[llvm] [VectorCombine] Add constant splat handling for shuffleToIdentity (PR #92797)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: David Green (davemgreen)

<details>
<summary>Changes</summary>

This just adds splat constants, which can be treated like any other splat which hopefully makes them very simple. It does not try to handle more complex constant vectors yet, just the more common splats.

I will rebase over #<!-- -->92766, but already had the patch for this version.

---
Full diff: https://github.com/llvm/llvm-project/pull/92797.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Vectorize/VectorCombine.cpp (+9) 
- (modified) llvm/test/Transforms/VectorCombine/AArch64/shuffletoidentity.ll (+2-10) 


``````````diff
diff --git a/llvm/lib/Transforms/Vectorize/VectorCombine.cpp b/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
index 15deaf908422d..0c88038734b04 100644
--- a/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
+++ b/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
@@ -1739,6 +1739,15 @@ bool VectorCombine::foldShuffleToIdentity(Instruction &I) {
       IdentityLeafs.insert(Item[0].first);
       continue;
     }
+    // Look for constants, for the moment only supporting constant splats.
+    if (isa<Constant>(Item[0].first) &&
+        cast<Constant>(Item[0].first)->getSplatValue() &&
+        all_of(drop_begin(Item), [&](InstLane &IL) {
+          return !IL.first || IL.first == Item[0].first;
+        })) {
+      SplatLeafs.insert(Item[0].first);
+      continue;
+    }
     // Look for a splat value.
     if (all_of(drop_begin(Item), [&](InstLane &IL) {
           return !IL.first ||
diff --git a/llvm/test/Transforms/VectorCombine/AArch64/shuffletoidentity.ll b/llvm/test/Transforms/VectorCombine/AArch64/shuffletoidentity.ll
index eb368471b1d84..815136afb4789 100644
--- a/llvm/test/Transforms/VectorCombine/AArch64/shuffletoidentity.ll
+++ b/llvm/test/Transforms/VectorCombine/AArch64/shuffletoidentity.ll
@@ -268,11 +268,7 @@ define <8 x i8> @undeflane(<8 x i8> %a, <8 x i8> %b) {
 
 define <8 x i8> @constantsplat(<8 x i8> %a) {
 ; CHECK-LABEL: @constantsplat(
-; CHECK-NEXT:    [[AB:%.*]] = shufflevector <8 x i8> [[A:%.*]], <8 x i8> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
-; CHECK-NEXT:    [[AT:%.*]] = shufflevector <8 x i8> [[A]], <8 x i8> poison, <4 x i32> <i32 7, i32 6, i32 5, i32 4>
-; CHECK-NEXT:    [[ABT:%.*]] = add <4 x i8> [[AT]], <i8 10, i8 10, i8 10, i8 10>
-; CHECK-NEXT:    [[ABB:%.*]] = add <4 x i8> [[AB]], <i8 10, i8 10, i8 10, i8 10>
-; CHECK-NEXT:    [[R:%.*]] = shufflevector <4 x i8> [[ABT]], <4 x i8> [[ABB]], <8 x i32> <i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0>
+; CHECK-NEXT:    [[R:%.*]] = add <8 x i8> [[A:%.*]], <i8 10, i8 10, i8 10, i8 10, i8 10, i8 10, i8 10, i8 10>
 ; CHECK-NEXT:    ret <8 x i8> [[R]]
 ;
   %ab = shufflevector <8 x i8> %a, <8 x i8> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
@@ -319,11 +315,7 @@ define <8 x i8> @constantdiff2(<8 x i8> %a) {
 
 define <8 x half> @constantsplatf(<8 x half> %a) {
 ; CHECK-LABEL: @constantsplatf(
-; CHECK-NEXT:    [[AB:%.*]] = shufflevector <8 x half> [[A:%.*]], <8 x half> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
-; CHECK-NEXT:    [[AT:%.*]] = shufflevector <8 x half> [[A]], <8 x half> poison, <4 x i32> <i32 7, i32 6, i32 5, i32 4>
-; CHECK-NEXT:    [[ABT:%.*]] = fadd <4 x half> [[AT]], <half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900>
-; CHECK-NEXT:    [[ABB:%.*]] = fadd <4 x half> [[AB]], <half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900>
-; CHECK-NEXT:    [[R:%.*]] = shufflevector <4 x half> [[ABT]], <4 x half> [[ABB]], <8 x i32> <i32 7, i32 6, i32 5, i32 4, i32 3, i32 2, i32 1, i32 0>
+; CHECK-NEXT:    [[R:%.*]] = fadd <8 x half> [[A:%.*]], <half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900, half 0xH4900>
 ; CHECK-NEXT:    ret <8 x half> [[R]]
 ;
   %ab = shufflevector <8 x half> %a, <8 x half> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>

``````````

</details>


https://github.com/llvm/llvm-project/pull/92797