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commit 50e1e417b5cc68a6fd280e0a67c5c4eba524b70b
Author: Richard Sandiford <richard.sandiford@arm.com>
Date: Sun Mar 22 18:51:48 2020 +0000
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.
--- a/src/gcc/config/rs6000/rs6000.cc
+++ b/src/gcc/config/rs6000/rs6000.cc
@@ -13399,6 +13399,15 @@ rs6000_can_change_mode_class (machine_mo
|| (to == SDmode && from == DDmode))
return true;
+ /* 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;
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