[llvm-dev] should we do this time-consuming transform in InstCombine? (original) (raw)

Zheng CZ Chen via llvm-dev llvm-dev at lists.llvm.org
Tue Dec 18 00:26:11 PST 2018

Hi Roman,

Thanks for your good idea. I think it can solve the abs issue very well. I can continue with my work now^-^.

But if it is not abs and there is no select, %res = OP i32 %b, %a %sub = sub i32 0, %b %res2 = OP i32 %sub, %a

theoretically, we can still do the following transform for the above pattern: %res2 = OP i32 %sub, %a ==> %res2 = sub i32 0, %res

Not sure whether we can do it in instCombine.

Thanks.

BRS// Chen Zheng Power Compiler Backend Developer

From: Roman Lebedev <lebedev.ri at gmail.com> To: Zheng CZ Chen <czhengsz at cn.ibm.com> Cc: llvm-dev at lists.llvm.org Date: 2018/12/18 03:45 PM Subject: Re: [llvm-dev] should we do this time-consuming transform in InstCombine?

On Tue, Dec 18, 2018 at 10:18 AM Zheng CZ Chen via llvm-dev <llvm-dev at lists.llvm.org> wrote:

Hi, Hi.

There is an opportunity in instCombine for following instruction pattern:

%mul = mul nsw i32 %b, %a %cmp = icmp sgt i32 %mul, -1 %sub = sub i32 0, %a %mul2 = mul nsw i32 %sub, %b %cond = select i1 %cmp, i32 %mul, i32 %mul2 Source code for above pattern: return (ab) >=0 ? (ab) : -a*b; Currently, llvm(-O3) can not recognize this as abs(a*b). I initially think we could do this in instCombine phase in opt. Below is what I think: %res = OP i32 %b, %a %sub = sub i32 0, %b %res2 = OP i32 %sub, %a We could do the transform: %res2 = OP i32 %sub, %a ==> %res2 = sub i32 0, %res Then we can get the advantage: 1: if %res2 is the only user of %sub, %sub can be eliminated; 2: if %res2 is not the only user of %sub, we could change some heavy instruction like div to sub; 3: expose more abs opportunity for later pass. But my concern is finding %res is a little compiling time-consuming. At least we need MIN(usercount(%b), usercount(%a)) times to check if instruction with same opcode and same operands exists. In instcombine, no user checking is performed/allowed. This should match that specific pattern (other than verifying the correct equal binop types), although i have not tested it:

ICmpInst::Predicate Pred; Value *A, *B, *Mul, *Sub, *Mul2; if (match(&SI, m_Select(m_ICmp(Pred, m_CombineAnd(m_BinOp(m_Value(A), m_Value(B)), m_Value(Mul)), m_AllOnes()), m_Deferred(Mul), m_CombineAnd( m_c_BinOp(m_CombineAnd(m_Sub(m_Zero(), m_Deferred (A)), m_Value(Sub)), m_Deferred(B)), m_Value(Mul2)))) && Pred == ICmpInst::Predicate::ICMP_SGT) { }

Could you guys give some comment? Is there any better idea for this transform?

Thanks. BRS// Chen Zheng Power Compiler Backend Developer Roman.

LLVM Developers mailing list llvm-dev at lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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