[PowerPC] avoid unprofitable Repl32 flag in BitPermutationSelector

BitPermutationSelector sets Repl32 flag for bit groups which can be (potentially) benefit from 32-bit rotate-and-mask instructions with bit replication, i.e. rlwinm/rlwimi copies lower 32 bits into upper 32 bits on 64-bit PowerPC before rotation.
However, enforcing 32-bit instruction sometimes results in redundant generated code.
For example, the following simple code is compiled into rotldi + rlwimi while it can be compiled into only rldimi instruction if Repl32 flag is not set on the bit group for (a & 0xFFFFFFFF).

uint64_t func(uint64_t a, uint64_t b) {
	return (a & 0xFFFFFFFF) | (b << 32) ;
}

To avoid such problem, this patch checks the potential benefit of Repl32 flag before setting it. If a bit group does not require rotation (i.e. RLAmt == 0) and won't be merged into another group, we do not benefit from Repl32 flag on this group.

Differential Revision: https://reviews.llvm.org/D47867

llvm-svn: 334195

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp b/llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp
index de2c2289251..4b0633142c1 100644
--- a/llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelDAGToDAG.cpp
@@ -1450,6 +1450,20 @@ class BitPermutationSelector {
     };
 
     for (auto &BG : BitGroups) {
+      // If this bit group has RLAmt of 0 and will not be merged with
+      // another bit group, we don't benefit from Repl32. We don't mark
+      // such group to give more freedom for later instruction selection.
+      if (BG.RLAmt == 0) {
+        auto PotentiallyMerged = [this](BitGroup & BG) {
+          for (auto &BG2 : BitGroups)
+            if (&BG != &BG2 && BG.V == BG2.V &&
+                (BG2.RLAmt == 0 || BG2.RLAmt == 32))
+              return true;
+          return false;
+        };
+        if (!PotentiallyMerged(BG))
+          continue;
+      }
       if (BG.StartIdx < 32 && BG.EndIdx < 32) {
         if (IsAllLow32(BG)) {
           if (BG.RLAmt >= 32) {