gcse: Skip hardreg pre when the hardreg is never live [PR121095]

r15-6789-ge7f98d9603808b added a new RTL pass for hardreg PRE for the hard register
of FPM_REGNUM, this pass could get expensive if you have a large number of basic blocks
and the hard register was never live so it does nothing in the end.
In the aarch64 case, FPM_REGNUM is only used for FP8 related code so it has a high probability
of not being used. So skipping the pass for that register can improve both compile time and memory
usage.

Build and tested for aarch64-linux-gnu.

	PR middle-end/121095
gcc/ChangeLog:

	* gcse.cc (execute_hardreg_pre): Skip if the hardreg which is never live.

Signed-off-by: Andrew Pinski <[email protected]>

Author: apinski-quic

gcc/gcse.cc

@@ -4238,8 +4238,15 @@ execute_hardreg_pre (void)
     {
       int changed;
       current_hardreg_regno = regnos[i];
+      if (!df_regs_ever_live_p (current_hardreg_regno))
+	{
+	  if (dump_file)
+	    fprintf (dump_file, "Skipping hardreg PRE for regno %d, which is never live\n",
+		     current_hardreg_regno);
+	  continue;
+	}
       if (dump_file)
-	fprintf(dump_file, "Entering hardreg PRE for regno %d\n",
+	fprintf (dump_file, "Entering hardreg PRE for regno %d\n",
 		current_hardreg_regno);
       delete_unreachable_blocks ();
       df_analyze ();