mirror of
https://github.com/c64scene-ar/llvm-6502.git
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fff6c5332f
the definition of the nsw and nuw flags to make use of it. nsw was introduced to help optimizers answer yes to the following: // Can we change i from i32 to i64 to eliminate the cast inside the loop? for (int i = 0; i < n; ++i) A[i] *= 0.1; // Can we assume that this loop will eventually terminate? for (int i = 0; i <= n; ++i) A[i] *= 0.1; In its current form, it isn't truly sufficient for either. In the first case, if the increment overflows, it'll still have some valid i32 value; sign-extending it will produce a value which is 33 homogeneous sign bits trailed by 31 independent undef bits. If i is promoted to i64, it won't have those same values when it reaches that point. (The compiler could recover here by reasoning about how i is used by the load, but that's a lot more complicated and isn't always possible.) In the second case, there is no value for i which will be greater than n, so having the increment return undef on overflow doesn't help. Trap values are a formalization of some existing concepts that we have about LLVM IR, and give the optimizers a better basis for answering yes to both questions above. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102140 91177308-0d34-0410-b5e6-96231b3b80d8 |
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