+{
+       asm("lea 40f(, %[slen], 4), %%r11\n\t"
+           "clc\n\t"
+           "jmpq *%%r11\n\t"
+           "adcq 7*8(%[src]),%[res]\n\t"
+           "adcq 6*8(%[src]),%[res]\n\t"
+           "adcq 5*8(%[src]),%[res]\n\t"
+           "adcq 4*8(%[src]),%[res]\n\t"
+           "adcq 3*8(%[src]),%[res]\n\t"
+           "adcq 2*8(%[src]),%[res]\n\t"
+           "adcq 1*8(%[src]),%[res]\n\t"
+           "adcq 0*8(%[src]),%[res]\n\t"
+           "nop\n\t"
+           "40: adcq $0,%[res]"
+                   : [res] "=r" (sum)
+                   : [src] "r" (buff),
+                     [slen] "r" (-((unsigned long)(len >> 3))), "[res]" (sum)
+                   : "r11");
+

With this patch I cannot mix/match different length checksums without
things failing.  In perf the jmpq in the loop above seems to be set to
a fixed value so perhaps it is something in how the compiler is
interpreting the inline assembler.

- Alex