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c112624b0b
As discussed in D101191, this patch adds a poison-safe folding of overflow bit check:
```
%Op0 = icmp ne i4 %X, 0
%Agg = call { i4, i1 } @llvm.[us]mul.with.overflow.i4(i4 %X, i4 %Y)
%Op1 = extractvalue { i4, i1 } %Agg, 1
%ret = select i1 %Op0, i1 %Op1, i1 false
=>
%Y.fr = freeze %Y
%Agg = call { i4, i1 } @llvm.[us]mul.with.overflow.i4(i4 %X, i4 %Y.fr)
%Op1 = extractvalue { i4, i1 } %Agg, 1
%ret = %Op1
```
https://alive2.llvm.org/ce/z/zgPUGT
https://alive2.llvm.org/ce/z/h2gZ_6
Note that there are cases where inserting freeze is not necessary: e.g. %Y is `noundef`.
In this case, LLVM is already good because `%ret` is already successfully folded into `and`,
triggering the pre-existing optimization in InstSimplify: https://godbolt.org/z/v6qena15K
Differential Revision: https://reviews.llvm.org/D101423
46 lines
1.7 KiB
C++
46 lines
1.7 KiB
C++
//===-- OverflowInstAnalysis.h - Utils to fold overflow insts ----*- C++ -*-==//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file holds routines to help analyse overflow instructions
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// and fold them into constants or other overflow instructions
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_ANALYSIS_OVERFLOWINSTANALYSIS_H
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#define LLVM_ANALYSIS_OVERFLOWINSTANALYSIS_H
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#include "llvm/IR/InstrTypes.h"
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namespace llvm {
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class Value;
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class Use;
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/// Match one of the patterns up to the select/logic op:
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/// %Op0 = icmp ne i4 %X, 0
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/// %Agg = call { i4, i1 } @llvm.[us]mul.with.overflow.i4(i4 %X, i4 %Y)
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/// %Op1 = extractvalue { i4, i1 } %Agg, 1
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/// %ret = select i1 %Op0, i1 %Op1, i1 false / %ret = and i1 %Op0, %Op1
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///
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/// %Op0 = icmp eq i4 %X, 0
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/// %Agg = call { i4, i1 } @llvm.[us]mul.with.overflow.i4(i4 %X, i4 %Y)
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/// %NotOp1 = extractvalue { i4, i1 } %Agg, 1
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/// %Op1 = xor i1 %NotOp1, true
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/// %ret = select i1 %Op0, i1 true, i1 %Op1 / %ret = or i1 %Op0, %Op1
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///
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/// Callers are expected to align that with the operands of the select/logic.
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/// IsAnd is set to true if the Op0 and Op1 are used as the first pattern.
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/// If Op0 and Op1 match one of the patterns above, return true and fill Y's
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/// use.
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bool isCheckForZeroAndMulWithOverflow(Value *Op0, Value *Op1, bool IsAnd,
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Use *&Y);
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bool isCheckForZeroAndMulWithOverflow(Value *Op0, Value *Op1, bool IsAnd);
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} // end namespace llvm
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#endif
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