[LLVMdev] Possible bug in ScalarEvolution

Zhou Sheng zhousheng00 at gmail.com
Wed Nov 12 01:30:57 PST 2008


I'm using pass ScalarEvolution to analyze the loop trip count on my
And I found a possible bug in the code, that is in function
Line 2905:

2904     // The exit value should be (End+A)/A.
2905     APInt ExitVal = (End + A).udiv(A);
2906     ConstantInt *ExitValue = ConstantInt::get(ExitVal);

The divide should be sdiv, right? otherwise, the ExitVal will be zero when A
= -1

For example,   for (int i = 15; i > 7; --i) {}   the exit value should be 7
not zero.

Also, I think we should consider the possible overflow situation,

for example,  for (uint7 i = 0; i < 64; i += 2) {}

The Range should be [0, 64)  (type i7), so

End = 63,  A = 2,

(End + A).sdiv(A) will be 97  instead of 32.

This is due to overflow of the addition of  End and A. They both are
positive value, and the result should be 65 which in i7 type may be

Any comments?
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