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

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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.
This commit is contained in:
Jeff Law 2020-03-25 10:37:31 -06:00
parent c7a252ba2d
commit 713ecb3d41
2 changed files with 15 additions and 0 deletions
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6
gcc/ChangeLog
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@ -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

9
gcc/config/rs6000/rs6000.c
View File

@ -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;