[llvm] [SDAG] Intersect poison-generating flags after CSE (PR #97434)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-selectiondag

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

This patch fixes a miscompilation when `N` gets CSEed to `Existing`:
```
Existing: t5: i32 = sub nuw Constant:i32<0>, t3
N: t30: i32 = sub Constant:i32<0>, t3
```

An alternative is to block CSE by taking SDFlags into account in `AddNodeIDNode`. It will enable more folds (e.g., `sub nuw 0, X -> 0` in this case).

Fixes https://github.com/llvm/llvm-project/issues/96366.


---
Full diff: https://github.com/llvm/llvm-project/pull/97434.diff


3 Files Affected:

- (modified) llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp (+1) 
- (added) llvm/test/CodeGen/AArch64/pr96366.ll (+25) 
- (added) llvm/test/CodeGen/RISCV/pr96366.ll (+29) 


``````````diff
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index bc16f885f6a04..96242305e9eab 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -1239,6 +1239,7 @@ SelectionDAG::AddModifiedNodeToCSEMaps(SDNode *N) {
       // If there was already an existing matching node, use ReplaceAllUsesWith
       // to replace the dead one with the existing one.  This can cause
       // recursive merging of other unrelated nodes down the line.
+      Existing->intersectFlagsWith(N->getFlags());
       ReplaceAllUsesWith(N, Existing);
 
       // N is now dead. Inform the listeners and delete it.
diff --git a/llvm/test/CodeGen/AArch64/pr96366.ll b/llvm/test/CodeGen/AArch64/pr96366.ll
new file mode 100644
index 0000000000000..392b7b66b0bb1
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/pr96366.ll
@@ -0,0 +1,25 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s -mtriple=aarch64 | FileCheck %s
+
+declare void @mumble(i32)
+
+define i32 @f(i32 %0) nounwind {
+; CHECK-LABEL: f:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    stp x30, x19, [sp, #-16]! // 16-byte Folded Spill
+; CHECK-NEXT:    mov w19, w0
+; CHECK-NEXT:    neg w0, w0
+; CHECK-NEXT:    bl mumble
+; CHECK-NEXT:    mov w8, #4 // =0x4
+; CHECK-NEXT:    sub w0, w8, w19
+; CHECK-NEXT:    ldp x30, x19, [sp], #16 // 16-byte Folded Reload
+; CHECK-NEXT:    ret
+  %2 = sub nuw i32 0, %0
+  call void @mumble(i32 %2)
+  %3 = sub i32 1, %0
+  %4 = sub i32 3, %0
+  %5 = mul i32 %0, 1
+  %6 = add i32 %3, %5
+  %7 = add i32 %6, %4
+  ret i32 %7
+}
diff --git a/llvm/test/CodeGen/RISCV/pr96366.ll b/llvm/test/CodeGen/RISCV/pr96366.ll
new file mode 100644
index 0000000000000..05563d7bc14af
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/pr96366.ll
@@ -0,0 +1,29 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; RUN: llc < %s -mtriple=riscv64 | FileCheck %s
+
+declare void @mumble(i32)
+
+define i32 @f(i32 %0) nounwind {
+; CHECK-LABEL: f:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    addi sp, sp, -16
+; CHECK-NEXT:    sd ra, 8(sp) # 8-byte Folded Spill
+; CHECK-NEXT:    sd s0, 0(sp) # 8-byte Folded Spill
+; CHECK-NEXT:    mv s0, a0
+; CHECK-NEXT:    negw a0, a0
+; CHECK-NEXT:    call mumble
+; CHECK-NEXT:    li a0, 4
+; CHECK-NEXT:    subw a0, a0, s0
+; CHECK-NEXT:    ld ra, 8(sp) # 8-byte Folded Reload
+; CHECK-NEXT:    ld s0, 0(sp) # 8-byte Folded Reload
+; CHECK-NEXT:    addi sp, sp, 16
+; CHECK-NEXT:    ret
+  %2 = sub nuw i32 0, %0
+  call void @mumble(i32 %2)
+  %3 = sub i32 1, %0
+  %4 = sub i32 3, %0
+  %5 = mul i32 %0, 1
+  %6 = add i32 %3, %5
+  %7 = add i32 %6, %4
+  ret i32 %7
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/97434