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[X86] Extend combinei64TruncSrlAdd to handle patterns with or and xor #128435

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Mar 1, 2025
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[X86] Extend combinei64TruncSrlAdd to handle patterns with or and xor #128435

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fe8151f
[X86] Add pre-commit tests.
joaotgouveia Feb 23, 2025
5574dce
[X86] Extend `combinei64TruncSrlAdd` to handle patterns with `or` and…
joaotgouveia Feb 23, 2025
55189cc
[X86] Add extra test cases
joaotgouveia Feb 24, 2025
d2f27bf
[X86] Address reviews
joaotgouveia Feb 26, 2025
1ba8e32
[X86] Improve readability
joaotgouveia Feb 27, 2025
696f3a5
[X86] Remove getNumOperands
joaotgouveia Feb 27, 2025
7caceb1
[X86] Remove unnecessary switch case
joaotgouveia Feb 28, 2025
4388e73
[X86] Address reviews
joaotgouveia Mar 1, 2025
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69 changes: 42 additions & 27 deletions llvm/lib/Target/X86/X86ISelLowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -53733,36 +53733,47 @@ static SDValue combineLRINT_LLRINT(SDNode *N, SelectionDAG &DAG,
return DAG.getNode(X86ISD::CVTP2SI, DL, VT, Src);
}

// Attempt to fold some (truncate (srl (add X, C1), C2)) patterns to
// (add (truncate (srl X, C2)), C1'). C1' will be smaller than C1 so we are able
// to avoid generating code with MOVABS and large constants in certain cases.
static SDValue combinei64TruncSrlAdd(SDValue N, EVT VT, SelectionDAG &DAG,
const SDLoc &DL) {
using namespace llvm::SDPatternMatch;
// Attempt to fold some (truncate (srl (add/or/xor X, C1), C2)) patterns to
// (add/or/xor (truncate (srl X, C2)), C1'). C1' will be smaller than C1 so we
// are able to avoid generating code with MOVABS and large constants in certain
// cases.
static SDValue combinei64TruncSrlConstant(SDValue N, EVT VT, SelectionDAG &DAG,
const SDLoc &DL) {

SDValue AddLhs;
APInt AddConst, SrlConst;
if (VT != MVT::i32 ||
!sd_match(N, m_AllOf(m_SpecificVT(MVT::i64),
m_Srl(m_OneUse(m_Add(m_Value(AddLhs),
m_ConstInt(AddConst))),
m_ConstInt(SrlConst)))))
return SDValue();
SDValue Op = N.getOperand(0);
APInt OpConst = Op.getConstantOperandAPInt(1);
APInt SrlConst = N.getConstantOperandAPInt(1);
unsigned Opcode = Op.getOpcode();

if (SrlConst.ule(32) || AddConst.countr_zero() < SrlConst.getZExtValue())
switch (Opcode) {
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@phoebewang phoebewang Feb 28, 2025

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No need to use switch since we have limit to ADD/OR/XOR in combineTruncatedArithmetic.

default:
return SDValue();
case ISD::ADD:
if (OpConst.countr_zero() < SrlConst.getZExtValue())
return SDValue();
[[fallthrough]];
case ISD::OR:
case ISD::XOR:
if (SrlConst.ule(32))
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@phoebewang phoebewang Feb 24, 2025

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For or/xor they don't need the second condition and without the limition https://alive2.llvm.org/ce/z/uEAYxG, the and I showed in https://godbolt.org/z/1za939PKc should also work.

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@joaotgouveia joaotgouveia Feb 24, 2025

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I've added test cases that cover the scenario you mentioned for AND, as well as equivalent cases for OR and XOR. It seems the fold works as-is for these cases, though I'm not entirely sure why.

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@phoebewang phoebewang Feb 25, 2025

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You are right. I mistook it with add.

return SDValue();
break;
}

SDValue AddLHSSrl =
DAG.getNode(ISD::SRL, DL, MVT::i64, AddLhs, N.getOperand(1));
SDValue Trunc = DAG.getNode(ISD::TRUNCATE, DL, VT, AddLHSSrl);

APInt NewAddConstVal = AddConst.lshr(SrlConst).trunc(VT.getSizeInBits());
SDValue NewAddConst = DAG.getConstant(NewAddConstVal, DL, VT);
SDValue NewAddNode = DAG.getNode(ISD::ADD, DL, VT, Trunc, NewAddConst);
SDValue OpLhsSrl =
DAG.getNode(ISD::SRL, DL, MVT::i64, Op.getOperand(0), N.getOperand(1));
SDValue Trunc = DAG.getNode(ISD::TRUNCATE, DL, VT, OpLhsSrl);

APInt NewOpConstVal = OpConst.lshr(SrlConst).trunc(VT.getSizeInBits());
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@RKSimon RKSimon Feb 27, 2025

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APInt::extractBits ?

