[llvm] [Hexagon] Fix extractHvxSubvectorPred shuffle mask for small predicates (PR #181364)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-hexagon

Author: Brian Cain (androm3da)

<details>
<summary>Changes</summary>

The loop generating the shuffle mask in extractHvxSubvectorPred used HwLen/ResLen as the iteration count, but each iteration produces 8 elements (ResLen * Rep where Rep = 8/ResLen). This means the total mask size was (HwLen/ResLen) * 8, which only equals HwLen when ResLen == 8. For smaller predicate subvectors (e.g., <4 x i1> or <2 x i1>), the mask was too large, causing an assertion failure in getVectorShuffle.

Fix by using HwLen/8 as the loop bound, which correctly produces HwLen elements regardless of ResLen.

---
Full diff: https://github.com/llvm/llvm-project/pull/181364.diff


2 Files Affected:

- (modified) llvm/lib/Target/Hexagon/HexagonISelLoweringHVX.cpp (+1-1) 
- (added) llvm/test/CodeGen/Hexagon/extract-hvx-subvector-pred-small.ll (+28) 


``````````diff
diff --git a/llvm/lib/Target/Hexagon/HexagonISelLoweringHVX.cpp b/llvm/lib/Target/Hexagon/HexagonISelLoweringHVX.cpp
index b1181dfa13a10..b42cd4e91938a 100644
--- a/llvm/lib/Target/Hexagon/HexagonISelLoweringHVX.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonISelLoweringHVX.cpp
@@ -1434,7 +1434,7 @@ HexagonTargetLowering::extractHvxSubvectorPred(SDValue VecV, SDValue IdxV,
   unsigned Rep = 8 / ResLen;
   // Make sure the output fill the entire vector register, so repeat the
   // 8-byte groups as many times as necessary.
-  for (unsigned r = 0; r != HwLen/ResLen; ++r) {
+  for (unsigned r = 0; r != HwLen / 8; ++r) {
     // This will generate the indexes of the 8 interesting bytes.
     for (unsigned i = 0; i != ResLen; ++i) {
       for (unsigned j = 0; j != Rep; ++j)
diff --git a/llvm/test/CodeGen/Hexagon/extract-hvx-subvector-pred-small.ll b/llvm/test/CodeGen/Hexagon/extract-hvx-subvector-pred-small.ll
new file mode 100644
index 0000000000000..e0aa6a680d20d
--- /dev/null
+++ b/llvm/test/CodeGen/Hexagon/extract-hvx-subvector-pred-small.ll
@@ -0,0 +1,28 @@
+; RUN: llc -mtriple=hexagon -mcpu=hexagonv73 -mattr=+hvxv73,+hvx-length128b \
+; RUN:   < %s | FileCheck %s
+;
+; Check that extracting a small predicate subvector (<8 x i1) from an HVX
+; predicate compiles correctly. The bug was in extractHvxSubvectorPred where
+; the loop generating the shuffle mask used HwLen/ResLen instead of HwLen/8,
+; producing a mask of wrong size for ResLen < 8.
+
+target datalayout = "e-m:e-p:32:32:32-a:0-n16:32-i64:64:64-i32:32:32-i16:16:16-i1:8:8-f32:32:32-f64:64:64-v32:32:32-v64:64:64-v512:512:512-v1024:1024:1024-v2048:2048:2048"
+target triple = "hexagon-unknown-linux-musl"
+
+; CHECK-LABEL: test_extract_v4i1:
+; CHECK-DAG:   vand(v{{[0-9]+}},r{{[0-9]+}})
+; CHECK-DAG:   vdelta(v{{[0-9]+}},v{{[0-9]+}})
+; CHECK:       dealloc_return
+define <4 x i1> @test_extract_v4i1(<128 x i1> %pred) {
+  %r = shufflevector <128 x i1> %pred, <128 x i1> undef, <4 x i32> <i32 0, i32 1, i32 2, i32 3>
+  ret <4 x i1> %r
+}
+
+; CHECK-LABEL: test_extract_v2i1:
+; CHECK-DAG:   vand(v{{[0-9]+}},r{{[0-9]+}})
+; CHECK-DAG:   vdelta(v{{[0-9]+}},v{{[0-9]+}})
+; CHECK:       dealloc_return
+define <2 x i1> @test_extract_v2i1(<128 x i1> %pred) {
+  %r = shufflevector <128 x i1> %pred, <128 x i1> undef, <2 x i32> <i32 0, i32 1>
+  ret <2 x i1> %r
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/181364