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

    g:497498c878d48754318e486428e2aa30854020b9 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.

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index b75ac74e73a37b380cd85701c22a6ce4084b0e39..7678776c344519ecfb4d20cea723c9245499181d 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+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

diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index 7505a0e1e8e18b34110c82d2fdc4dca6f6796720..5b36593bfb715b5239ec01ad0843bd6fd6db6c8c 100644 (file)
--- a/gcc/config/rs6000/rs6000.c
+++ b/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;