[CIR][CIRGen][builtin] handle `__lzcnt` #1382

FantasqueX · 2025-02-21T17:01:26Z

Traditional clang implementation:

Lines 3618 to 3632 in a1ab6bf

    
           case Builtin::BI__lzcnt16: 
        
           case Builtin::BI__lzcnt: 
        
           case Builtin::BI__lzcnt64: { 
        
             Value *ArgValue = EmitScalarExpr(E->getArg(0)); 
        
             llvm::Type *ArgType = ArgValue->getType(); 
        
             Function *F = CGM.getIntrinsic(Intrinsic::ctlz, ArgType); 
        
             llvm::Type *ResultType = ConvertType(E->getType()); 
        
             Value *Result = Builder.CreateCall(F, {ArgValue, Builder.getFalse()}); 
        
             if (Result->getType() != ResultType) 
        
               Result = Builder.CreateIntCast(Result, ResultType, /*isSigned*/true, 
        
                                              "cast"); 
        
             return RValue::get(Result); 
        
           }

The problem here is that __builtin_clz allows undefined result, while __lzcnt doesn't. As a result, I have to create a new CIR for __lzcnt. Since the return type of those two builtin differs, I decided to change return type of current CIR_BitOp to allow new CIR_LzcntOp to inherit from it.

I would like to hear your suggestions. C.c. @Lancern

Lancern · 2025-02-22T12:35:02Z

The problem here is that __builtin_clz allows undefined result, while __lzcnt doesn't. As a result, I have to create a new CIR for __lzcnt.

Have you considered updating the existing cir.bit.clz operation and add a unit attribute that specifies whether an input value of 0 is poisoned?

bcardosolopes · 2025-02-27T01:54:24Z

clang/include/clang/CIR/Dialect/IR/CIROps.td

@@ -1714,6 +1714,30 @@ def BitPopcountOp
  }];
 }

+def BitLzcntOp : CIR_BitOp<"bit.lzcnt", AnyTypeOf<[UInt16, UInt32, UInt64]>> {


This can share a tablegen class with BitPopcountOp.

Why BitPopcount? Do you mean BitClzOp? The reason is their different poison bit.

I was actually curious if you could reuse the CIR_BitOp if you make it a bit more generic.

github-actions · 2025-03-01T14:21:19Z

✅ With the latest revision this PR passed the C/C++ code formatter.

FantasqueX · 2025-03-01T14:25:05Z

The new LzcntOp doesn't inherent BitOp. So, I can avoid touching original code.

Lancern

I'm still unclear about why we don't just update cir.bit.clz for __lzcnt. AFAIK, we may have two problems in doing so:

The result type of cir.bit.clz is fixed to be !s32i. Have you tried update the TableGen definition of cir.bit.clz so you could relax this constraint?
cir.bit.clz would invoke undefined behaviors if the operand is 0. To overcome this, we could add a UnitAttr to the parameters of cir.bit.clz that indicates whether the operation should poison an input value of 0. Have you tried this?

Are there any other concerns I'm missing here?

Lancern · 2025-03-01T17:17:19Z

clang/include/clang/CIR/Dialect/IR/CIROps.td

+    Compute the number of leading 0-bits in the input.
+
+    The input integer must be an unsigned integer. The `cir.bit.lzcnt` operation
+    returns the number of consecutive 0-bits at the most significant bit
+    position in the input.
+
+    If the input value is 0, the return value is the size of the input value.


Add some more description about the differences between the cir.bit.clz operation.

Yea, that'd be great

bcardosolopes · 2025-03-04T03:26:31Z

+1 on all comments from @Lancern

FantasqueX · 2025-03-04T19:53:47Z

@Lancern @bcardosolopes Thanks for your review. I'm going to make some changes. First, changing the return type of CIR_BitOp to match input. Second, adding a new base class for CIR_ClzOp and CIR_CtzOp which contains a unit attribute to indicate poison. For future work, specifically handling BI__builtin_clzg, we can add more logic in CodeGen stage and reuse CIR_ClzOp. I would like to hear your suggestions❤️

FantasqueX · 2025-03-06T07:29:56Z

I have reworked the patch. Creating a base class for ClzOp and CtzOp which contains an I1Attr just like MLIR LLVMIR. And change BitOp to require same operand and result type. So, at CodeGen stage, clz will emit a ClzOp and a CastOp. Also changing lowering stage to emit llvm.intr.ctlz directly install of calling intrinsics. This patch also fixes an IR difference for ffs like op. Before, CIR casts after ctlz and uses sint32 to calculate. However, OG calculates first and then casts. This patch removes ThroughMLIR lowering for ClzOp and CtzOp if is_zero_poison is true.

