[llvm] [InstCombine] Do not combine shuffle+bitcast if the bitcast is eliminable. (PR #135769)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Ricardo Jesus (rj-jesus)

<details>
<summary>Changes</summary>

If we are attempting to combine shuffle+bitcast but the bitcast is
pairable with a subsequent bitcast, we should not fold the shuffle as
doing so can block further simplifications.

The motivation for this is a long standing regression affecting SIMDe on
AArch64 introduced indirectly by the AlwaysInliner (1a2e77cf). Some
reproducers:
* https://godbolt.org/z/53qx18s6M
* https://godbolt.org/z/o5e43h5M7


---
Full diff: https://github.com/llvm/llvm-project/pull/135769.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp (+12-4) 
- (modified) llvm/test/Transforms/InstCombine/shufflevec-bitcast.ll (+15) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index f897cc7855d2d..f6423cb40492e 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -3029,10 +3029,18 @@ Instruction *InstCombinerImpl::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
     SmallVector<BitCastInst *, 8> BCs;
     DenseMap<Type *, Value *> NewBCs;
     for (User *U : SVI.users())
-      if (BitCastInst *BC = dyn_cast<BitCastInst>(U))
-        if (!BC->use_empty())
-          // Only visit bitcasts that weren't previously handled.
-          BCs.push_back(BC);
+      if (BitCastInst *BC = dyn_cast<BitCastInst>(U)) {
+        // Only visit bitcasts that weren't previously handled.
+        if (BC->use_empty())
+          continue;
+        // Prefer to combine bitcasts of bitcasts before attempting this fold.
+        if (BC->hasOneUse()) {
+          auto *BC2 = dyn_cast<BitCastInst>(BC->user_back());
+          if (BC2 && isEliminableCastPair(BC, BC2))
+            continue;
+        }
+        BCs.push_back(BC);
+      }
     for (BitCastInst *BC : BCs) {
       unsigned BegIdx = Mask.front();
       Type *TgtTy = BC->getDestTy();
diff --git a/llvm/test/Transforms/InstCombine/shufflevec-bitcast.ll b/llvm/test/Transforms/InstCombine/shufflevec-bitcast.ll
index f20077243273c..c6152368f06fd 100644
--- a/llvm/test/Transforms/InstCombine/shufflevec-bitcast.ll
+++ b/llvm/test/Transforms/InstCombine/shufflevec-bitcast.ll
@@ -235,3 +235,18 @@ define <3 x i4> @shuf_bitcast_wrong_size(<2 x i8> %v, i8 %x) {
   %r = shufflevector <4 x i4> %b, <4 x i4> undef, <3 x i32> <i32 0, i32 1, i32 2>
   ret <3 x i4> %r
 }
+
+; Negative test - chain of bitcasts.
+
+define <16 x i8> @shuf_bitcast_chain(<8 x i32> %v) {
+; CHECK-LABEL: @shuf_bitcast_chain(
+; CHECK-NEXT:    [[S:%.*]] = shufflevector <8 x i32> [[V:%.*]], <8 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
+; CHECK-NEXT:    [[C:%.*]] = bitcast <4 x i32> [[S]] to <16 x i8>
+; CHECK-NEXT:    ret <16 x i8> [[C]]
+;
+  %s = shufflevector <8 x i32> %v, <8 x i32> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
+  %a = bitcast <4 x i32> %s to <2 x i64>
+  %b = bitcast <2 x i64> %a to i128
+  %c = bitcast i128 %b to <16 x i8>
+  ret <16 x i8> %c
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/135769