struct interrupt_frame *frame)
{
  char fxsave_region [512] __attribute__((aligned(16)));
  _fxsave64 (fxsave_region);
}
[hjl@gnu-skl-1 pr79793]$ make x.s
/export/build/gnu/gcc-x32-7/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-x32-7/build-x86_64-linux/gcc/ -O2 -mgeneral-regs-only
-S -o x.s x.c
[hjl@gnu-skl-1 pr79793]$ cat x.s
        .file   "x.c"
        .text
        .p2align 4,,15
        .globl  fn
        .type   fn, @function
fn:
.LFB4:
        .cfi_startproc
        subq    $400, %rsp
        .cfi_def_cfa_offset 408
        fxsave64        -120(%rsp)
        addq    $400, %rsp
        .cfi_def_cfa_offset 8
        iretq
        .cfi_endproc
.LFE4:
        .size   fn, .-fn
        .ident  "GCC: (GNU) 7.1.1 20170724"
        .section        .note.GNU-stack,"",@progbits
[hjl@gnu-skl-1 pr79793]$ 

-120(%rsp) is aligned at 16 bytes.