[X86]: Rewrite demorgan rule for ANDN #163789

kper · 2025-10-16T14:00:20Z

The pattern ~(a | b | c) should use the ANDN instruction when BMI is available. We do that by rewritting it to ~a & ~b & ~c.

Fixes: #163516
Proof: https://alive2.llvm.org/ce/z/vwoZo6

llvmbot · 2025-10-16T14:01:05Z

@llvm/pr-subscribers-llvm-selectiondag

@llvm/pr-subscribers-backend-x86

Author: None (kper)

Changes

The pattern ~(a | b) should use the ANDN instruction when BMI is available. We do that by reassociating to ~a & ~b. The b is then wrapped with a NOT and ANDN already negates the first operand.

Fixes: #163516
Proof: https://alive2.llvm.org/ce/z/vwoZo6

Full diff: https://github.com/llvm/llvm-project/pull/163789.diff

2 Files Affected:

(modified) llvm/lib/Target/X86/X86ISelLowering.cpp (+28)
(added) llvm/test/CodeGen/X86/bmi-reassoc-demorgan.ll (+98)

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index a0b64ff370b10..e2632d114ce0b 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -51651,6 +51651,31 @@ static SDValue combineAndXorSubWithBMI(SDNode *And, const SDLoc &DL,
   return AndN;
 }
 
+// fold (not (or A, B)) -> nand(A, not(B)) if BMI
+static SDValue
+combineReassocDemorganWithNANDWithBMI(SDNode *Xor, const SDLoc &DL,
+                                      SelectionDAG &DAG,
+                                      const X86Subtarget &Subtarget) {
+  using namespace llvm::SDPatternMatch;
+
+  EVT VT = Xor->getValueType(0);
+  // Make sure this node is a candidate for BMI instructions.
+  if (!Subtarget.hasBMI() || (VT != MVT::i32 && VT != MVT::i64))
+    return SDValue();
+
+  SDValue A;
+  SDValue B;
+  APInt Cst;
+  if (!(sd_match(Xor, m_Xor(m_Or(m_Value(A), m_Value(B)), m_ConstInt(Cst))) &&
+        Cst.isAllOnes()))
+    return SDValue();
+
+  auto Opcode =
+      Subtarget.is64Bit() && VT == MVT::i64 ? X86::ANDN64rr : X86::ANDN32rr;
+  auto AndN = DAG.getMachineNode(Opcode, DL, VT, A, DAG.getNOT(DL, B, VT));
+  return SDValue(AndN, 0);
+}
+
 static SDValue combineX86SubCmpForFlags(SDNode *N, SDValue Flag,
                                         SelectionDAG &DAG,
                                         TargetLowering::DAGCombinerInfo &DCI,
@@ -55150,6 +55175,9 @@ static SDValue combineXor(SDNode *N, SelectionDAG &DAG,
   if (SDValue R = combineBMILogicOp(N, DAG, Subtarget))
     return R;
 
+  if (SDValue R = combineReassocDemorganWithNANDWithBMI(N, DL, DAG, Subtarget))
+    return R;
+
   return combineFneg(N, DAG, DCI, Subtarget);
 }
 
