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On Tue, Jan 27, 2015 at 7:23 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote: > Hi David, > > I spent some time thinking about poison semantics this way, but here > is where I always get stuck: > > Consider the IR fragment > > %x = zext i32 %maybe_poison to i64 > %y = lshr i64 %x 32 > %ptr = gep %global, %y > store 42 to %ptr > > If

...t. > > Right, which is why I initially chose zext. :) > > But with sexts with you a similar problem: > > > %t = add nsw <known positive>, <known positive> > %t1 = and %t, 0x7fff... > %t2 = sext %t1 > %t3 = lshr %t2, <width of typeof %t> > store_to (gep %global, %t2) > > Now %t3 is always zero and this program is well-defined if even if %t > is poison. However, I cannot reason "%t never overflows, so it is > always >0 and hence %t1 == %t" since substituting %t1 with %t will > change the meaning of a well-defined p...