This simple patch implements Richard Biener's suggestion in comment #6
of PR tree-optimization/52171 (from February 2013) that the insn-preds
code generated by genpreds can avoid using strncmp when matching constant
strings of length one.
The effect of this patch is best explained by the diff of insn-preds.cc:
< if (!strncmp (str + 1, "g", 1))
---
> if (str[1] == 'g')
3104c3104
< if (!strncmp (str + 1, "m", 1))
---
> if (str[1] == 'm')
3106c3106
< if (!strncmp (str + 1, "c", 1))
---
> if (str[1] == 'c')
...
The equivalent optimization is performed by GCC (but perhaps not by the
host compiler), but generating simpler/smaller code may encourage further
optimizations (such as use of a switch statement).
2022-05-24 Roger Sayle <roger@nextmovesoftware.com>
gcc/ChangeLog
* genpreds.cc (write_lookup_constraint_1): Avoid generating a call
to strncmp for strings of length one.
{
do
{
- printf (" if (!strncmp (str + 1, \"%s\", %lu))\n"
- " return CONSTRAINT_%s;\n",
- c->name + 1, (unsigned long int) c->namelen - 1,
- c->c_name);
+ if (c->namelen > 2)
+ printf (" if (!strncmp (str + 1, \"%s\", %lu))\n"
+ " return CONSTRAINT_%s;\n",
+ c->name + 1, (unsigned long int) c->namelen - 1,
+ c->c_name);
+ else
+ printf (" if (str[1] == '%c')\n"
+ " return CONSTRAINT_%s;\n",
+ c->name[1], c->c_name);
c = c->next_this_letter;
}
while (c);