[PATCH] D150943: [SimplifyCFG] add nsw on BuildLookuptable LinearMap calculation

[Kohei Asano via Phabricator via llvm-commits]

khei4 added a comment.

TODO: use ConstantRange, 
TODO: add test case which have select case value ranges [2^n-1, 2^n].



================
Comment at: llvm/lib/Transforms/Utils/SimplifyCFG.cpp:6122
+    LinearOffset->getValue().sadd_ov(
+        LinearMultiplier->getValue().smul_ov(MaxIndex, MulSW), AddSW);
     if (!LinearMultiplier->isOne())
----------------
nikic wrote:
> I don't think this is sufficient. Let's say we have multiplier 2 and MaxIndex is -1 (that is, full range). Then 2*-1 does not overflow, but there are still plenty of values in the range that do overflow (e.g. INT_MAX).
> 
> A possible way to properly handle this would be using ConstantRange. I think another possibility is to used ssub_ov for the `Val - PrevVal` calculation above and set NSW based on the overflow flag there.
> 
> In any case, we need additional tests to cover this.
> Let's say we have multiplier 2 and MaxIndex is -1 (that is, full range). Then 2*-1 does not overflow, but there are still plenty of values in the range that do overflow (e.g. INT_MAX).

Yeah, its case matters. Overflowed MaxIndex should be handled. I'll use ConstantRange! 

Also, I'll add a linear map test that uses full range.


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  https://reviews.llvm.org/D150943/new/

https://reviews.llvm.org/D150943