rs6000: Allow FPRs to change between SDmode and DDmode [PR94254]

    g:497498c8 caused lra to cycle
    on some SDmode reloads for power6.  As explained in more detail
    in the PR comments, the problem was a conflict between two target
    hooks: rs6000_secondary_memory_needed_mode required SDmode FPR
    reloads to use DDmode memory (rightly, since using SDmode memory
    wouldn't make progress) but rs6000_can_change_mode_class didn't
    allow FPRs to change from SDmode to DDmode.  Previously lra
    ignored that and changed the mode anyway.

    From what Segher says, it sounds like the "from_size < 8 || to_size < 8"
    check is mostly there for SF<->64-bit subregs, and that SDmode is stored
    in the way that target-independent code expects.  This patch therefore
    allows SD<->DD changes.

    I wondered about checking for SD<->64-bit changes instead, but that
    seemed like an unnecessary generalisation for this stage.

    2020-03-23  Richard Sandiford  <richard.sandiford@arm.com>

    gcc/
            PR target/94254
            * config/rs6000/rs6000.c (rs6000_can_change_mode_class): Allow
            FPRs to change between SDmode and DDmode.

gcc/ChangeLog

+2020-03-25  Richard Sandiford  <richard.sandiford@arm.com>
+
+	PR target/94254
+	* config/rs6000/rs6000.c (rs6000_can_change_mode_class): Allow
+	FPRs to change between SDmode and DDmode.
+
 2020-03-25  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/94131

gcc/config/rs6000/rs6000.c

@@ -12307,6 +12307,15 @@ rs6000_can_change_mode_class (machine_mode from,
 	  if (!BYTES_BIG_ENDIAN && (to == TDmode || from == TDmode))
 	    return false;
 
+	  /* Allow SD<->DD changes, since SDmode values are stored in
+	     the low half of the DDmode, just like target-independent
+	     code expects.  We need to allow at least SD->DD since
+	     rs6000_secondary_memory_needed_mode asks for that change
+	     to be made for SD reloads.  */
+	  if ((to == DDmode && from == SDmode)
+	      || (to == SDmode && from == DDmode))
+	    return true;
+
 	  if (from_size < 8 || to_size < 8)
 	    return false;