[llvm-branch-commits] [llvm] release/18.x: [AArch64][GISel] Don't pointlessly lower G_TRUNC (#81479) (PR #81581)

[via llvm-branch-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-aarch64

Author: None (llvmbot)

<details>
<summary>Changes</summary>

Backport 070848c17c2944afa494d42d3ad42929f3379842

Requested by: @<!-- -->nikic

---
Full diff: https://github.com/llvm/llvm-project/pull/81581.diff


2 Files Affected:

- (modified) llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp (+2-3) 
- (modified) llvm/test/CodeGen/AArch64/GlobalISel/legalize-xtn.mir (+24) 


``````````diff
diff --git a/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp b/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp
index fd69a7d6c33d03..4b9d549e791142 100644
--- a/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp
+++ b/llvm/lib/Target/AArch64/GISel/AArch64LegalizerInfo.cpp
@@ -622,9 +622,8 @@ AArch64LegalizerInfo::AArch64LegalizerInfo(const AArch64Subtarget &ST)
       .lowerIf([=](const LegalityQuery &Query) {
         LLT DstTy = Query.Types[0];
         LLT SrcTy = Query.Types[1];
-        return DstTy.isVector() && (SrcTy.getSizeInBits() > 128 ||
-                                    (DstTy.getScalarSizeInBits() * 2 <
-                                     SrcTy.getScalarSizeInBits()));
+        return DstTy.isVector() && SrcTy.getSizeInBits() > 128 &&
+               DstTy.getScalarSizeInBits() * 2 <= SrcTy.getScalarSizeInBits();
       })
 
       .alwaysLegal();
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/legalize-xtn.mir b/llvm/test/CodeGen/AArch64/GlobalISel/legalize-xtn.mir
index 16b780a8397347..661265173ae82b 100644
--- a/llvm/test/CodeGen/AArch64/GlobalISel/legalize-xtn.mir
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/legalize-xtn.mir
@@ -529,3 +529,27 @@ body:             |
     RET_ReallyLR implicit $q0
 
 ...
+
+---
+name:            pr81244
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $d0
+    ; CHECK-LABEL: name: pr81244
+    ; CHECK: liveins: $d0
+    ; CHECK-NEXT: {{  $}}
+    ; CHECK-NEXT: [[COPY:%[0-9]+]]:_(<2 x s32>) = COPY $d0
+    ; CHECK-NEXT: [[TRUNC:%[0-9]+]]:_(<2 x s8>) = G_TRUNC [[COPY]](<2 x s32>)
+    ; CHECK-NEXT: [[CONCAT_VECTORS:%[0-9]+]]:_(<4 x s8>) = G_CONCAT_VECTORS [[TRUNC]](<2 x s8>), [[TRUNC]](<2 x s8>)
+    ; CHECK-NEXT: [[ANYEXT:%[0-9]+]]:_(<4 x s16>) = G_ANYEXT [[CONCAT_VECTORS]](<4 x s8>)
+    ; CHECK-NEXT: $d0 = COPY [[ANYEXT]](<4 x s16>)
+    ; CHECK-NEXT: RET_ReallyLR implicit $d0
+    %0:_(<2 x s32>) = COPY $d0
+    %1:_(<2 x s8>) = G_TRUNC %0(<2 x s32>)
+    %2:_(<4 x s8>) = G_CONCAT_VECTORS %1(<2 x s8>), %1(<2 x s8>)
+    %3:_(<4 x s16>) = G_ANYEXT %2(<4 x s8>)
+    $d0 = COPY %3(<4 x s16>)
+    RET_ReallyLR implicit $d0
+
+...

``````````

</details>


https://github.com/llvm/llvm-project/pull/81581