diff --git a/llvm/test/CodeGen/X86/bmi-reassoc-demorgan.ll b/llvm/test/CodeGen/X86/bmi-reassoc-demorgan.ll
new file mode 100644
index 0000000000000..ea81d08cd2e6d
--- /dev/null
+++ b/llvm/test/CodeGen/X86/bmi-reassoc-demorgan.ll
@@ -0,0 +1,98 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=i686-unknown-unknown -mattr=+bmi | FileCheck %s --check-prefix=X86
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+bmi | FileCheck %s --check-prefix=X64
+
+define i32 @reassoc_demorgan_i32(i32 %a, i32 %b) nounwind {
+; X86-LABEL: reassoc_demorgan_i32:
+; X86:       # %bb.0:
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %ecx
+; X86-NEXT:    notl %ecx
+; X86-NEXT:    andnl %ecx, %eax, %eax
+; X86-NEXT:    retl
+;
+; X64-LABEL: reassoc_demorgan_i32:
+; X64:       # %bb.0:
+; X64-NEXT:    notl %edi
+; X64-NEXT:    andnl %edi, %esi, %eax
+; X64-NEXT:    retq
+  %temp = or i32 %b, %a
+  %res = xor i32 %temp, -1
+  ret i32 %res
+}
+
+define i32 @reassoc_demorgan_three_arguments_i32(i32 %a, i32 %b, i32 %c) nounwind {
+; X86-LABEL: reassoc_demorgan_three_arguments_i32:
+; X86:       # %bb.0:
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %ecx
+; X86-NEXT:    orl {{[0-9]+}}(%esp), %ecx
+; X86-NEXT:    notl %eax
+; X86-NEXT:    andnl %eax, %ecx, %eax
+; X86-NEXT:    retl
+;
+; X64-LABEL: reassoc_demorgan_three_arguments_i32:
+; X64:       # %bb.0:
+; X64-NEXT:    orl %esi, %edi
+; X64-NEXT:    notl %edx
+; X64-NEXT:    andnl %edx, %edi, %eax
+; X64-NEXT:    retq
+  %and.demorgan = or i32 %b, %a
+  %and3.demorgan = or i32 %and.demorgan, %c
+  %and3 = xor i32 %and3.demorgan, -1
+  ret i32 %and3
+}
+
+define i64 @reassoc_demorgan_i64(i64 %a, i64 %b) nounwind {
+; X86-LABEL: reassoc_demorgan_i64:
+; X86:       # %bb.0:
+; X86-NEXT:    pushl %esi
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %ecx
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %edx
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %esi
+; X86-NEXT:    notl %edx
+; X86-NEXT:    andnl %edx, %eax, %eax
+; X86-NEXT:    notl %esi
+; X86-NEXT:    andnl %esi, %ecx, %edx
+; X86-NEXT:    popl %esi
+; X86-NEXT:    retl
+;
+; X64-LABEL: reassoc_demorgan_i64:
+; X64:       # %bb.0:
+; X64-NEXT:    notq %rdi
+; X64-NEXT:    andnq %rdi, %rsi, %rax
+; X64-NEXT:    retq
+  %temp = or i64 %b, %a
+  %res = xor i64 %temp, -1
+  ret i64 %res
+}
+
+define i64 @reassoc_demorgan_three_arguments_i64(i64 %a, i64 %b, i64 %c) nounwind {
+; X86-LABEL: reassoc_demorgan_three_arguments_i64:
+; X86:       # %bb.0:
+; X86-NEXT:    pushl %esi
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %ecx
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %edx
+; X86-NEXT:    movl {{[0-9]+}}(%esp), %esi
+; X86-NEXT:    orl {{[0-9]+}}(%esp), %esi
+; X86-NEXT:    orl {{[0-9]+}}(%esp), %edx
+; X86-NEXT:    notl %eax
+; X86-NEXT:    andnl %eax, %edx, %eax
+; X86-NEXT:    notl %ecx
+; X86-NEXT:    andnl %ecx, %esi, %edx
+; X86-NEXT:    popl %esi
+; X86-NEXT:    retl
+;
+; X64-LABEL: reassoc_demorgan_three_arguments_i64:
+; X64:       # %bb.0:
+; X64-NEXT:    orq %rsi, %rdi
+; X64-NEXT:    notq %rdx
+; X64-NEXT:    andnq %rdx, %rdi, %rax
+; X64-NEXT:    retq
+  %and.demorgan = or i64 %b, %a
+  %and3.demorgan = or i64 %and.demorgan, %c
+  %and3 = xor i64 %and3.demorgan, -1
+  ret i64 %and3
+}

llvm/lib/Target/X86/X86ISelLowering.cpp

kper · 2025-10-16T14:03:00Z

fyi @RKSimon

RKSimon · 2025-10-16T14:10:14Z

llvm/lib/Target/X86/X86ISelLowering.cpp

+
+  EVT VT = Xor->getValueType(0);
+  // Make sure this node is a candidate for BMI instructions.
+  if (!Subtarget.hasBMI() || (VT != MVT::i32 && VT != MVT::i64))


Please can this be handled generically in DAGCombine

But there is no ISD::ANDN (at least that I know of)
So do you want to generally reassociate it? For all cases?

Search DAGCombiner.cpp for hasAndNot

kper · 2025-10-17T06:49:03Z

At the moment, there is an infinity loop with Andnp for some tests.

/// Do target-specific dag combines on X86ISD::ANDNP nodes.
static SDValue combineAndnp(SDNode *N, SelectionDAG &DAG,
                            TargetLowering::DAGCombinerInfo &DCI,
                            const X86Subtarget &Subtarget) {
...

// Folds for better commutativity:
  if (N1->hasOneUse()) {
    // ANDNP(x,NOT(y)) -> AND(NOT(x),NOT(y)) -> NOT(OR(X,Y)).
    if (SDValue Not = IsNOT(N1, DAG))
      return DAG.getNOT(
          DL, DAG.getNode(ISD::OR, DL, VT, N0, DAG.getBitcast(VT, Not)), VT);

// ...

