[PATCH] D143786: [X86] Add `TuningPreferShiftShuffle` for when Shifts are preferable to shuffles.

[Noah Goldstein via Phabricator via llvm-commits]

goldstein.w.n added inline comments.


================
Comment at: llvm/lib/Target/X86/X86ISelLowering.cpp:18288
 
-  if (V2.isUndef()) {
-    // When the shuffle is mirrored between the 128-bit lanes of the unit, we
-    // can use lower latency instructions that will operate on both lanes.
-    SmallVector<int, 2> RepeatedMask;
-    if (is128BitLaneRepeatedShuffleMask(MVT::v4i64, Mask, RepeatedMask)) {
-      SmallVector<int, 4> PSHUFDMask;
-      narrowShuffleMaskElts(2, RepeatedMask, PSHUFDMask);
-      return DAG.getBitcast(
-          MVT::v4i64,
-          DAG.getNode(X86ISD::PSHUFD, DL, MVT::v8i32,
-                      DAG.getBitcast(MVT::v8i32, V1),
-                      getV4X86ShuffleImm8ForMask(PSHUFDMask, DL, DAG)));
-    }
+  for (unsigned Order = 0; Order < 2; ++Order) {
+    if (Subtarget.hasFasterShiftThanShuffle() ? (Order == 1) : (Order == 0)) {
----------------
RKSimon wrote:
> This approach isn't particularly easy to grok - why not just add an additional lowerShuffleAsShift check before behind a hasFasterShiftThanShuffle check?
Was to avoid duplicating ~30 lines of code, but will do for v2.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D143786/new/

https://reviews.llvm.org/D143786