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@joaotgouveia joaotgouveia Feb 27, 2025

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If I'm not mistaken, by using APInt::extractBits the constant transformation becomes
APInt NewOpConstVal = OpConst.extractBits(64 - SrlConst.getZExtValue(), SrlConst.getZExtValue()).zext(VT.getSizeInBits());. I believe the current implementation is simpler and easier to understand.

SDValue NewOpConst = DAG.getConstant(NewOpConstVal, DL, VT);
SDValue NewOpNode = DAG.getNode(Opcode, DL, VT, Trunc, NewOpConst);
EVT CleanUpVT =
EVT::getIntegerVT(*DAG.getContext(), 64 - SrlConst.getZExtValue());
return DAG.getZeroExtendInReg(NewAddNode, DL, CleanUpVT);

if (Opcode == ISD::ADD)
return DAG.getZeroExtendInReg(NewOpNode, DL, CleanUpVT);

SDValue CleanUp = DAG.getAnyExtOrTrunc(NewOpNode, DL, CleanUpVT);
return DAG.getAnyExtOrTrunc(CleanUp, DL, VT);
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@phoebewang phoebewang Feb 28, 2025

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Can we return DAG.getAnyExtOrTrunc(NewOpNode, DL, VT); directly?

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@joaotgouveia joaotgouveia Feb 28, 2025

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That causes a crash because of an assert in SelectionDAG::ReplaceAllUsesWith.

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@phoebewang phoebewang Feb 28, 2025 •
edited
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I see the problem, so we should return NewOpNode directly because we don't need to clean up the upper bits.

Proof: https://alive2.llvm.org/ce/z/AXuaQu

}

/// Attempt to pre-truncate inputs to arithmetic ops if it will simplify
Expand Down Expand Up @@ -53810,11 +53821,15 @@ static SDValue combineTruncatedArithmetic(SDNode *N, SelectionDAG &DAG,
if (!Src.hasOneUse())
return SDValue();

if (SDValue R = combinei64TruncSrlAdd(Src, VT, DAG, DL))
return R;
if (VT == MVT::i32 && SrcVT == MVT::i64 && SrcOpcode == ISD::SRL &&
Src.getOperand(0).getNumOperands() == 2 &&
isa<ConstantSDNode>(Src.getOperand(1)) &&
isa<ConstantSDNode>(Src.getOperand(0).getOperand(1))) {
if (SDValue R = combinei64TruncSrlConstant(Src, VT, DAG, DL))
return R;
return SDValue();
}
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@phoebewang phoebewang Feb 27, 2025

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Do not need to check NumOperands = 2.

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@joaotgouveia joaotgouveia Feb 27, 2025

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Removing that check causes LLVM to crash on a few regression tests (CodeGen/X86/arg-cast.ll, CodeGen/X86/pr49162.ll, CodeGen/X86/vector-reduce-xor-bool.ll, and ExecutionEngine/MCJIT/non-extern-addend.ll). I might be mistaken, but at the time combineTruncatedArithmetic is called, we have no information about whether the first operand of Src is actually a binary operation. Calling Src.getOperand(0).getOperand(1) might cause us to attempt to access an invalid operand.

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@phoebewang phoebewang Feb 27, 2025

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Oh, right! We can check opcode = ADD/OR/AND before it then.


// Only support vector truncation for now.
// TODO: i64 scalar math would benefit as well.
if (!VT.isVector())
return SDValue();
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@phoebewang phoebewang Feb 27, 2025

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Maybe combinei64TruncSrlConstant?


Expand Down
101 changes: 99 additions & 2 deletions llvm/test/CodeGen/X86/combine-i64-trunc-srl-add.ll
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Expand Up @@ -128,6 +128,103 @@ define i32 @test_trunc_add(i64 %x) {
ret i32 %conv
}

define i32 @test_trunc_sub(i64 %x) {
; X64-LABEL: test_trunc_sub:
; X64: # %bb.0:
; X64-NEXT: shrq $48, %rdi
; X64-NEXT: addl $65522, %edi # imm = 0xFFF2
; X64-NEXT: movzwl %di, %eax
; X64-NEXT: retq
%sub = sub i64 %x, 3940649673949184
%shr = lshr i64 %sub, 48
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@phoebewang phoebewang Feb 28, 2025

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Let's avoid to use 48 to test for the problem #128353 solving.