Once the demorgan is applied and we have an ANDNP, it tries to revert it.
@RKSimon I suppose that I can remove this, right?

jayfoad · 2025-10-17T09:35:53Z

This should not be described as "reassociation".

jayfoad · 2025-10-17T12:51:29Z

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

+    // If we have AndNot then it is profitable to apply demorgan to make use
+    // of the machine instruction.


I would argue that this fold is not profitable at all on its own. It is only profitable when combined into a larger pattern like ~(A|B|C).

Yeah that's also my impression :/
I have been updating the rest of the .ll after I "fixed" the infinite loop. And, it doesn't look so good.
I will try the larger pattern and let's see what the result is.

RKSimon · 2025-10-17T13:29:16Z

Once the demorgan is applied and we have an ANDNP, it tries to revert it.
@RKSimon I suppose that I can remove this, right?

IIRC this is to help with register pressure as we can only fold the second operand on ANDNP which can cause headaches - X86's IsNOT attempts to peek though chains to see if we can remove a NOT entirely (or in this case move it after the OR) - if you're going to improve this patch to look for more complex patterns this fold shouldn't infinite loop any more.

This reverts commit e31e5ab.

This reverts commit cef0067.

llvm/test/CodeGen/AArch64/eon.ll

kper · 2025-10-18T08:02:25Z

Let's the disregard the fact that I had to update some test files needlessly for now, e.g. llvm/test/CodeGen/PowerPC/xxeval-and-nand.ll and llvm/test/CodeGen/PowerPC/vec_veqv_vnand_vorc.ll

Looking at llvm/test/CodeGen/X86/unfold-masked-merge-vector-variablemask-const.ll,
llvm/test/CodeGen/X86/bool-ext-inc.ll, llvm/test/CodeGen/X86/unfold-masked-merge-vector-variablemask-const.ll, llvm/test/CodeGen/PowerPC/xxeval-eqv-nor-or-xor.ll, the three operand variant does not seem very profitable as well. I had also to remove the snippet llvm/lib/Target/X86/X86ISelLowering.cpp to avoid a loop.

kper · 2025-10-21T13:48:29Z

@RKSimon @jayfoad what do you think? How should we proceed here?

RKSimon

Failed Tests (4):
  LLVM :: CodeGen/LoongArch/ctlz-cttz-ctpop.ll
  LLVM :: CodeGen/PowerPC/xxeval-and-nand.ll
  LLVM :: CodeGen/SystemZ/scalar-ctlz-02.ll
  LLVM :: CodeGen/SystemZ/vec-eval.ll

[X86]: Reassoc demorgan rule for ANDN

ee35653

llvmbot added the backend:X86 label Oct 16, 2025

kper commented Oct 16, 2025

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RKSimon self-requested a review October 16, 2025 14:08

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kper added 5 commits October 16, 2025 18:24

[X86]: Removed obsolete code

deda338

[DAG]: Reassoc demorgan rule for ANDN

cdb57ef

[DAG]: Fixed type

9e4103d

[X86]: Updated tests

cf6ee58

[DAG]: Updated tests

cef0067

llvmbot added the llvm:SelectionDAG SelectionDAGISel as well label Oct 16, 2025

[DAG]: Updated tests

e31e5ab

RKSimon requested a review from jayfoad October 17, 2025 10:21

kper changed the title ~~[X86]: Reassoc demorgan rule for ANDN~~ [X86]: Rewrite demorgan rule for ANDN Oct 17, 2025

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kper added 10 commits October 18, 2025 06:22

Revert "[DAG]: Updated tests"

cf9da83

This reverts commit e31e5ab.

Revert "[DAG]: Updated tests"

3a90a69

This reverts commit cef0067.

[DAG]: Rewrite ~(a | b | c) into ~a & ~b & ~c

1245b6b

[X86]: Created new test

746b101

[DAG]: Run fmt

f237020

[AArch64]: Updated tests

956b849

[X86]: Updated tests

643e4d5

[PowerPC]: Updated tests

e2032ef

[X86]: Removed reversing of rewriting demorgan

5218588

[X86][PowerPC][AArch64]: Updated tests

de10f4a

kper commented Oct 18, 2025

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RKSimon requested changes Oct 22, 2025

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kper added 3 commits October 22, 2025 18:48

[LoongArch][SystemZ]: Updated tests

c95c10a

[AArch64][PowerPC]: Reverting some updates

3681932

[X86]: Removed comment

b3ec648

		// If we have AndNot then it is profitable to apply demorgan to make use
		// of the machine instruction.

[X86]: Rewrite demorgan rule for ANDN #163789

Are you sure you want to change the base?

[X86]: Rewrite demorgan rule for ANDN #163789

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