[LLVMdev] Probable error in InstCombine

[Stephen Canon]

Worth noting that if C is INTTYPE_MIN, (X/C) is equivalent to (X == INTTYPE_MIN), and CMP + SETcc/CMOV is generally cheaper than integer division.

– Steve

> On Jul 1, 2014, at 6:10 PM, Marcello Maggioni <hayarms at gmail.com> wrote:
> 
> Hmm, I believe you are right there.
> 
> There is no representable respective positive value for INT_MIN in i32 , which means the transformation is not possible in this case (in particular seems ConstantExpr::getNeg(C) in this case == C itself).
> 
> It should be checked that C->isMinValue(true) does not return true before applying the optimization I believe (C should be always a ConstantInt) .
> I'm not an expert on this though :)
> 
> Marcello
> 
> 
> 
> 
> 2014-07-01 23:01 GMT+02:00 David Menendez <davemm at cs.rutgers.edu>:
> I've found what appears to be a bug in instcombine. Specifically, the transformation of -(X/C) to X/(-C) is invalid if C == INT_MIN.
> 
> Specifically, if I have
> 
> > define i32 @foo(i32 %x) #0 {
> > entry:
> >  %div = sdiv i32 %x, -2147483648
> >  %sub = sub nsw i32 0, %div
> >  ret i32 %sub
> > }
> 
> then opt -instcombine will produce
> 
> > define i32 @foo(i32 %x) #0 {
> > entry:
> >  %sub = sdiv i32 %x, -2147483648
> >  ret i32 %sub
> > }
> 
> 
> You can observe this with the following test case:
> 
> > #include <stdio.h>
> > #include <limits.h>
> >
> > int foo(int x)
> > {
> >  return -(x/INT_MIN);
> > }
> >
> > int main (void)
> > {
> >  printf ("%d\n", foo(INT_MIN));
> > }
> 
> This will print -1 or 1, depending on the optimization level.
> 
> This appears to be the relevant code:
> 
> InstCombineAddSub.cpp:1556
> >     // 0 - (X sdiv C)  -> (X sdiv -C)
> >     if (match(Op1, m_SDiv(m_Value(X), m_Constant(C))) &&
> >         match(Op0, m_Zero()))
> >       return BinaryOperator::CreateSDiv(X, ConstantExpr::getNeg(C));
> 
> - David Menendez
> 
> 
> 
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