Roman Lebedev
|
473a063a5e
|
[InstCombine] Fold '((%x * %y) u/ %x) != %y' to '@llvm.umul.with.overflow' + overflow bit extraction
Summary:
`((%x * %y) u/ %x) != %y` is one of (3?) common ways to check that
some unsigned multiplication (will not) overflow.
Currently, we don't catch it. We could:
```
$ /repositories/alive2/build-Clang-unknown/alive -root-only ~/llvm-patch1.ll
Processing /home/lebedevri/llvm-patch1.ll..
----------------------------------------
Name: no overflow
%o0 = mul i4 %y, %x
%o1 = udiv i4 %o0, %x
%r = icmp ne i4 %o1, %y
ret i1 %r
=>
%n0 = umul_overflow i4 %x, %y
%o0 = extractvalue {i4, i1} %n0, 0
%o1 = udiv %o0, %x
%r = extractvalue {i4, i1} %n0, 1
ret %r
Done: 1
Optimization is correct!
----------------------------------------
Name: no overflow
%o0 = mul i4 %y, %x
%o1 = udiv i4 %o0, %x
%r = icmp eq i4 %o1, %y
ret i1 %r
=>
%n0 = umul_overflow i4 %x, %y
%o0 = extractvalue {i4, i1} %n0, 0
%o1 = udiv %o0, %x
%n1 = extractvalue {i4, i1} %n0, 1
%r = xor %n1, -1
ret i1 %r
Done: 1
Optimization is correct!
```
Reviewers: nikic, spatel, efriedma, xbolva00, RKSimon
Reviewed By: nikic
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D65144
llvm-svn: 370348
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2019-08-29 12:47:20 +00:00 |