Messages in this thread | | | Date | Thu, 29 Nov 2012 13:13:28 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCH] lib/raid6: Add AVX2 optimized recovery functions |
| |
On 11/29/2012 12:09 PM, Andi Kleen wrote: > Jim Kukunas <james.t.kukunas@linux.intel.com> writes: >> + >> + /* ymm0 = x0f[16] */ >> + asm volatile("vpbroadcastb %0, %%ymm7" : : "m" (x0f)); >> + >> + while (bytes) { >> +#ifdef CONFIG_X86_64 >> + asm volatile("vmovdqa %0, %%ymm1" : : "m" (q[0])); >> + asm volatile("vmovdqa %0, %%ymm9" : : "m" (q[32])); >> + asm volatile("vmovdqa %0, %%ymm0" : : "m" (p[0])); >> + asm volatile("vmovdqa %0, %%ymm8" : : "m" (p[32])); > > This is somewhat dangerous to assume registers do not get changed > between assembler statements or assembler statements do not get > reordered. Better always put such values into explicit variables or > merge them into a single asm statement. > > asm volatile is also not enough to prevent reordering. If anything > you would need a memory clobber. >
The code is compiled so that the xmm/ymm registers are not available to the compiler. Do you have any known examples of asm volatiles being reordered *with respect to each other*? My understandings of gcc is that volatile operations are ordered with respect to each other (not necessarily with respect to non-volatile operations, though.)
Either way, this implementatin technique was used for the MMX/SSE implementations without any problems for 9 years now.
-h[a
| |