[llvm] [SelectionDAG] Proper poison value for 32bit statepoint lowering on unused return (PR #214709)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-arm

Author: Berstanio

<details>
<summary>Changes</summary>

I started working on arm32 statepoint support. 
While working on that, I encountered the following issue, generalised to 32bit:
When getIntPtrConstant is called with -1, the uint64_t type ensures it is expanded to 64bit. The 64bit representation (0xFFFFFFFFFFFFFFFF) truncation to 32bit is rejected by APInt.

I considered two possible fixes:
- Using `getAllOnesConstant` 
- Using `getSignedConstant`

I opted for `getSignedConstant`, because it seemed closer to the original intention to me. Result is identical anyway according to my tests.
64bit lowering is not be affected according to my tests.

A test for arm32 is submitted. The test generation was assisted by generative AI. 4 tests unrelated to this changeset fail on this branch.

As I understand it, I need to ping reviewers explicitly, I hope this is correct:
@<!-- -->RKSimon @<!-- -->topperc 

---
Full diff: https://github.com/llvm/llvm-project/pull/214709.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp (+3-1) 
- (added) llvm/test/CodeGen/ARM/statepoint-unused-result.ll (+13) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp
index d66d6f5a99416..3243dda3284c5 100644
--- a/llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp
@@ -1108,7 +1108,9 @@ SelectionDAGBuilder::LowerStatepoint(const GCStatepointInst &I,
   if (!GCResultLocality.first && !GCResultLocality.second) {
     // The return value is not needed, just generate a poison value.
     // Note: This covers the void return case.
-    setValue(&I, DAG.getIntPtrConstant(-1, getCurSDLoc()));
+    const auto &TLI = DAG.getTargetLoweringInfo();
+    setValue(&I, DAG.getSignedConstant(-1, getCurSDLoc(),
+                                       TLI.getPointerTy(DAG.getDataLayout())));
     return;
   }
 
diff --git a/llvm/test/CodeGen/ARM/statepoint-unused-result.ll b/llvm/test/CodeGen/ARM/statepoint-unused-result.ll
new file mode 100644
index 0000000000000..af3e565be758d
--- /dev/null
+++ b/llvm/test/CodeGen/ARM/statepoint-unused-result.ll
@@ -0,0 +1,13 @@
+; RUN: llc -mtriple=armv7-unknown-linux-gnueabihf -stop-before=finalize-isel < %s | FileCheck %s
+
+declare void @safepoint_poll()
+
+define void @statepoint_unused_result() gc "statepoint-example" {
+; CHECK-LABEL: name: statepoint_unused_result
+; CHECK: STATEPOINT 0, 0, 0, @safepoint_poll
+entry:
+  call token (i64, i32, ptr, i32, i32, ...)
+    @llvm.experimental.gc.statepoint.p0(i64 0, i32 0,
+      ptr elementtype(void ()) @safepoint_poll, i32 0, i32 0, i32 0, i32 0)
+  ret void
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/214709