Juneyoung Lee
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1977c53b2a
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[InstCombine] Fold overflow bit of [u|s]mul.with.overflow in a poison-safe way
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
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2021-05-02 11:54:12 +09:00 |