Extended Description

[hjl@gnu-6 interrupt-8]$ cat z.i
typedef short __v4hi attribute ((vector_size (8)));
typedef int __m64 attribute ((vector_size (8), may_alias));

extern __m64 y, x;

void
attribute((interrupt))
fn1 (void *frame)
{
x = (__m64) __builtin_ia32_packsswb ((__v4hi) x, (__v4hi) y);
}
[hjl@gnu-6 interrupt-8]$ make z.s
/export/build/gnu/llvm-clang-bootstrap/stage1/build-x86_64-linux/bin/clang -O2 -Wall -march=nehalem -S -o z.s z.i
[hjl@gnu-6 interrupt-8]$ cat z.s
.text
.file "z.i"
.globl fn1
.p2align 4, 0x90
.type fn1,@function
fn1: # @​fn1
.cfi_startproc

BB#0:

movq	x(%rip), %mm0
packsswb	y(%rip), %mm0
movq	%mm0, x(%rip)
iretq

.Lfunc_end0:
.size fn1, .Lfunc_end0-fn1
.cfi_endproc

.ident	"clang version 3.9.0 (http://llvm.org/git/clang.git 1f64ddbc4c5d1036b68ec896765a7535537ded85) (http://llvm.org/git/llvm.git 43b517fe4e0a181b1cf20f36fd9eb92f7b32946c)"
.section	".note.GNU-stack","",@progbits

[hjl@gnu-6 interrupt-8]$

Interrupt handler must preserve MMX and X87 states. It shouldn"t be
allowed to use MMX nor x87 registers.