FantasqueX · 2025-03-06T07:31:54Z

I also noticed that MLIR LLVMIR doesn't require SameOperandsAndResultType for llvm.intr.ctlz, llvm.intr.cttz and llvm.intr.ctpop.

Lancern

Another round of review! Some comments inline.

Lancern · 2025-03-06T13:17:29Z

clang/include/clang/CIR/Dialect/IR/CIROps.td


  let assemblyFormat = [{
    `(` $input `:` type($input) `)` `:` type($result) attr-dict
  }];
 }

+class CIR_CountZerosBitOp<string mnemonic, TypeConstraint inputTy>
+    : CIR_BitOp<mnemonic, inputTy> {
+  let arguments = (ins inputTy:$input, I1Attr:$is_zero_poison);


Do not use I1Attr; use UnitAttr instead. Then use optional groups in the assembly format for is_zero_poison.

I think we should use I1Attr here to be consist with MLIR

clangir/mlir/include/mlir/Dialect/LLVMIR/LLVMIntrinsicOps.td

Line 93 in 52fd4a2

I1Attr:$is_zero_poison);

I believe CIR always uses UnitAttr for representing boolean flags.

clang/lib/CIR/CodeGen/CIRGenBuiltin.cpp

clang/lib/CIR/Lowering/ThroughMLIR/LowerCIRToMLIR.cpp

FantasqueX requested review from lanza and bcardosolopes as code owners February 21, 2025 17:01

FantasqueX marked this pull request as draft February 23, 2025 09:58

FantasqueX mentioned this pull request Feb 23, 2025

[RFC] Refine design of CIR_BitOp #1392

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bcardosolopes reviewed Feb 27, 2025

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FantasqueX force-pushed the builtin-lzcnt branch from 0e571e7 to 17557ab Compare March 1, 2025 14:14

FantasqueX marked this pull request as ready for review March 1, 2025 14:17

FantasqueX requested a review from Lancern March 1, 2025 14:18

Lancern reviewed Mar 1, 2025

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FantasqueX force-pushed the builtin-lzcnt branch from 2d8eca3 to 0fbbce2 Compare March 6, 2025 07:23

Lancern requested changes Mar 6, 2025

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FantasqueX added 9 commits March 7, 2025 15:56

[CIR][CIRGen][builtin] handle __lzcnt

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fix bit.cir test

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Redesign LzcntOp

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Restore last line

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Redine CIR clz

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FantasqueX force-pushed the builtin-lzcnt branch from 0fbbce2 to 445b348 Compare March 7, 2025 07:59

FantasqueX added 2 commits March 7, 2025 23:54

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[CIR][CIRGen][builtin] handle `__lzcnt` #1382

[CIR][CIRGen][builtin] handle `__lzcnt` #1382

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	case Builtin::BI__lzcnt16:
	case Builtin::BI__lzcnt:
	case Builtin::BI__lzcnt64: {
	Value *ArgValue = EmitScalarExpr(E->getArg(0));

	llvm::Type *ArgType = ArgValue->getType();
	Function *F = CGM.getIntrinsic(Intrinsic::ctlz, ArgType);

	llvm::Type *ResultType = ConvertType(E->getType());
	Value *Result = Builder.CreateCall(F, {ArgValue, Builder.getFalse()});
	if (Result->getType() != ResultType)
	Result = Builder.CreateIntCast(Result, ResultType, /isSigned/true,
	"cast");
	return RValue::get(Result);
	}

[CIR][CIRGen][builtin] handle __lzcnt #1382

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[CIR][CIRGen][builtin] handle __lzcnt #1382

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[CIR][CIRGen][builtin] handle `__lzcnt` #1382

[CIR][CIRGen][builtin] handle `__lzcnt` #1382

FantasqueX commented Feb 21, 2025 •

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