On 11/29/2012 12:09 PM, Andi Kleen wrote:

Jim Kukunas [off-list ref] writes:

quoted

+
+	/* 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