[llvm] [X86][ISel][FMA] Get a handle on operand nodes when negating FMA (PR #130176)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-x86

Author: Vineet Kumar (vntkmr)

<details>
<summary>Changes</summary>

When negaitng an FMA opcode, a new node created for a negated FMA operand may be deleted while recursively negating another FMA operand. This causes the following assertion to fail:
```
llc: /root/llvm-project/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp:7686: llvm::SDValue llvm::SelectionDAG::getNode(unsigned int, const llvm::SDLoc&, llvm::EVT, llvm::SDValue, llvm::SDValue, llvm::SDValue, llvm::SDNodeFlags): Assertion `N1.getOpcode() != ISD::DELETED_NODE && N2.getOpcode() != ISD::DELETED_NODE && N3.getOpcode() != ISD::DELETED_NODE && "Operand is DELETED_NODE!"' failed.
```
This patch adds a temporary handle on the new negated nodes to prevent them from being deleted.
For eg. see https://godbolt.org/z/Tq4PvnKM4 .


---
Full diff: https://github.com/llvm/llvm-project/pull/130176.diff


2 Files Affected:

- (modified) llvm/lib/Target/X86/X86ISelLowering.cpp (+5) 
- (added) llvm/test/CodeGen/X86/combine-fma-negate.ll (+25) 


``````````diff
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index deab638b7e546..1b56826667329 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -55673,7 +55673,12 @@ static SDValue combineFMA(SDNode *N, SelectionDAG &DAG,
   // Do not convert the passthru input of scalar intrinsics.
   // FIXME: We could allow negations of the lower element only.
   bool NegA = invertIfNegative(A);
+    // Create a dummy use for A so that in the process of negating B or C
+    // recursively, it is not deleted.
+    HandleSDNode NegAHandle(A);
   bool NegB = invertIfNegative(B);
+    // Similar to A, get a handle on B.
+    HandleSDNode NegBHandle(B);
   bool NegC = invertIfNegative(C);
 
   if (!NegA && !NegB && !NegC)
diff --git a/llvm/test/CodeGen/X86/combine-fma-negate.ll b/llvm/test/CodeGen/X86/combine-fma-negate.ll
new file mode 100644
index 0000000000000..945df32781464
--- /dev/null
+++ b/llvm/test/CodeGen/X86/combine-fma-negate.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=x86_64-unknown-linux-gnu -mattr=+avx512f -mattr=+fma | FileCheck %s
+
+define void @fma_neg(<8 x i1> %r280, ptr %pp1, ptr %pp2)  {
+; CHECK-LABEL: fma_neg:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vpmovsxwq %xmm0, %zmm0
+; CHECK-NEXT:    vpsllq $63, %zmm0, %zmm0
+; CHECK-NEXT:    vptestmq %zmm0, %zmm0, %k1
+; CHECK-NEXT:    vmovdqu64 (%rdi), %zmm0
+; CHECK-NEXT:    vpxorq {{\.?LCPI[0-9]+_[0-9]+}}(%rip){1to8}, %zmm0, %zmm1 {%k1} {z}
+; CHECK-NEXT:    vxorpd %xmm2, %xmm2, %xmm2
+; CHECK-NEXT:    vfnmadd213pd {{.*#+}} zmm2 = -(zmm0 * zmm2) + zmm1
+; CHECK-NEXT:    vmovupd %zmm2, (%rsi)
+; CHECK-NEXT:    vzeroupper
+; CHECK-NEXT:    retq
+  %r290 = load <8 x double>, ptr %pp1, align 8
+  %r307 = fneg <8 x double> %r290
+  %r309 = select <8 x i1> %r280, <8 x double> %r307, <8 x double> zeroinitializer
+  %r311 = tail call <8 x double> @llvm.x86.avx512.vfmadd.pd.512(<8 x double> %r307, <8 x double> zeroinitializer, <8 x double> %r309, i32 4)
+  store <8 x double> %r311, ptr %pp2, align 8
+  ret void
+}
+
+declare <8 x double> @llvm.x86.avx512.vfmadd.pd.512(<8 x double>, <8 x double>, <8 x double>, i32 immarg)

``````````

</details>


https://github.com/llvm/llvm-project/pull/130176