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@joaotgouveia joaotgouveia Feb 28, 2025

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Sure. Is changing one test enough, or should I change more than one?

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@phoebewang phoebewang Mar 1, 2025

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I'd like change them all.

%conv = trunc i64 %shr to i32
ret i32 %conv
}

define i32 @test_trunc_and_1(i64 %x) {
; X64-LABEL: test_trunc_and_1:
; X64: # %bb.0:
; X64-NEXT: movq %rdi, %rax
; X64-NEXT: shrq $48, %rax
; X64-NEXT: andl $14, %eax
; X64-NEXT: # kill: def $eax killed $eax killed $rax
; X64-NEXT: retq
%and = and i64 %x, 3940649673949184
%shr = lshr i64 %and, 48
%conv = trunc i64 %shr to i32
ret i32 %conv
}

define i32 @test_trunc_or_1(i64 %x) {
; X64-LABEL: test_trunc_or_1:
; X64: # %bb.0:
; X64-NEXT: movq %rdi, %rax
; X64-NEXT: shrq $48, %rax
; X64-NEXT: orl $14, %eax
; X64-NEXT: # kill: def $eax killed $eax killed $rax
; X64-NEXT: retq
%or = or i64 %x, 3940649673949184
%shr = lshr i64 %or, 48
%conv = trunc i64 %shr to i32
ret i32 %conv
}

define i32 @test_trunc_xor_1(i64 %x) {
; X64-LABEL: test_trunc_xor_1:
; X64: # %bb.0:
; X64-NEXT: movq %rdi, %rax
; X64-NEXT: shrq $48, %rax
; X64-NEXT: xorl $14, %eax
; X64-NEXT: # kill: def $eax killed $eax killed $rax
; X64-NEXT: retq
%xor = xor i64 %x, 3940649673949184
%shr = lshr i64 %xor, 48
%conv = trunc i64 %shr to i32
ret i32 %conv
}

define i32 @test_trunc_and_2(i64 %x) {
; X64-LABEL: test_trunc_and_2:
; X64: # %bb.0:
; X64-NEXT: movq %rdi, %rax
; X64-NEXT: shrq $48, %rax
; X64-NEXT: andl $13, %eax
; X64-NEXT: # kill: def $eax killed $eax killed $rax
; X64-NEXT: retq
%and = and i64 %x, 3940649673949183
%shr = lshr i64 %and, 48
%conv = trunc i64 %shr to i32
ret i32 %conv
}

define i32 @test_trunc_or_2(i64 %x) {
; X64-LABEL: test_trunc_or_2:
; X64: # %bb.0:
; X64-NEXT: movq %rdi, %rax
; X64-NEXT: shrq $48, %rax
; X64-NEXT: orl $13, %eax
; X64-NEXT: # kill: def $eax killed $eax killed $rax
; X64-NEXT: retq
%or = or i64 %x, 3940649673949183
%shr = lshr i64 %or, 48
%conv = trunc i64 %shr to i32
ret i32 %conv
}

define i32 @test_trunc_xor_2(i64 %x) {
; X64-LABEL: test_trunc_xor_2:
; X64: # %bb.0:
; X64-NEXT: movq %rdi, %rax
; X64-NEXT: shrq $48, %rax
; X64-NEXT: xorl $13, %eax
; X64-NEXT: # kill: def $eax killed $eax killed $rax
; X64-NEXT: retq
%xor = xor i64 %x, 3940649673949183
%shr = lshr i64 %xor, 48
%conv = trunc i64 %shr to i32
ret i32 %conv
}

; Make sure we don't crash on this test case.

define i32 @pr128158(i64 %x) {
Expand All @@ -137,10 +234,10 @@ define i32 @pr128158(i64 %x) {
; X64-NEXT: addq %rdi, %rax
; X64-NEXT: shrq $32, %rax
; X64-NEXT: .p2align 4
; X64-NEXT: .LBB9_1: # %for.body
; X64-NEXT: .LBB16_1: # %for.body
; X64-NEXT: # =>This Inner Loop Header: Depth=1
; X64-NEXT: cmpl $9, %eax
; X64-NEXT: jb .LBB9_1
; X64-NEXT: jb .LBB16_1
; X64-NEXT: # %bb.2: # %exit
; X64-NEXT: xorl %eax, %eax
; X64-NEXT: retq
Expand Down