[PATCH] D126040: [InstCombine] Fold a mul with bool value into and

[Allen zhong via Phabricator via llvm-commits]

Allen added inline comments.


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp:312
+        match(Op1, m_And(m_Value(Y), m_One())))
+      return BinaryOperator::CreateAnd(Op0, Op1);
+  }
----------------
Allen wrote:
> craig.topper wrote:
> > xbolva00 wrote:
> > > grandinj wrote:
> > > > Surely the more general case is something like:
> > > > 
> > > >     if (ComputeMaxSignificantBits(Op0) ==1 ||
> > > >         ComputeMaxSignificantBits(Op1)==1)
> > > > 
> > > > ?
> > > > 
> > > Yeah, please more general case.
> > ComputeMaxSignificantBits(Op0) ==1 means the input is 0 or -1.
> > 
> > You would need computeKnownBits or MaskedValueIsZero.
> Thanks very much , I'll try to use the API ComputeMaxSignificantBits.
> 
> and there is some failure cases, for example, it 'll turn the -42 into 42.
> ```
> ; X * C (when X is a sext boolean) --> X ? -C : 0
> 
> define i32 @mul_sext_bool(i1 %x) {
> ; CHECK-LABEL: @mul_sext_bool(   
> ; CHECK-NEXT:    [[M:%.*]] = select i1 [[X:%.*]], i32 -42, i32 0
> ; CHECK-NEXT:    ret i32 [[M]]
> ;            
>   %s = sext i1 %x to i32
>   %m = mul i32 %s, 42
>   ret i32 %m
> }
> ```
hi @craig.topper, base your comment, I tried the following change, and it works. 
```
+    KnownBits XKnown = computeKnownBits(Op0, 0, &I);
+    KnownBits YKnown = computeKnownBits(Op1, 0, &I);
+    if ((XKnown.countMaxPopulation() == 1) &&
+        (YKnown.countMaxPopulation() == 1))
+      return BinaryOperator::CreateAnd(Op0, Op1);
```

but there is a new failure case **mul_bools_use3 **in file llvm/test/Transforms/InstCombine/mul.ll. And I can't sure which version is better, should I add extra condition to avoid touch this case ?
* without above change
```
%r = select i1 %x, i32 %zy, i32 0
```
* with above change
```
  %r1 = and i1 %x, %y
  %r = zext i1 %r1 to i32
```



CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D126040/new/

https://reviews.llvm.org/D126040