[gcc r13-738] Minor improvement to genpreds.cc

[gcc r13-738] Minor improvement to genpreds.cc

[Roger Sayle]

https://gcc.gnu.org/g:9e7a0e42a15eb53850496e91f2e484ed74ac3617

commit r13-738-g9e7a0e42a15eb53850496e91f2e484ed74ac3617
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Tue May 24 14:29:27 2022 +0100

    Minor improvement to genpreds.cc
    
    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.

Diff:
---
 gcc/genpreds.cc | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/gcc/genpreds.cc b/gcc/genpreds.cc
index f71da094e25..4571ac7ec6b 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);