[PATCH] Minor improvement to genpreds.cc

[Roger Sayle]

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).

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check with no new failures.  Ok for mainline?


2022-05-22  Roger Sayle  <ro...@nextmovesoftware.com>

gcc/ChangeLog
        * genpreds.cc (write_lookup_constraint_1): Avoid generating a call
        to strncmp for strings of length one.

Roger
--


diff --git a/gcc/genpreds.cc b/gcc/genpreds.cc
index f71da09..4571ac7 100644
--- a/gcc/genpreds.cc
+++ b/gcc/genpreds.cc
@@ -1089,10 +1089,15 @@ write_lookup_constraint_1 (void)
        {
          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);