[PATCH] D47867: [PowerPC] avoid redundant Repl32 flag in BitPermutationSelector

[Hiroshi Inoue via Phabricator via llvm-commits]

inouehrs created this revision.
inouehrs added reviewers: hfinkel, echristo, kbarton, nemanjai.

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 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.


https://reviews.llvm.org/D47867

Files:
  lib/Target/PowerPC/PPCISelDAGToDAG.cpp
  test/CodeGen/PowerPC/bperm.ll


Index: test/CodeGen/PowerPC/bperm.ll
===================================================================
--- test/CodeGen/PowerPC/bperm.ll
+++ test/CodeGen/PowerPC/bperm.ll
@@ -271,6 +271,18 @@
 ; CHECK: blr
 }
 
+define i64 @test16(i64 %a, i64 %b) #0 {
+entry:
+  %and = and i64 %a, 4294967295
+  %shl = shl i64 %b, 32
+  %or = or i64 %and, %shl
+  ret i64 %or
+
+; CHECK-LABEL: @test16
+; CHECK: rldimi 3, 4, 32, 0
+; CHECK: blr
+}
+
 ; Function Attrs: nounwind readnone
 declare i32 @llvm.bswap.i32(i32) #0
 declare i64 @llvm.bswap.i64(i64) #0
Index: lib/Target/PowerPC/PPCISelDAGToDAG.cpp
===================================================================
--- lib/Target/PowerPC/PPCISelDAGToDAG.cpp
+++ lib/Target/PowerPC/PPCISelDAGToDAG.cpp
@@ -1450,6 +1450,20 @@
     };
 
     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 insrtruction 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) {


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