On Mar 24, 2007, at 9:55 PM, Nick Lewycky wrote:
> Chris Lattner wrote:
>> On Mar 23, 2007, at 11:46 AM, Reid Spencer wrote:
>>
>>> // shl uint X, 32 = 0 and shr ubyte Y, 9 = 0, ... just don't
>>> eliminate shr
>>> // of a signed value.
>>> //
>>> - unsigned TypeBits = Op0->getType()->getPrimitiveSizeInBits();
>>> - if (Op1->getZExtValue() >= TypeBits) {
>>> + if (Op1->getZExtValue() >= TypeBits) { // shift amount always
>>> <= 32 bits
>>
>>
>> This isn't safe. Code like:
>>
>> shl i1024 %A, 123456789123456789123456789123456789
>>
>> is undefined but legal. The compiler should not crash on it.
>
> What exactly is undefined? Just the result of that single instruction,
> or the behaviour of any program that attempts to execute it?
The behavior of any program that attempts to execute it.
> Would it be
> acceptable to replace such an instruction with "unreachable"?
Yes, in theory, but I'm not sure that's a good idea.
> If so, then it gives me a new source of information for predsimplify.
> After the instruction executes, I can assume that the RHS of a
> shift is
> within range.
I don't think this is appropriate. The failure mode is just too
great. You can treat the shift as though it returns an undef though.
-Chris
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