[llvm] [DAGCombiner] Fold bswap of single-byte-known-nonzero value to a shift (PR #193473)

[Simon Pilgrim via llvm-commits]

=?utf-8?q?Pawe=C5=82?= Bylica <pawel at hepcolgum.band>,
=?utf-8?q?Pawe=C5=82?= Bylica <pawel at hepcolgum.band>,
=?utf-8?q?Pawe=C5=82?= Bylica <pawel at hepcolgum.band>,
=?utf-8?q?Pawe=C5=82?= Bylica <pawel at hepcolgum.band>,
=?utf-8?q?Pawe=C5=82?= Bylica <pawel at hepcolgum.band>,
=?utf-8?q?Pawe=C5=82?= Bylica <pawel at hepcolgum.band>,
=?utf-8?q?Pawe=C5=82?= Bylica <pawel at hepcolgum.band>,
=?utf-8?q?Pawe=C5=82?= Bylica <pawel at hepcolgum.band>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/193473 at github.com>


================
@@ -12133,6 +12133,32 @@ SDValue DAGCombiner::visitBSWAP(SDNode *N) {
   if (SDValue V = foldBitOrderCrossLogicOp(N, DAG))
     return V;
 
+  // Folds that depend on computeKnownBits of the operand.
+  KnownBits Known = DAG.computeKnownBits(N0);
+  // bswap(0) = 0. Catch cases that computeKnownBits can prove are zero but
+  // that structural combines haven't simplified to a constant yet
+  // (e.g. and of disjoint byte masks).
+  if (Known.isZero())
+    return DAG.getConstant(0, DL, VT);
+  // If only one byte of the operand may be nonzero, bswap becomes a shift
+  // to the mirror byte.
+  unsigned Lo = Known.countMinTrailingZeros();
+  unsigned Hi = BW - Known.countMinLeadingZeros();
+  if (unsigned SrcByte = Lo / 8; SrcByte == (Hi - 1) / 8) {
+    unsigned DstByte = BW / 8 - 1 - SrcByte;
----------------
RKSimon wrote:

```suggestion
    unsigned DstByte = (BW / 8) - 1 - SrcByte;
```

https://github.com/llvm/llvm-project/pull/193473